101 Core Java Interview Questions
1) What is Java?
Java is the high-level, object-oriented, robust, secure programming language, platform-independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.
2) List the features of Java Programming language.
There are the following features in Java Programming Language.
-Simple: Java is easy to learn. The syntax of Java is based on C++ which makes it easier to write the program in it.
-Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different types of objects that incorporates both data and behavior.
-Portable: Java supports the read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine.
-Platform Independent: Java is a platform-independent programming language. It is different from other programming languages like C and C++ which need a platform to be executed. Java comes with its platform on which its code is executed. Java doesn't depend upon the operating system to be executed.
-Secured: Java is secured because it doesn't use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secure.
-Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust.
-Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data *types may vary according to the architecture (32 bit or 64 bit) which doesn't exist in Java.
-Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.
-High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is "close" to native code. It is still a little bit slower than a compiled language (e.g., C++).
-Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn't occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc.
-Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet.
-Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.
4) What do you understand by Java virtual machine?
Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification that must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine-independent and close to the native code.
5) What is the difference between JDK, JRE, and JVM?
JVM
JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification that specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE.
JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance that is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.
JRE
JRE stands for Java Runtime Environment. It is the implementation of JVM. The Java Runtime Environment is a set of software tools that are used for developing Java applications. It is used to provide a runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.
JDK
JDK is an acronym for Java Development Kit. It is a software development environment that is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:
Standard Edition Java Platform
Enterprise Edition Java Platform
Micro Edition Java Platform
6) How many types of memory areas are allocated by JVM?
Many types:
Class(MeAreathod): Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
Heap: It is the runtime data area in which the memory is allocated to the objects
Stack: Java Stack store frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
Native Method Stack: It contains all the native methods used in the application.
7) What is the JIT compiler?
Just-In-Time(JIT) compiler: It is used to improve performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
8) What is the platform?
A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides a software-based platform.
9) What are the main differences between the Java platform and other platforms?
There are the following differences between the Java platform and other platforms.
*Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
*Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.
10) What gives Java its 'write once and run anywhere' nature?
The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform-specific and can be executed on any computer.
11) What is the classloader?
Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.
Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java. util package classes, java.io package classes, java.sql package classes, etc.
Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/JRE/lib/ext directory.
System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using the "-cp" or "-classpath" switch. It is also known as the Application classloader.
12) Is Empty .java file name a valid source file name?
Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let's take a simple example:
//save by .java only
class A{
public static void main(String args[]){
System.out.println("Hello java");
}
}
//compile by javac .java
//run by java A
13) Is delete, next, main, exit or null keyword in java?
No.
14) If I don't provide any arguments on the command line, then what will the value stored in the String array passed into the main() method, empty or NULL?
It is empty, but not null.
15) What if I write static public void instead of the public static void?
The program compiles and runs correctly because the order of specifiers doesn't matter in Java.
16) What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references.
17) What are the various access specifiers in Java?
In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.
Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
The default is accessible within the package only. By default, all the classes, methods, and variables are of default scope.
Private The private class, methods, or variables defined as private can be accessed within the class only.
18) What is the purpose of static methods and variables?
The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.
For example, In the class simulating the collection of the students in a college, the name of the college is a common attribute to all the students. Therefore, the college name will be defined as static.
19) What are the advantages of Packages in Java?
There are various advantages to defining packages in Java.
Packages avoid name clashes.
The Package provides easier access control.
We can also have the hidden classes that are not visible outside and used by the package.
It is easier to locate related classes.
20) What is the output of the following Java program?
class Test
{
public static void main (String args[])
{
System.out.println(10 + 20 + "Javatpoint");
System.out.println("Javatpoint" + 10 + 20);
}
}
The output of the above code will be
30Javatpoint
Javatpoint1020
Explanation
In the first case, 10 and 20 are treated as numbers and added to be 30. Now, their sum 30 is treated as the string and concatenated with the string Javatpoint. Therefore, the output will be 30Javatpoint.
In the second case, the string Javatpoint is concatenated with 10 to be the string Javatpoint10 which will then be concatenated with 20 to be Javatpoint1020.
21) What is the output of the following Java program?
class Test
{
public static void main (String args[])
{
System.out.println(10 * 20 + "Javatpoint");
System.out.println("Javatpoint" + 10 * 20);
}
}
The output of the above code will be
200Javatpoint
Javatpoint200
Explanation
In the first case, The numbers 10 and 20 will be multiplied first and then the result 200 is treated as the string and concatenated with the string Javatpoint to produce the output 200Javatpoint.
In the second case, The numbers 10 and 20 will be multiplied first to be 200 because the precedence of the multiplication is higher than in addition. The result 200 will be treated as the string and concatenated with the string Javatpointto produce the output as Javatpoint200.
22) What is the output of the following Java program?
class Test
{
public static void main (String args[])
{
for(int i=0; 0; i++)
{
System.out.println("Hello Javatpoint");
}
}
}
The above code will give the compile-time error because the for loop demands a boolean value in the second part and we are providing an integer value, i.e., 0.
23) What is an object-oriented paradigm?
It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. The object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes that interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.
Follows the bottom-up approach in program design.
Focus on data with methods to operate upon the object's data
Includes the concept like Encapsulation and abstraction which hides the complexities from the user and shows only functionality.
Implements the real-time approach like inheritance, abstraction, etc.
The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.
24) What is an object?
The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.
25) What is the difference between an object-oriented programming language and an object-based programming language?
There are the following basic differences between the object-oriented language and object-based language.
Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn't follow all the concepts of OOPs like inheritance and polymorphism.
Object-oriented languages do not have the inbuilt objects whereas Object-based languages have inbuilt objects, for example, JavaScript has window objects.
Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.
26) What will be the initial value of an object reference which is defined as an instance variable?
All object references are initialized to null in Java.
27) What is the constructor?
The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.
28) How many types of constructors are used in Java?
Based on the parameters passed in the constructors, there are two types of constructors in Java.
Default Constructor: The default constructor is the one that does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful tasks on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class.
Parameterized Constructor: The parameterized constructor is the one that can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.
29) What is the purpose of a default constructor?
The purpose of the default constructor is to assign the default value to the objects. The java compiler creates a default constructor implicitly if there is no constructor in the class.
class Student3{
int id;
String name;
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student3 s1=new Student3();
Student3 s2=new Student3();
s1.display();
s2.display();
}
}
Output:
0 null
0 null
Explanation: In the above class, you are not creating any constructor, so the compiler provides you a default constructor. Here 0 and null values are provided by default constructor.
30) Does the constructor return any value?
Ans: yes, The constructor implicitly returns the current instance of the class (You can't use an explicit return type with the constructor).
31)Is constructor inherited?
No, The constructor is not inherited.
32) Can you make a constructor final?
No, the constructor can't be final.
33) Can we overload the constructors?
Yes, the constructors can be overloaded by changing the number of arguments accepted by the constructor or by changing the data type of the parameters. Consider the following example.
class Test
{
int i;
public Test(int k)
{
i=k;
}
public Test(int k, int m)
{
System.out.println("Hi I am assigning the value max(k, m) to i");
if(k>m)
{
i=k;
}
else
{
i=m;
}
}
}
public class Main
{
public static void main (String args[])
{
Test test1 = new Test(10);
Test test2 = new Test(12, 15);
System.out.println(test1.i);
System.out.println(test2.i);
}
}
In the above program, The constructor Test is overloaded with another constructor. In the first call to the constructor, The constructor with one argument is called, and i will be initialized with the value 10. However, In the second call to the constructor, The constructor with the 2 arguments is called, and i will be initialized with the value 15.
34) What do you understand by copy constructor in Java?
There is no copy constructor in java. However, we can copy the values from one object to another like a
copy constructor in C++.
There are many ways to copy the values of one object into another in java. They are:
By constructor
By assigning the values of one object into another
By clone() method of Object class
In this example, we are going to copy the values of one object into another using java constructor.
//Java program to initialize the values from one object to another class
Student6{
int id;
String name;
//constructor to initialize integer and string
Student6(int i,String n){
id = i;
name = n;
}
//constructor to initialize another object
Student6(Student6 s){
id = s.id;
name =s.name;
}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student6 s1 = new Student6(111,"Karan");
Student6 s2 = new Student6(s1);
s1.display();
s2.display();
}
}
40) What is the static method?
A static method belongs to the class rather than the object.
There is no need to create the object to call the static methods.
A static method can access and change the value of the static variable.
41) What are the restrictions that are applied to the Java static methods?
Two main restrictions are applied to static methods.
The static method can not use non-static data member or call the non-static method directly.
this and super cannot be used in static context as they are non-static.
42) Why is the main method static?
Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call a main() method which will lead to the extra memory allocation.
43) Can we override the static methods?
No, we can't override static methods.
44) What is the static block?
A static block is used to initialize the static data member. It is executed before the main method, at the time of classloading.
class A2{
static{System.out.println("static block is invoked");}
public static void main(String args[]){
System.out.println("Hello main");
}
}
Output: static block is invoked
Hello main
48) Can we make constructors static?
As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.
49) Can we make the abstract methods static in Java?
In Java, if we make the abstract methods static, It will become part of the class, and we can directly call it which is unnecessary. Calling an undefined method is completely useless therefore it is not allowed.
50) Can we declare the static variables and methods in an abstract class?
Yes, we can declare static variables and methods in an abstract method. As we know that there is no requirement to make the object to access the static context, therefore, we can access the static context declared inside the abstract class by using the name of the abstract class. Consider the following example.
abstract class Test
{
static int i = 102;
static void TestMethod()
{
System.out.println("hi !! I am good !!");
}
}
public class TestClass extends Test
{
public static void main (String args[])
{
Test.TestMethod();
System.out.println("i = "+Test.i);
}
}
Output
hi !! I am good !!
i = 102
51) What is this keyword in java?
This keyword is a reference variable that refers to the current object. There are various uses of this keyword in Java. It can be used to refer to current class properties such as for instance methods, variables, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.
java this keyword
More Details.
52) What are the main uses of this keyword?
There are the following uses of this keyword.
this can be used to refer to the current class instance variable.
this can be used to invoke current class method (implicitly)
this() can be used to invoke the current class constructor.
this can be passed as an argument in the method call.
this can be passed as an argument in the constructor call.
this can be used to return the current class instance from the method.
53) Can we assign the reference to this variable?
No, this cannot be assigned to any value because it always points to the current class object and this is the final reference in Java. However, if we try to do so, the compiler error will be shown. Consider the following example.
public class Test
{
public Test()
{
this = null;
System.out.println("Test class constructor called");
}
public static void main (String args[])
{
Test t = new Test();
}
}
Output
Test.java:5: error: cannot assign a value to final variable this
this = null;
^
1 error
54) Can this keyword be used to refer to static members?
Yes, It is possible to use this keyword to refer to static members because this is just a reference variable that refers to the current class object. However, as we know that, it is unnecessary to access static variables through objects, therefore, it is not the best practice to use this to refer static members. Consider the following example.
public class Test
{
static int i = 10;
public Test ()
{
System.out.println(this.i);
}
public static void main (String args[])
{
Test t = new Test();
}
}
Output
10
55) How can constructor chaining be done using this keyword?
Constructor chaining enables us to call one constructor from another constructor of the class with respect to the current class object. We can use this keyword to perform constructor chaining within the same class. Consider the following example which illustrates how can we use this keyword to achieve constructor chaining.
public class Employee
{
int id,age;
String name, address;
public Employee (int age)
{
this.age = age;
}
public Employee(int id, int age)
{
this(age);
this.id = id;
}
public Employee(int id, int age, String name, String address)
{
this(id, age);
this.name = name;
this.address = address;
}
public static void main (String args[])
{
Employee emp = new Employee(105, 22, "Vikas", "Delhi");
System.out.println("ID: "+emp.id+" Name:"+emp.name+" age:"+emp.age+" address: "+emp.address);
}
}
Output
ID: 105 Name:Vikas age:22 address: Delhi
56) What are the advantages of passing this into a method instead of the current class object itself?
As we know, this refers to the current class object, therefore, it must be similar to the current class object. However, there can be two main advantages of passing this into a method instead of the current class object.
this is a final variable. Therefore, this cannot be assigned to any new value whereas the current class object might not be final and can be changed.
this can be used in the synchronized block.
57) What is Inheritance?
Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship.
There are five types of inheritance in Java.
Single-level inheritance
Multi-level inheritance
Multiple Inheritance
Hierarchical Inheritance
Hybrid Inheritance
Multiple inheritances is not supported in Java through the class.
58) Why is Inheritance used in Java?
There are various advantages of using inheritance in Java that is given below.
Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method.
Runtime polymorphism cannot be achieved without using inheritance.
We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic.
Inheritance provides data hiding. The base class can hide some data from the derived class by making it private.
Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic methods contained by the base class.
59) Which class is the superclass for all the classes?
The object class is the superclass of all other classes in Java.
60) Why are multiple inheritances not supported in java?
To reduce the complexity and simplify the language, multiple inheritances is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.
Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile-time error.
class A{
void msg(){System.out.println("Hello");}
}
class B{
void msg(){System.out.println("Welcome");}
}
class C extends A,B{//suppose if it were
Public Static void main(String args[]){
C obj=new C();
obj.msg();//Now which msg() method would be invoked?
}
}
Compile Time Error
61) What is aggregation?
Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that the Employee (class) has an object of Address class. Consider the following example.
(Address.java)
public class Address {
String city,state,country;
public Address(String city, String state, String country) {
this.city = city;
this.state = state;
this.country = country;
}
}
(Employee.java)
public class Emp {
int id;
String name;
Address address;
public Emp(int id, String name,Address address) {
this.id = id;
this.name = name;
this.address=address;
}
void display(){
System.out.println(id+" "+name);
System.out.println(address.city+" "+address.state+" "+address.country);
}
public static void main(String[] args) {
Address address1=new Address("gzb","UP","india");
Address address2=new Address("gno","UP","india");
Emp e=new Emp(111,"varun",address1);
Emp e2=new Emp(112,"arun",address2);
e.display();
e2.display();
}
}
Output
111 varun
gzb UP india
112 arun
gno UP india
62) What is composition?
Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of a container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists a composition between class and students.
63) What is the difference between aggregation and composition?
Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).
64) Why does Java not support pointers?
The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.
65) What is super in java?
The super keyword in Java is a reference variable that is used to refer to the immediate parent class object. Whenever you create the instance of the subclass, an instance of the parent class is created implicitly which is referred by a super reference variable. The super() is called in the class constructor implicitly by the compiler if there is no super or this.
class Animal{
Animal(){System.out.println("animal is created");}
}
class Dog extends Animal{
Dog(){
System.out.println("dog is created");
}
}
class TestSuper4{
public static void main(String args[]){
Dog d=new Dog();
}
}
animal is created
dog is created
66) How can constructor chaining be done by using the super keyword?
class Person
{
String name,address;
int age;
public Person(int age, String name, String address)
{
this.age = age;
this.name = name;
this.address = address;
}
}
class Employee extends Person
{
float salary;
public Employee(int age, String name, String address, float salary)
{
super(age,name,address);
this.salary = salary;
}
}
public class Test
{
public static void main (String args[])
{
Employee e = new Employee(22, "Mukesh", "Delhi", 90000);
System.out.println("Name: "+e.name+" Salary: "+e.salary+" Age: "+e.age+" Address: "+e.address);
}
}
Output
Name: Mukesh Salary: 90000.0 Age: 22 Address: Delhi
67) What are the main uses of the super keyword?
There are the following uses of the super keyword.
super can be used to refer to the immediate parent class instance variable.
super can be used to invoke the immediate parent class method.
super() can be used to invoke immediate parent class constructor.
68) What are the differences between this and super keyword?
There are the following differences between this and super keyword.
The super keyword always points to the parent class contexts whereas this keyword always points to the current class context.
The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor.
The super and this must be the first statement inside constructor otherwise the compiler will throw an error.
69) What is the output of the following Java program?
class Person
{
public Person()
{
System.out.println("Person class constructor called");
}
}
public class Employee extends Person
{
public Employee()
{
System.out.println("Employee class constructor called");
}
public static void main (String args[])
{
Employee e = new Employee();
}
}
Output
Person class constructor called
Employee class constructor called
Explanation
The super() is implicitly invoked by the compiler if no super() or this() is included explicitly within the derived class constructor. Therefore, in this case, The Person class constructor is called first and then the Employee class constructor is called.
70) Can you use this() and super() both in a constructor?
No, because this() and super() must be the first statement in the class constructor.
Example:
public class Test{
Test()
{
super();
this();
System.out.println("Test class object is created");
}
public static void main(String []args){
Test t = new Test();
}
}
Output:
Test.java:5: error: call to this must be first statement in constructor
71)What is object cloning?
The object cloning is used to create the exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.The cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone() method generates CloneNotSupportedException.
protected Object clone() throws CloneNotSupportedException
72) What is method overloading?
Method overloading is the polymorphism technique that allows us to create multiple methods with the same name but a different signature. We can achieve method overloading in two ways.
Changing the number of arguments
Changing the return type
Method overloading increases the readability of the program. Method overloading is performed to figure out the program quickly.
the main() method is the entry point of any standalone java application. The syntax of the main method is
1) What is Java?
Java is the high-level, object-oriented, robust, secure programming language, platform-independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.
2) List the features of Java Programming language.
There are the following features in Java Programming Language.
-Simple: Java is easy to learn. The syntax of Java is based on C++ which makes it easier to write the program in it.
-Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different types of objects that incorporates both data and behavior.
-Portable: Java supports the read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine.
-Platform Independent: Java is a platform-independent programming language. It is different from other programming languages like C and C++ which need a platform to be executed. Java comes with its platform on which its code is executed. Java doesn't depend upon the operating system to be executed.
-Secured: Java is secured because it doesn't use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secure.
-Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust.
-Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data *types may vary according to the architecture (32 bit or 64 bit) which doesn't exist in Java.
-Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.
-High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is "close" to native code. It is still a little bit slower than a compiled language (e.g., C++).
-Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn't occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc.
-Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet.
-Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.
4) What do you understand by Java virtual machine?
Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification that must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine-independent and close to the native code.
5) What is the difference between JDK, JRE, and JVM?
JVM
JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification that specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE.
JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance that is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.
JRE
JRE stands for Java Runtime Environment. It is the implementation of JVM. The Java Runtime Environment is a set of software tools that are used for developing Java applications. It is used to provide a runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.
JDK
JDK is an acronym for Java Development Kit. It is a software development environment that is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:
Standard Edition Java Platform
Enterprise Edition Java Platform
Micro Edition Java Platform
6) How many types of memory areas are allocated by JVM?
Many types:
Class(MeAreathod): Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
Heap: It is the runtime data area in which the memory is allocated to the objects
Stack: Java Stack store frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
Native Method Stack: It contains all the native methods used in the application.
7) What is the JIT compiler?
Just-In-Time(JIT) compiler: It is used to improve performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
8) What is the platform?
A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides a software-based platform.
9) What are the main differences between the Java platform and other platforms?
There are the following differences between the Java platform and other platforms.
*Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
*Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.
10) What gives Java its 'write once and run anywhere' nature?
The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform-specific and can be executed on any computer.
11) What is the classloader?
Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.
Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java. util package classes, java.io package classes, java.sql package classes, etc.
Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/JRE/lib/ext directory.
System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using the "-cp" or "-classpath" switch. It is also known as the Application classloader.
12) Is Empty .java file name a valid source file name?
Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let's take a simple example:
//save by .java only
class A{
public static void main(String args[]){
System.out.println("Hello java");
}
}
//compile by javac .java
//run by java A
13) Is delete, next, main, exit or null keyword in java?
No.
14) If I don't provide any arguments on the command line, then what will the value stored in the String array passed into the main() method, empty or NULL?
It is empty, but not null.
15) What if I write static public void instead of the public static void?
The program compiles and runs correctly because the order of specifiers doesn't matter in Java.
16) What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references.
17) What are the various access specifiers in Java?
In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.
Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
The default is accessible within the package only. By default, all the classes, methods, and variables are of default scope.
Private The private class, methods, or variables defined as private can be accessed within the class only.
18) What is the purpose of static methods and variables?
The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.
For example, In the class simulating the collection of the students in a college, the name of the college is a common attribute to all the students. Therefore, the college name will be defined as static.
19) What are the advantages of Packages in Java?
There are various advantages to defining packages in Java.
Packages avoid name clashes.
The Package provides easier access control.
We can also have the hidden classes that are not visible outside and used by the package.
It is easier to locate related classes.
20) What is the output of the following Java program?
class Test
{
public static void main (String args[])
{
System.out.println(10 + 20 + "Javatpoint");
System.out.println("Javatpoint" + 10 + 20);
}
}
The output of the above code will be
30Javatpoint
Javatpoint1020
Explanation
In the first case, 10 and 20 are treated as numbers and added to be 30. Now, their sum 30 is treated as the string and concatenated with the string Javatpoint. Therefore, the output will be 30Javatpoint.
In the second case, the string Javatpoint is concatenated with 10 to be the string Javatpoint10 which will then be concatenated with 20 to be Javatpoint1020.
21) What is the output of the following Java program?
class Test
{
public static void main (String args[])
{
System.out.println(10 * 20 + "Javatpoint");
System.out.println("Javatpoint" + 10 * 20);
}
}
The output of the above code will be
200Javatpoint
Javatpoint200
Explanation
In the first case, The numbers 10 and 20 will be multiplied first and then the result 200 is treated as the string and concatenated with the string Javatpoint to produce the output 200Javatpoint.
In the second case, The numbers 10 and 20 will be multiplied first to be 200 because the precedence of the multiplication is higher than in addition. The result 200 will be treated as the string and concatenated with the string Javatpointto produce the output as Javatpoint200.
22) What is the output of the following Java program?
class Test
{
public static void main (String args[])
{
for(int i=0; 0; i++)
{
System.out.println("Hello Javatpoint");
}
}
}
The above code will give the compile-time error because the for loop demands a boolean value in the second part and we are providing an integer value, i.e., 0.
23) What is an object-oriented paradigm?
It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. The object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes that interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.
Follows the bottom-up approach in program design.
Focus on data with methods to operate upon the object's data
Includes the concept like Encapsulation and abstraction which hides the complexities from the user and shows only functionality.
Implements the real-time approach like inheritance, abstraction, etc.
The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.
24) What is an object?
The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.
25) What is the difference between an object-oriented programming language and an object-based programming language?
There are the following basic differences between the object-oriented language and object-based language.
Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn't follow all the concepts of OOPs like inheritance and polymorphism.
Object-oriented languages do not have the inbuilt objects whereas Object-based languages have inbuilt objects, for example, JavaScript has window objects.
Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.
26) What will be the initial value of an object reference which is defined as an instance variable?
All object references are initialized to null in Java.
27) What is the constructor?
The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.
28) How many types of constructors are used in Java?
Based on the parameters passed in the constructors, there are two types of constructors in Java.
Default Constructor: The default constructor is the one that does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful tasks on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class.
Parameterized Constructor: The parameterized constructor is the one that can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.
29) What is the purpose of a default constructor?
The purpose of the default constructor is to assign the default value to the objects. The java compiler creates a default constructor implicitly if there is no constructor in the class.
class Student3{
int id;
String name;
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student3 s1=new Student3();
Student3 s2=new Student3();
s1.display();
s2.display();
}
}
Output:
0 null
0 null
Explanation: In the above class, you are not creating any constructor, so the compiler provides you a default constructor. Here 0 and null values are provided by default constructor.
30) Does the constructor return any value?
Ans: yes, The constructor implicitly returns the current instance of the class (You can't use an explicit return type with the constructor).
31)Is constructor inherited?
No, The constructor is not inherited.
32) Can you make a constructor final?
No, the constructor can't be final.
33) Can we overload the constructors?
Yes, the constructors can be overloaded by changing the number of arguments accepted by the constructor or by changing the data type of the parameters. Consider the following example.
class Test
{
int i;
public Test(int k)
{
i=k;
}
public Test(int k, int m)
{
System.out.println("Hi I am assigning the value max(k, m) to i");
if(k>m)
{
i=k;
}
else
{
i=m;
}
}
}
public class Main
{
public static void main (String args[])
{
Test test1 = new Test(10);
Test test2 = new Test(12, 15);
System.out.println(test1.i);
System.out.println(test2.i);
}
}
In the above program, The constructor Test is overloaded with another constructor. In the first call to the constructor, The constructor with one argument is called, and i will be initialized with the value 10. However, In the second call to the constructor, The constructor with the 2 arguments is called, and i will be initialized with the value 15.
34) What do you understand by copy constructor in Java?
There is no copy constructor in java. However, we can copy the values from one object to another like a
There are many ways to copy the values of one object into another in java. They are:
By constructor
By assigning the values of one object into another
By clone() method of Object class
In this example, we are going to copy the values of one object into another using java constructor.
//Java program to initialize the values from one object to another class
Student6{
int id;
String name;
//constructor to initialize integer and string
Student6(int i,String n){
id = i;
name = n;
}
//constructor to initialize another object
Student6(Student6 s){
id = s.id;
name =s.name;
}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student6 s1 = new Student6(111,"Karan");
Student6 s2 = new Student6(s1);
s1.display();
s2.display();
}
}
40) What is the static method?
A static method belongs to the class rather than the object.
There is no need to create the object to call the static methods.
A static method can access and change the value of the static variable.
41) What are the restrictions that are applied to the Java static methods?
Two main restrictions are applied to static methods.
The static method can not use non-static data member or call the non-static method directly.
this and super cannot be used in static context as they are non-static.
42) Why is the main method static?
Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call a main() method which will lead to the extra memory allocation.
43) Can we override the static methods?
No, we can't override static methods.
44) What is the static block?
A static block is used to initialize the static data member. It is executed before the main method, at the time of classloading.
class A2{
static{System.out.println("static block is invoked");}
public static void main(String args[]){
System.out.println("Hello main");
}
}
Output: static block is invoked
Hello main
48) Can we make constructors static?
As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.
49) Can we make the abstract methods static in Java?
In Java, if we make the abstract methods static, It will become part of the class, and we can directly call it which is unnecessary. Calling an undefined method is completely useless therefore it is not allowed.
50) Can we declare the static variables and methods in an abstract class?
Yes, we can declare static variables and methods in an abstract method. As we know that there is no requirement to make the object to access the static context, therefore, we can access the static context declared inside the abstract class by using the name of the abstract class. Consider the following example.
abstract class Test
{
static int i = 102;
static void TestMethod()
{
System.out.println("hi !! I am good !!");
}
}
public class TestClass extends Test
{
public static void main (String args[])
{
Test.TestMethod();
System.out.println("i = "+Test.i);
}
}
Output
hi !! I am good !!
i = 102
51) What is this keyword in java?
This keyword is a reference variable that refers to the current object. There are various uses of this keyword in Java. It can be used to refer to current class properties such as for instance methods, variables, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.
java this keyword
More Details.
52) What are the main uses of this keyword?
There are the following uses of this keyword.
this can be used to refer to the current class instance variable.
this can be used to invoke current class method (implicitly)
this() can be used to invoke the current class constructor.
this can be passed as an argument in the method call.
this can be passed as an argument in the constructor call.
this can be used to return the current class instance from the method.
53) Can we assign the reference to this variable?
No, this cannot be assigned to any value because it always points to the current class object and this is the final reference in Java. However, if we try to do so, the compiler error will be shown. Consider the following example.
public class Test
{
public Test()
{
this = null;
System.out.println("Test class constructor called");
}
public static void main (String args[])
{
Test t = new Test();
}
}
Output
Test.java:5: error: cannot assign a value to final variable this
this = null;
^
1 error
54) Can this keyword be used to refer to static members?
Yes, It is possible to use this keyword to refer to static members because this is just a reference variable that refers to the current class object. However, as we know that, it is unnecessary to access static variables through objects, therefore, it is not the best practice to use this to refer static members. Consider the following example.
public class Test
{
static int i = 10;
public Test ()
{
System.out.println(this.i);
}
public static void main (String args[])
{
Test t = new Test();
}
}
Output
10
55) How can constructor chaining be done using this keyword?
Constructor chaining enables us to call one constructor from another constructor of the class with respect to the current class object. We can use this keyword to perform constructor chaining within the same class. Consider the following example which illustrates how can we use this keyword to achieve constructor chaining.
public class Employee
{
int id,age;
String name, address;
public Employee (int age)
{
this.age = age;
}
public Employee(int id, int age)
{
this(age);
this.id = id;
}
public Employee(int id, int age, String name, String address)
{
this(id, age);
this.name = name;
this.address = address;
}
public static void main (String args[])
{
Employee emp = new Employee(105, 22, "Vikas", "Delhi");
System.out.println("ID: "+emp.id+" Name:"+emp.name+" age:"+emp.age+" address: "+emp.address);
}
}
Output
ID: 105 Name:Vikas age:22 address: Delhi
56) What are the advantages of passing this into a method instead of the current class object itself?
As we know, this refers to the current class object, therefore, it must be similar to the current class object. However, there can be two main advantages of passing this into a method instead of the current class object.
this is a final variable. Therefore, this cannot be assigned to any new value whereas the current class object might not be final and can be changed.
this can be used in the synchronized block.
57) What is Inheritance?
Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship.
There are five types of inheritance in Java.
Single-level inheritance
Multi-level inheritance
Multiple Inheritance
Hierarchical Inheritance
Hybrid Inheritance
Multiple inheritances is not supported in Java through the class.
58) Why is Inheritance used in Java?
There are various advantages of using inheritance in Java that is given below.
Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method.
Runtime polymorphism cannot be achieved without using inheritance.
We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic.
Inheritance provides data hiding. The base class can hide some data from the derived class by making it private.
Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic methods contained by the base class.
59) Which class is the superclass for all the classes?
The object class is the superclass of all other classes in Java.
60) Why are multiple inheritances not supported in java?
To reduce the complexity and simplify the language, multiple inheritances is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.
Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile-time error.
class A{
void msg(){System.out.println("Hello");}
}
class B{
void msg(){System.out.println("Welcome");}
}
class C extends A,B{//suppose if it were
Public Static void main(String args[]){
C obj=new C();
obj.msg();//Now which msg() method would be invoked?
}
}
Compile Time Error
61) What is aggregation?
Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that the Employee (class) has an object of Address class. Consider the following example.
(Address.java)
public class Address {
String city,state,country;
public Address(String city, String state, String country) {
this.city = city;
this.state = state;
this.country = country;
}
}
(Employee.java)
public class Emp {
int id;
String name;
Address address;
public Emp(int id, String name,Address address) {
this.id = id;
this.name = name;
this.address=address;
}
void display(){
System.out.println(id+" "+name);
System.out.println(address.city+" "+address.state+" "+address.country);
}
public static void main(String[] args) {
Address address1=new Address("gzb","UP","india");
Address address2=new Address("gno","UP","india");
Emp e=new Emp(111,"varun",address1);
Emp e2=new Emp(112,"arun",address2);
e.display();
e2.display();
}
}
Output
111 varun
gzb UP india
112 arun
gno UP india
62) What is composition?
Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of a container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists a composition between class and students.
63) What is the difference between aggregation and composition?
Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).
64) Why does Java not support pointers?
The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.
65) What is super in java?
The super keyword in Java is a reference variable that is used to refer to the immediate parent class object. Whenever you create the instance of the subclass, an instance of the parent class is created implicitly which is referred by a super reference variable. The super() is called in the class constructor implicitly by the compiler if there is no super or this.
class Animal{
Animal(){System.out.println("animal is created");}
}
class Dog extends Animal{
Dog(){
System.out.println("dog is created");
}
}
class TestSuper4{
public static void main(String args[]){
Dog d=new Dog();
}
}
animal is created
dog is created
66) How can constructor chaining be done by using the super keyword?
class Person
{
String name,address;
int age;
public Person(int age, String name, String address)
{
this.age = age;
this.name = name;
this.address = address;
}
}
class Employee extends Person
{
float salary;
public Employee(int age, String name, String address, float salary)
{
super(age,name,address);
this.salary = salary;
}
}
public class Test
{
public static void main (String args[])
{
Employee e = new Employee(22, "Mukesh", "Delhi", 90000);
System.out.println("Name: "+e.name+" Salary: "+e.salary+" Age: "+e.age+" Address: "+e.address);
}
}
Output
Name: Mukesh Salary: 90000.0 Age: 22 Address: Delhi
67) What are the main uses of the super keyword?
There are the following uses of the super keyword.
super can be used to refer to the immediate parent class instance variable.
super can be used to invoke the immediate parent class method.
super() can be used to invoke immediate parent class constructor.
68) What are the differences between this and super keyword?
There are the following differences between this and super keyword.
The super keyword always points to the parent class contexts whereas this keyword always points to the current class context.
The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor.
The super and this must be the first statement inside constructor otherwise the compiler will throw an error.
69) What is the output of the following Java program?
class Person
{
public Person()
{
System.out.println("Person class constructor called");
}
}
public class Employee extends Person
{
public Employee()
{
System.out.println("Employee class constructor called");
}
public static void main (String args[])
{
Employee e = new Employee();
}
}
Output
Person class constructor called
Employee class constructor called
Explanation
The super() is implicitly invoked by the compiler if no super() or this() is included explicitly within the derived class constructor. Therefore, in this case, The Person class constructor is called first and then the Employee class constructor is called.
70) Can you use this() and super() both in a constructor?
No, because this() and super() must be the first statement in the class constructor.
Example:
public class Test{
Test()
{
super();
this();
System.out.println("Test class object is created");
}
public static void main(String []args){
Test t = new Test();
}
}
Output:
Test.java:5: error: call to this must be first statement in constructor
71)What is object cloning?
The object cloning is used to create the exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.The cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone() method generates CloneNotSupportedException.
protected Object clone() throws CloneNotSupportedException
72) What is method overloading?
Method overloading is the polymorphism technique that allows us to create multiple methods with the same name but a different signature. We can achieve method overloading in two ways.
Changing the number of arguments
Changing the return type
Method overloading increases the readability of the program. Method overloading is performed to figure out the program quickly.
73 )Which class is the superclass of all classes?
java.lang.Object is the root class for all the java classes and we don’t need to extend it
74)Why Java doesn’t support multiple inheritances?
Java doesn’t support multiple inheritances in classes because of the “Diamond Problem”. To know more about a diamond problem with an example, read Multiple Inheritance in Java.
However multiple inheritances are supported in interfaces. An interface can extend multiple interfaces because they just declare the methods and implementation will be present in the implementing class. So there is no issue of the diamond problem with interfaces
75)What is the difference between path and classpath variables?
PATH is an environment variable used by the operating system to locate the executables. That’s why when we install Java or want any executable to be found by OS, we need to add the directory location in the PATH variable. If you work on Windows OS, read this post to learn how to set up a PATH variable on Windows.
Classpath is specific to Java and used by java executables to locate class files. We can provide the classpath location while running java application and it can be a directory, ZIP files, JAR files, etc.
76)What is the importance of the main method in Java?
the main() method is the entry point of any standalone java application. The syntax of the main method is public static void main(String args[]).
Java's main method is public and static so that Java runtime can access it without initializing the class. The input parameter is an array of String through which we can pass runtime arguments to the java program. Check this post to learn how to compile and run java programs.
77)What is the multi-catch block in java?
Java 7 one of the improvement was a multi-catch block where we can catch multiple exceptions in a single catch block. This makes our code shorter and cleaner when every catch block has similar code.
If a catch block handles multiple exceptions, you can separate them using a pipe (|) and in this case, the exception parameter (ex) is final, so you can’t change it.
78)What are the Wrapper classes?
Java wrapper classes are the Object representation of eight primitive types in java. All the wrapper classes in java are immutable and final. Java 5 autoboxing and unboxing allows easy conversion between primitive types and their corresponding wrapper classes.
79)What is Enum in Java?
Enum was introduced in Java 1.5 as a new type whose fields consist of a fixed set of constants. For example, in Java, we can create Direction as an enum with fixed fields as EAST, WEST, NORTH, SOUTH.
enum is the keyword to create an enum type and similar to the class. Enum constants are implicitly static and final.
80)What is an inner class in java?
We can define a class inside a class and they are called nested classes. Any non-static nested class is known as an inner class. Inner classes are associated with the object of the class and they can access all the variables and methods of the outer class. Since inner classes are associated with the instance, we can’t have any static variables in them.
We can have a local inner class or anonymous inner class inside a class.
81)What is Classloader in Java?
Java Classloader is the program that loads the byte code program into memory when we want to access any class. We can create our own classloader by extending the ClassLoader class and overriding the loadClass(String name) method.
82)What is break and continue statement?
We can use break statement to terminate for, while, or do-while loop. We can use the break statement in the switch statement to exit the switch case. You can see the example of a break statement at the java break. We can use a break with a label to terminate the nested loops.
The continue statement skips the current iteration of a for, while or do-while loop. We can use the continue statement with the label to skip the current iteration of the outermost loop.
83)What is the default constructor?
No argument constructor of a class is known as the default constructor. When we don’t define any constructor for the class, the java compiler automatically creates the default no-args constructor for the class. If there are other constructors defined, then the compiler won’t create a default constructor for us.
84)What are Serialization and Deserialization?
We can convert a Java object to a Stream that is called Serialization. Once an object is converted to Stream, it can be saved to file or send over the network or used in socket connections.
The object should implement a Serializable interface and we can use java.io.ObjectOutputStream to write objects to file or to any OutputStream object.
85)What is the platform?
A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform
86)What are the main differences between the Java platform and other platforms?
- Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
- Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components
87)What do you understand by copy constructor in Java?
- By constructor
- By assigning the values of one object into another
- By clone() method of Object class
- Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
- Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
- The default is accessible within the package only. By default, all the classes, methods, and variables are of default scope.
- Private The private class, methods, or variables defined as private can be accessed within the class only
| Java Constructor | Java Method |
|---|---|
| A constructor is used to initialize the state of an object. | A method is used to expose the behavior of an object. |
| A constructor must not have a return type. | A method must have a return type. |
| The constructor is invoked implicitly. | The method is invoked explicitly. |
| The Java compiler provides a default constructor if you don't have any constructor in a class. | The method is not provided by the compiler in any case. |
| The constructor name must be the same as the class name. | The method name may or may not be same as the class name. |
90)what is the JIT compiler?
Just-In-Time(JIT) compiler: It is used to improve performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
91)What do you understand by Java virtual machine?
Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification that must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine-independent and close to the native code.
92)What is the Java IDE’s?
Ans: Eclipse and NetBeans are the IDE's of JAVA.
93)What are the two ways of implementing multi-threading in Java?
Ans: Multi-threaded applications can be developed in Java by using any of the following two methodologies:
By using Java.Lang.Runnable Interface. Classes implement this interface to enable multithreading. There is a Run() method in this interface that is implemented.
By writing a class that extends Java.Lang.Thread class.
94)How garbage collection is done in Java?
Ans: In java, when an object is not referenced anymore, garbage collection takes place and the object is destroyed automatically. For automatic garbage collection java calls either System.gc() method or Runtime.gc() method.
95) Which types of exceptions are caught at compile time?
Ans: Checked exceptions can be caught at the time of program compilation. Checked exceptions must be handled by using a try-catch block in the code in order to successfully compile the code.
96) Can variables be used in Java without initialization?
Ans: In Java, if a variable is used in code without prior initialization by a valid value, the program doesn't compile and gives an error as no default value is assigned to variables in Java.
97)How are destructors defined in Java?
Ans: In Java, there are no destructors defined in the class as there is no need to do so. Java has its own garbage collection mechanism which does the job automatically by destroying the objects when no longer referenced.
98)What is the multi-catch block in Java?
Multi-catch block makes the code shorter and cleaner when every catch block has similar code. We can catch multiple exceptions in a single catch block using this feature.
99)Differentiate between String Buffer and String Builder in Java?
The only difference between StringBuffer and StringBuilder is that StringBuffer methods are synchronized while StringBuilder is not synchronized. StringBuilder in Java was introduced in Java 5.
100)DifferExplain the Request Dispatcher?
Request Dispatcher is a Java interface that is used to forward a request to some other resource which can be HTML, JSP or any other Java servlet within the same application.
101)Differentiate JAR and WAR files
We are going to share the Difference between JAR & WAR File:
| JAR FILES | WAR FILES |
| The full form of JAR files is Java Archive Files. | The full form of WAR files is Web Archive Files. |
| Aggregating many files into one is allowed in JAR files | XML, Java classes, and JavaServer Pages are stored in WAR |
| The JAR is usually used to hold Java classes in a library. | Mainly used for Web Application purposes.` |
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