Java Interview Questions and Answers

In this collection, I have chosen 60+ mostly asked Java interview questions and answers, which will surely help you to get success in interviews.

 

1. What are the benefits of using Java?
Java offers a number of benefits, including: platform independence, ease of use, scalability, robustness, security, portability, and advanced features such as garbage collection, threading, and automated memory management.

 

2. What are the major differences between C++ and Java?
The major differences between C++ and Java include: Java is platform-independent, while C++ is not; Java is based on an interpreted language, while C++ is based on a compiled language; Java has built-in memory management, while C++ does not; Java has no pointers, while C++ does; Java supports multi-threading, while C++ does not; and Java does not support operator overloading, while C++ does.

 

3. What is a static variable in Java?
A static variable in Java is a variable that is associated with a class, rather than an instance of the class. It is declared with the static keyword and is only initialized once, when the class is loaded. Static variables maintain their value across instances of the class, and are accessible without creating an instance of the class.

 

4. What is the purpose of garbage collection?
A process in which automatically free up unused process(no longer required)from the memory, is called Garbage Collection. Java uses garbage collection to manage memory in the object lifecycle.

java interview questions

5. What is platform independence in java?
Platform independence is the most important feature of Java, which allows developers to write code once and run it anywhere. Any computer with Java Virtual Machine (JVM) installed, can run java code. This allows programs written in Java to run on any operating system.

 

6. Explain difference between a JDK and a JRE?
 JDK stands for Java Development Kit. It is a set of tools for developing Java applications, including a compiler, a debugger, and other tools. JRE stands for Java Runtime Environment. It is an implementation of the Java Virtual Machine and is used to run Java applications.

 

7. What is String class in Java?
For the creation and modification of strings, use the String class. It provides a number of methods for comparing, searching, and manipulating strings. It is also used to store text data in Java programs.

 

8. What is JVM?
JVM stands for Java Virtual Machine. It is a virtual machine that provides an environment to execute Java code and programs. JVM is platform-independent and is responsible for the ability to run a Java application on multiple operating systems.

 

9. What is super keyword?
The super keyword is used to give access to methods and properties of a parent or base class in the current context of a class.

 

10. What is final keyword?
The final keyword is used to indicate that a variable or a method cannot be overridden in a subclass.

 

11. What is synchronized keyword?
The synchronized keyword is a Java keyword used to provide exclusive access to an object or method. When a method is synchronized, only one thread at a time can access the method or object, ensuring thread safety and preventing race conditions.

 

12. Explain static and a non-static inner class in Java?
A static inner class is associated with the class in which it is defined, while a non-static inner class is associated with an instance of its enclosing class.

 

13. What is interface in Java?
Interface in Java is a code structure that defines methods and variables. It can be used as a contract between a class and an outside code, stating that the class must implement certain methods if it is to be used in a program.

 

14. What is an abstract class in Java?
An abstract class in Java is a class that is declared with the “abstract” keyword and is used to provide a basic structure for Java classes. Abstract classes cannot be instantiated but are extended by other classes that provide implementations for its abstract methods. Abstract classes typically contain only abstract methods, although they can also contain non-abstract methods.

 

15. What is assert keyword?
The assert keyword in Java is used to create simple assertions in a program in order to verify correctness. It is used to prevent errors from occurring during run-time.

 

16. What is transient keyword?
The transient keyword in Java is a modifier used in serialization. When a field is marked as transient, it is not serialized by the Java runtime during object serialization. This applies to instance and class variables.

 

17. Explain difference between a while loop and a do-while loop?
A while loop checks a condition before executing the loop body, thus the loop body may never be executed. A do-while loop executes the loop body at least once before checking the condition, ensuring that the loop body always runs at least once.

 

18. What is array and collections in java?
Array: An array in Java is a type of object that holds a fixed number of values of a single type. It is an indexed collection of elements, with each element corresponding to a numeric index, and the elements can be referred to using their index values.

Collection: A Collection is an object in Java that represents a group of objects. It is an abstraction that is implemented by various concrete data structures, such as lists, sets, queues, and others. While an array has a fixed size, a Collection can grow or shrink dynamically to accommodate elements that are added or removed.

 

19. What is Abstract class and Interface in java?
An abstract class is a class that cannot be instantiated and typically contains only abstract methods. An interface defines a contract that all implementing classes must adhere to. An interface does not contain any implementation code and is used to define methods that must be implemented by the classes that implement the interface.

 

20. What is the static and non-static variables?
Static variables are class variables, meaning they are associated with the class and shared by all instances of the class. Non-static variables are instance variables, meaning they are unique to each instance of a class.

 

21. What is the difference between inner class and subclass in java
An Inner Class is a nested class, meaning the class is declared within another class, whereas a Sub-Class is a class that inherits from another class.

 

22. What is thread and process in Java?
A Thread is a lightweight process that can be managed independently by the operating system. A Process is a more heavyweight process that requires more resources and is typically managed by the operating system.

 

23. What is the difference between immutable and final?
Immutable objects cannot be changed after they have been created, while final objects cannot be overridden or extended.

 

24. What is finally clause in try-catch-finally statement?
The finally clause is used to provide a block of code that will always be executed, regardless of whether an exception has been thrown or not. This is usually used for cleanup operations, such as closing a file or releasing resources.

 

25. Explain difference between a constructor and a method in java
A constructor is a special type of method that is used to create an instance of the class. It does not return any value and has the same name as the class.Constructors are called when an instance of an object is created while methods are called when a certain action is to be performed on that instance. Constructors have no return type while methods can have a return type.

 

26. What are the different types of constructors explain?
Java has three types of constructors:

Default Constructor: It has no parameters and is automatically provided by Java if no other constructor is defined.

Parameterized Constructor: It accepts parameters, allowing developers to set initial values during object creation.

Copy Constructor: It creates a new object by copying the values of another object.

 

27. Explain the main difference between a public and a private method?
A public method can be accessed by any other class, while a private method can only be accessed by methods within the same class.

 

28. Explain the difference between break statement and continue statement in Java?
A break statement is used to exit a loop or a switch statement, while a continue statement is used to skip the current iteration of a loop and continue to the next iteration.
A continue statement is used to skip the rest of the code inside a loop for the current iteration and immediately jump to the next iteration of the loop.

 

29. What is package in Java and its types?
A group of related classes, interfaces, and sub-packages are called package. It provides access protection and namespace management. The types of packages are following:-

1.Built-in Packages: These are the packages that come with the JDK and are located in the Java’s home directory. They provide various utility classes and interfaces for a wide range of tasks.

2.User-defined packages: These are the packages that are created by the users. They are used to group related classes and interfaces that are written by the user. The user-defined packages bring order and structure to the code and make it easier to reuse the code. It provides access protection and namespace management.

 

30. Explain the difference between Java applet and Java application?
A Java applet is a small application written in Java and accessed through a web browser. Applets are typically used to increase the functionality of a web page and to make small applications available to a wide audience.
A Java application is a stand-alone program written in Java that runs in a virtual machine or browser. Applications are designed to be used by a single user, have access to the entire system and can be run at any time. Applications are typically used to perform complex tasks and can be distributed to multiple devices.

 

31. What are the access modifiers in Java?

Java has four access modifiers: public, protected, private, and the default (no explicit modifier).

 

32. What is HashMap and HashTable in Java?
HashMap:- It is not synchronized and allows null values.
HashTable:- It is synchronized and does not allow null keys or values.

 

33. What is a constructor in Java?
A constructor is a special method used to initialize objects of a class. Constructor and Class has the same name. It is invoked when the class object is created.

 

34. Explain the concept of encapsulation in Java.
Encapsulation is a principle of wrapping data and methods together in a class. It protects the data from direct access and ensures controlled access through methods.

 

35. What are the concepts of inheritance?
Inheritance is the concept of OOP’s. In which a class inherits properties and behaviors from another class. The subclass can extend the functionalities of the superclass and implement additional features.

 

36. Explain the use of the ‘this’ keyword in Java.
The current instance of the class is referred to using the Java keyword “this”. It differentiate between method parameters and instance variables that have the same name.

 

37. How do you handle exceptions in Java?
In Java, exceptions are managed using try, catch, and finally blocks.
Try:- The code that may cause an exception is placed inside the try block.
Catch:- The corresponding exception is caught and handled in the catch block.
Finally:- The finally block contains code that is executed regardless of whether an exception occurs.

 

38. What are the different loops in Java?
Java supports three types of loops:

for loop: A block of code is repeated a particular number of times.
while loop: A block of code is repeated as long as a condition is met.
do-while loop: It is similar to the while loop but guarantees the execution of the block of code at least once before checking the condition.

 

39. How can you achieve multithreading in Java?
Multithreading in Java is achieved by creating multiple threads within a single process. Developers can use Thread class(Java’s built-in ) or implement the Runnable interface to create and manage threads.

 

40. What is the concept of synchronization in Java?
Synchronization in Java is used to prevent multiple threads from accessing shared resources simultaneously. It ensures that only one thread can access the synchronized block of code at a time, avoiding data inconsistency and conflicts.

 

41. Explain the ‘equals’ and ‘hashCode’ methods in Java.
The ‘equals’ method compares the content of two objects for equality. It should be overridden in custom classes to ensure meaningful comparisons.

The ‘hashCode’ method returns a unique integer value associated with an object. It is primarily used for hash-based data structures like HashMap.

 

42. What is the ‘StringBuilder’ class used for?
The ‘StringBuilder’ class in Java is used to create and manipulate strings efficiently.

 

43. Differentiate between ‘String’ and ‘StringBuffer’ in Java?
‘String’ is immutable, which means its content cannot be changed after creation. ‘StringBuffer’ is mutable and allows for dynamic modification of its content.

 

44. Explain the ‘ArrayList’ and ‘LinkedList’ classes.
‘ArrayList’: It is an array-based dynamic data structure, allowing fast random access but slower insertion and deletion of elements.

‘LinkedList’: It is a doubly-linked list-based data structure, offering fast insertion and deletion but slower random access.

 

45. What is ‘HashMap’ class in Java?
‘HashMap’ is an implementation of the Map interface in Java and stores key-value pairs. Based on their keys, it allows quick retrieval of values.

 

46. How do you implement sorting in Java?
Sorting in Java can be accomplished using the Collections.sort() method for collections of objects or by implementing the sorting logic manually for custom objects.

 

47. Explain the concept of JavaBeans.
JavaBeans are Java classes that adhere to specific naming conventions and design patterns. They are used to encapsulate data and expose their functionalities as properties through getter and setter methods.

 

48. What is Garbage Collection in Java?
Garbage collection is an automatic process, where the JVM (Java Virtual Machine) identifies and frees up the occupied memory by objects.

 

49. How do you handle file input and output in Java?
File input and output operations are performed by using following classes:-
“FileInputStream,”
“FileOutputStream,”
“BufferedReader,”
“BufferedWriter.”
These above classes provide methods to read and write to the files.

 

50. How do you sort elements in an array in Java?
“Arrays.sort()” method is used to sort elements in an array.

 

51. What is the purpose of the “Collections” framework in Java?
The “Collections” framework provides a set of classes and interfaces to work with collections of objects. It offers various utility methods for sorting, searching, and manipulating collections.

 

52. Explain the concept of JavaBeans.
JavaBeans are Java classes that adhere to specific naming conventions and design patterns. They are used to encapsulate data and expose their functionalities as properties through getter and setter methods.

 

53. How do you handle synchronization in Java?
To stop several threads from accessing shared resources at the same time, synchronization is utilized. It ensures that only one thread can access the synchronized block of code at a time, avoiding data inconsistency and conflicts.

 

 

 

 

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