Top Java Interview Questions and Answers

Here are top Java interview questions,

1. What is Java?

Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is designed to be platform-independent and can run on any operating system that has a Java Virtual Machine (JVM).

 

2. What are the main features of Java?

Key features of Java include platform independence, object-oriented programming, automatic memory management (garbage collection), strong type checking, and exception handling.

 

3. What is the difference between JDK, JRE, and JVM?

JDK (Java Development Kit) is a software development kit that includes tools for developing and running Java programs. JRE (Java Runtime Environment) provides the necessary runtime environment for executing Java applications. JVM (Java Virtual Machine) is the runtime instance that executes Java bytecode.

 

4. What is the difference between an object and a class in Java?

A class is a blueprint or template for creating objects, while an object is an instance of a class. Classes define the properties and behavior of objects.

 

5. What is the difference between the stack and the heap?

The stack is used for storing local variables and method call information, while the heap is used for dynamic memory allocation, including objects and arrays.

 

6. What is the difference between a checked exception and an unchecked exception?

Checked exceptions are checked at compile-time and must be either caught or declared in the method signature using the "throws" keyword. Unchecked exceptions (RuntimeExceptions) do not require explicit handling.

 

7. What is the difference between an abstract class and an interface?

An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. A class can implement multiple interfaces but can inherit from only one abstract class.

 

8. What is the difference between method overloading and method overriding?

Method overloading involves defining multiple methods with the same name but different parameters within the same class. Method overriding occurs when a subclass provides its own implementation of a method defined in the superclass.

 

9. What is the difference between a HashMap and a Hashtable?

Both HashMap and Hashtable are used to store key-value pairs, but HashMap is not synchronized and allows null values, whereas Hashtable is synchronized and does not allow null values.

 

10. What is the difference between StringBuilder and StringBuffer?

Both StringBuilder and StringBuffer are used to create and manipulate strings, but StringBuilder is not synchronized and is more efficient in a single-threaded environment. StringBuffer is synchronized and suitable for multithreaded environments.

 

11. What is the purpose of the "final" keyword in Java?

The "final" keyword can be applied to variables, methods, and classes. A final variable cannot be reassigned, a final method cannot be overridden, and a final class cannot be inherited.

 

12. What are the access modifiers in Java?

Java provides four access modifiers: public, private, protected, and default (no modifier). These modifiers control the visibility and accessibility of classes, methods, and variables.

 

13. What is the difference between composition and inheritance?

Composition is the practice of combining objects of different classes to create more complex objects, while inheritance is the process of creating a new class by inheriting properties and methods from an existing class.

 

14. What is the purpose of the "static" keyword in Java?

The "static" keyword is used to declare variables, methods, and nested classes that belong to the class itself rather than specific instances. Static members can be accessed without creating an object of the class.

 

15. What is the purpose of the "super" keyword in Java?

The "super" keyword is used to refer to the superclass or parent class. It is used to access superclass members, invoke superclass constructors, and differentiate between superclass and subclass members.

 

16. What is the difference between a shallow copy and a deep copy?

A shallow copy creates a new object but references the same memory locations for internal members, while a deep copy creates a new object and copies the values of all internal members to new memory locations.

 

17. What is the purpose of the "this" keyword in Java?

The "this" keyword is used to refer to the current object instance. It can be used to differentiate between instance variables and method parameters with the same name or to invoke other constructors within the same class.

 

18. What is the purpose of the "try-catch-finally" block in Java?

The "try-catch-finally" block is used for exception handling. The code that may throw an exception is enclosed in a try block, and if an exception occurs, it is caught in a catch block. The finally block is used to execute cleanup code.

 

19. What is the difference between "==" and ".equals()" in Java?

"==" is used to compare the equality of primitive types and the reference equality of objects, while ".equals()" is a method that can be overridden by classes to define their own equality criteria based on object content.

 

20. What is multithreading in Java?

Multithreading is the concurrent execution of two or more parts of a program for maximum utilization of CPU time. Java provides built-in support for multithreading through the Thread class and the Runnable interface.

Above are few top Java interview questions. Remember to prepare and expand on these answers.

Good luck with your interview!  👍