These top 50 core Java interview questions evaluate a candidate’s foundational Java programming knowledge comprehensively. They encompass crucial topics such as object-oriented programming, data types, control structures, exception handling, and multithreading, providing an extensive assessment of the candidate’s Java expertise.

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

  • JDK (Java Development Kit) includes JRE (Java Runtime Environment) and development tools, while JRE includes the JVM (Java Virtual Machine).

2. Explain the concept of OOP in Java.

  • Object-Oriented Programming (OOP) is a programming paradigm that uses objects to model real-world entities, with principles like encapsulation, inheritance, and polymorphism.

3. What is the difference between stack and heap memory in Java?

  • Stack is used for method execution and local variables, while heap is used for objects and dynamically allocated memory.

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

  • A class is a blueprint for creating objects, and an object is an instance of a class.

5. Explain the ‘final’ keyword in Java.

  • ‘final’ is used to make a variable, method, or class unmodifiable. It ensures the value of a variable cannot be changed, or a method/class cannot be extended or overridden.

6. What are constructors in Java, and why are they used?

  • Constructors are special methods used to initialize objects. They have the same name as the class and are called when an object is created.

7.  What is method overloading in Java?

  •  Method overloading allows a class to have multiple methods with the same name but different parameter lists.

8. Explain the purpose of the ‘static’ keyword in Java.

  •  ‘static’ is used to create class-level variables and methods, which are shared among all instances of the class.

9. What is the ‘super’ keyword used for in Java?

  • ‘super’ is used to call a superclass’s constructor or access a superclass’s member in a subclass.

10. What is an interface in Java, and why is it used?

  •  An interface defines a contract of methods that implementing classes must adhere to. It’s used to achieve multiple inheritance and establish a common API.

11. Explain the ‘this’ keyword in Java.

  •  ‘this’ is a reference to the current object. It’s used to distinguish between instance variables and method parameters with the same name.

12. What is method overriding in Java?

  •  Method overriding allows a subclass to provide a specific implementation for a method that is already defined in its superclass.

13. What is the ‘private’ access modifier in Java?

  •  ‘private’ makes a class member (variable or method) accessible only within the same class and not in subclasses or other classes.

14. What is the ‘protected’ access modifier in Java?

  •  ‘protected’ makes a class member accessible within the same class, its subclasses, and classes in the same package.

15. Explain the ‘abstract’ keyword in Java.

  •  ‘abstract’ is used to declare abstract classes and methods that must be implemented by concrete subclasses.

16. What is method chaining in Java, and how is it achieved?

  •  Method chaining is the practice of calling multiple methods in a single line. It’s achieved by having methods return the object itself (this).

17. What is a static initializer block in Java?

  • A static initializer block is used to initialize static variables or perform one-time setup for a class. It runs when the class is loaded.

18. Explain the ‘transient’ keyword in Java.

  •  ‘transient’ is used to mark instance variables that should not be serialized when an object is converted to a byte stream (e.g., for object storage or network transfer).

19. What is the ‘volatile’ keyword in Java, and when is it used?

  •  ‘volatile’ is used for variables that are shared among multiple threads, ensuring that reads and writes are always done directly to and from memory, avoiding thread inconsistencies.

20. Explain the ‘equals’ and ‘hashCode’ methods in Java.

  •  ‘equals’ is used to compare objects for equality, while ‘hashCode’ generates a hash code that is often used in data structures like HashSets and HashMaps.