Core Java Interview Questions and Answers for Beginners

Java remains one of the most popular programming languages, known for its versatility, platform independence, and robustness. If you’re preparing for a Core Java interview question as a beginner, understanding these fundamental questions and their answers can greatly enhance your chances of success. Let’s delve into the essential topics you need to know:

Q1. What is Java?

Ans: Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It was designed to have minimal implementation dependencies, making it suitable for writing code that can run on any platform with a Java Virtual Machine (JVM).

Q2. What are the key features of Java?

Ans: Java is known for its key features:

1. Object-oriented: Java models everything as an object.
2. Platform-independent: Java programs can run on any platform with JVM.
3. Simple: Java syntax is clean and easy to understand.
4. Secure: Java provides a secure environment with its bytecode verification.
5. Robust: Java emphasizes strong memory management with automatic garbage collection.
6. Multithreaded: Java supports concurrent execution of multiple threads.
7. Portable: Bytecode can be carried to any platform.
8. Dynamic: Java programs can carry extensive runtime information.

Q3. What are the data types in Java?

Ans: Java supports two categories of data types:

1. Primitive types: int, char, boolean, etc.
2. Reference types: Objects of classes and interfaces.

Q4. Explain the difference between == and .equals() method in Java.

Ans:

1. == compares references (memory addresses) of objects.
2. .equals() method compares the contents of objects for equality as defined by the method overridden in the class.

Q5. What is the difference between final, finally, and finalize in Java?

Ans:

1. final: Used to declare constants, prevent method overriding, and inheritance.
2. finally: Used in exception handling to execute code after try-catch blocks.
3. finalize: A method in Object class used for garbage collection, though it’s less commonly used.

Q6. Explain the concept of inheritance in Java.

Ans: Inheritance allows one class (subclass/derived class) to inherit properties and behaviors from another class (superclass/base class). It promotes code reusability and hierarchy in Java programs.

Q7. What is method overloading? Provide an example.

Ans: Method overloading is defining multiple methods in a class with the same name but different parameters. It allows the same method name to perform different tasks based on the number and type of parameters.

Example:

java

public class Calculator {

public int add(int a, int b) {

return a + b;

}

public double add(double a, double b) {

return a + b;

}

}

Q8. What is method overriding? Provide an example.

Ans: Method overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass. It allows a subclass to provide a specialized version of a method that is already provided by its superclass.

Example:

java

class Animal {

void sound() {

System.out.println(“Animal makes a sound”);

}

}

class Dog extends Animal {

@Override

void sound() {

System.out.println(“Dog barks”);

}

}

Q9. What is the difference between ArrayList and LinkedList?

Ans:

1. ArrayList: Implements dynamic array, faster for accessing elements, slower for manipulation.
2. LinkedList: Implements doubly linked list, faster for manipulation (add, remove), slower for accessing elements.

Q10. Explain static keyword in Java.

Ans: The static keyword in Java is used to create class-level variables and methods that belong to the class rather than instances of the class. It can be accessed without creating an instance of the class.

Q11. What is the purpose of final keyword in Java?

Ans: The final keyword in Java is used to declare constants, prevent method overriding, and inheritance. It ensures that a variable can be assigned only once, a method cannot be overridden, and a class cannot be extended.

Q12. What is the difference between public, private, protected, and default access modifiers?

Ans:

1. public: Accessible from anywhere.
2. private: Accessible only within the same class.
3. protected: Accessible within the same package and subclasses.
4. Default (no modifier): Accessible within the same package.

Q13. Explain the this keyword in Java.

Ans: The this keyword in Java refers to the current instance of the class. It is used to differentiate between instance variables and parameters with the same name, and to invoke current class constructors.

Q14. What are constructors in Java?

Ans: Constructors in Java are special methods used to initialize objects. They have the same name as the class and do not have a return type. Constructors are called when an object of a class is created.

Q15. Differentiate between throw and throws in Java.

Ans:

1. throw: Used to explicitly throw an exception.
2. throws: Used in method signature to declare the exceptions that can be thrown by the method.

Q16. Explain the try, catch, finally blocks in Java.

Ans:

1. try: Encloses code that may throw exceptions.
2. catch: Handles exceptions thrown in the try block.
3. finally: Executes code after try-catch blocks, whether an exception is thrown or not.

Q17. What is the super keyword in Java?

Ans: The super keyword in Java is used to refer to the superclass of the current object instance. It can be used to invoke superclass constructors and methods hidden by a subclass.

Q18. What is the abstract keyword in Java?

Ans: The abstract keyword in Java is used to declare abstract classes and methods. Abstract classes cannot be instantiated and may contain abstract methods that must be implemented by subclasses.

Q19. Explain the difference between interface and abstract class.

Ans:

1. Interface: Contains only abstract methods and constants, supports multiple inheritance, used to achieve abstraction and multiple inheritance.
2. Abstract class: Can contain abstract and non-abstract methods, can have constructors, cannot support multiple inheritance, used to provide a common interface and code reuse among related classes.

Q20. What is the volatile keyword in Java?

Ans: The volatile keyword in Java is used to indicate that a variable’s value may be changed by multiple threads simultaneously. It ensures visibility of changes to variables across threads.

Q21. How does Java handle memory management?

Ans: Java uses automatic garbage collection to manage memory. Garbage collector automatically deallocates memory occupied by objects that are no longer referenced.

Q22. Explain the difference between == and .equals() method in Java.

Ans:

1. ==: Compares references (memory addresses) of objects.
2. .equals(): Method compares the contents of objects for equality as defined by the method overridden in the class.

Q23. What are the different ways to create threads in Java?

Ans: Threads in Java can be created by:

1. Extending the Thread class.
2. Implementing the Runnable interface.

Q24. How does exception handling work in Java?

Ans: Exception handling in Java is managed using try, catch, finally, and throw keywords. Exceptions are objects that are thrown at runtime and can be caught and handled using appropriate exception handling blocks.

Q25. What are Java annotations?

Ans: Java annotations provide metadata about a program that can be processed by tools at compile time or runtime. They can be used for providing additional information or specifying how code should be processed.

Summary

These core Java interview questions and answers cover foundational concepts that every fresher should be familiar with. Practice these questions to gain confidence and ace your Java interviews. Read more: Online Interview Questions