10 basic Java interview questions with their answers

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1. What is Java?

Answer:
Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is platform-independent due to its "Write Once, Run Anywhere" (WORA) capability achieved through the Java Virtual Machine (JVM).

2. What are the key features of Java?

Answer:

  • Object-Oriented: Everything in Java is treated as an object.
  • Platform-Independent: Java code runs on any platform that supports JVM.
  • Robust: Provides strong memory management and exception handling.
  • Multithreaded: Supports concurrent execution of two or more threads.
  • Secure: Includes security features like cryptography and runtime checks.



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

Answer:

  • JDK (Java Development Kit): Includes tools for developing Java applications, like compilers and libraries.
  • JRE (Java Runtime Environment): Provides libraries and JVM to run Java applications.
  • JVM (Java Virtual Machine): Executes Java bytecode and ensures platform independence.

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

Answer:

  • ==: Compares memory references (address) of two objects.
  • .equals(): Compares the content of two objects (e.g., strings).

5. What are the data types in Java?

Answer: Java has two types of data types:

  • Primitive: int, float, double, boolean, char, byte, short, long.
  • Non-Primitive: Arrays, Strings, Classes, Interfaces, etc.

6. What is the difference between Array and ArrayList?

Answer:

  • Array: Fixed size, can store elements of the same type.
  • ArrayList: Dynamic size, part of Java's Collection framework, can store objects.

7. What is an object in Java?

Answer: An object is an instance of a class that has states (fields/attributes) and behaviors (methods). For example, a car object has attributes like color and speed, and methods like drive and brake.

8. What is a constructor in Java?

Answer: A constructor is a special method used to initialize objects. It has the same name as the class and no return type.
Example:

public class Example {
    int value;
    public Example() {
        value = 10; // Constructor initializing value
    }
}

9. What is the difference between abstract class and interface?

Answer:

  • Abstract Class: Can have both abstract and concrete methods. Supports inheritance (one class).
  • Interface: All methods are abstract by default (before Java 8). A class can implement multiple interfaces.

10. What is the purpose of the static keyword in Java?

Answer: The static keyword is used to define members (variables or methods) that belong to the class rather than a specific instance.
Example:

class Example {
    static int counter = 0; // Shared across all instances
}

These are fundamental questions often asked in beginner-level Java interviews.

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