1. What are the main features of Java?
Answer:
Java is a versatile, object-oriented programming language with several key features:
- Platform Independence: Write once, run anywhere (WORA) using JVM.
- Object-Oriented: Follows OOP principles like encapsulation, inheritance, and polymorphism.
- Automatic Memory Management: Uses garbage collection to free unused memory.
- Multi-threading Support: Enables concurrent programming using threads.
- Security: Provides a secure execution environment.
- Rich API and Libraries: Comes with a comprehensive standard library for various applications.
2. What is the difference between JDK, JRE, and JVM?
Answer:
- JDK (Java Development Kit): A software development kit that includes JRE, compilers, and tools for developing Java applications.
- JRE (Java Runtime Environment): Contains JVM and libraries required to run Java applications.
- JVM (Java Virtual Machine): An abstract machine that executes Java bytecode.
3. Explain the difference between Stack and Heap memory in Java.
Answer:
- Stack Memory: Stores method-specific local variables and function calls. It follows the Last In, First Out (LIFO) principle.
- Heap Memory: Used for dynamic memory allocation where objects are stored.
Example:
public class MemoryDemo {
public static void main(String[] args) {
int a = 10; // Stored in Stack
String str = new String("Hello"); // Stored in Heap
}
}
4. What is the difference between an interface and an abstract class?
Answer:
| Feature | Abstract Class | Interface |
|---|---|---|
| Methods | Can have both abstract and concrete methods | Only abstract methods (before Java 8) |
| Variables | Can have instance variables | Only static and final variables |
| Inheritance | Can extend another class | Can implement multiple interfaces |
Example:
abstract class Animal {
abstract void sound();
}
interface Pet {
void play();
}
5. Explain method overloading and method overriding in Java.
Answer:
- Method Overloading: Defining multiple methods with the same name but different parameters.
- Method Overriding: Redefining a method in a subclass that already exists in the parent class.
Example:
class Parent {
void show() {
System.out.println("Parent Class");
}
}
class Child extends Parent {
@Override
void show() {
System.out.println("Child Class");
}
}
6. What is the difference between == and .equals() method in Java?
Answer:
==Operator: Compares memory addresses..equals()Method: Compares the actual values.
Example:
String s1 = new String("Hello");
String s2 = new String("Hello");
System.out.println(s1 == s2); // false
System.out.println(s1.equals(s2)); // true
7. What is the purpose of the ‘final’ keyword in Java?
Answer:
The final keyword can be used in three ways:
- Final Variable: Value cannot be changed.
- Final Method: Cannot be overridden.
- Final Class: Cannot be extended.
Example:
final class Constants {
final int MAX_VALUE = 100;
}
8. What are checked and unchecked exceptions in Java?
Answer:
- Checked Exceptions: Must be handled at compile-time (e.g., IOException, SQLException).
- Unchecked Exceptions: Occur at runtime and do not require handling (e.g., NullPointerException, ArithmeticException).
Example:
try {
FileReader file = new FileReader("test.txt"); // Checked Exception
} catch (IOException e) {
e.printStackTrace();
}
9. Explain the difference between ArrayList and LinkedList.
Answer:
| Feature | ArrayList | LinkedList |
|---|---|---|
| Implementation | Uses dynamic array | Uses doubly linked list |
| Insertion/Deletion | Slow | Fast |
| Access Time | Fast | Slow |
Example:
ArrayList<String> list = new ArrayList<>();
list.add("Java");
10. What is the purpose of the volatile keyword in Java?
Answer:
The volatile keyword ensures that the value of a variable is always read from main memory, preventing thread-local caching.
Example:
class SharedResource {
volatile int counter = 0;
}