1. What is the difference between == and .equals() in Java?
Answer:
==is used to compare the memory reference of objects..equals()is used to compare the content of objects.- For example:
String str1 = new String("Java"); String str2 = new String("Java"); System.out.println(str1 == str2); // false System.out.println(str1.equals(str2)); // true
2. Explain the concept of JVM, JRE, and JDK.
Answer:
- JVM (Java Virtual Machine): Executes Java bytecode.
- JRE (Java Runtime Environment): Provides libraries and JVM for running Java applications.
- JDK (Java Development Kit): Includes JRE and development tools for Java programming.
3. What are wrapper classes in Java?
Answer:
- Wrapper classes provide object representations of primitive data types.
- Example:
int num = 10; Integer obj = Integer.valueOf(num); - Common wrapper classes:
Integer,Double,Character,Boolean, etc.
4. What is the difference between ArrayList and LinkedList?
Answer:
| Feature | ArrayList | LinkedList |
|---|---|---|
| Implementation | Uses dynamic array | Uses doubly linked list |
| Performance | Fast in retrieval, slow in insertion/deletion | Slow in retrieval, fast in insertion/deletion |
| Memory Overhead | Less | More due to extra pointers |
5. Explain method overloading and method overriding.
Answer:
- Method Overloading: Multiple methods with the same name but different parameters within a class.
- Method Overriding: A subclass redefines a method from the parent class.
- Example:
class Parent { void show() { System.out.println("Parent class method"); } } class Child extends Parent { void show() { System.out.println("Child class method"); } }
6. What is the difference between String, StringBuilder, and StringBuffer?
Answer:
| Feature | String | StringBuilder | StringBuffer |
|---|---|---|---|
| Mutability | Immutable | Mutable | Mutable |
| Thread Safety | No | No | Yes |
| Performance | Slow | Fast | Moderate |
7. What is a Java interface, and how does it differ from an abstract class?
Answer:
- Interface: A contract that defines abstract methods without implementation.
- Abstract Class: Can have both abstract and concrete methods.
- Example:
interface Animal { void makeSound(); } class Dog implements Animal { public void makeSound() { System.out.println("Bark"); } }
8. Explain the concept of exception handling in Java.
Answer:
- Java provides
try,catch,finally,throw, andthrowsfor handling exceptions. - Example:
try { int a = 5 / 0; } catch (ArithmeticException e) { System.out.println("Cannot divide by zero"); } finally { System.out.println("Execution completed"); }
9. What are Java annotations?
Answer:
- Annotations provide metadata to code.
- Common examples:
@Override,@Deprecated,@FunctionalInterface. - Example:
@Override public void display() { System.out.println("Overridden method"); }
10. What is the difference between HashMap and HashTable?
Answer:
| Feature | HashMap | HashTable |
|---|---|---|
| Thread Safety | No | Yes |
| Null Keys/Values | Allows one null key and multiple null values | Does not allow null keys or values |
| Performance | Faster | Slower |