Java + Spring Boot + Microservices Interview Questions with Examples

🔹 Section 1: Core Java (10 Questions)

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

String a = new String("test");
String b = new String("test");
System.out.println(a == b); // false (reference comparison)
System.out.println(a.equals(b)); // true (value comparison)

== compares references; .equals() compares values (unless overridden).

2. What is the difference between HashMap and Hashtable?

• HashMap is not synchronized (faster, not thread-safe)
• Hashtable is synchronized (thread-safe, but slower)

3. What is the purpose of the final, finally, and finalize()?

• final: Prevent modification
• finally: Always executes after try-catch
• finalize(): Called by GC before object removal

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

abstract class Animal {
    abstract void makeSound();
}

interface Walkable {
    void walk();
}

Abstract class can have constructors and state; interface is contract only.

5. What are functional interfaces and give an example?

@FunctionalInterface
interface Greeting {
    void sayHello(String name);
}