Top 15 Java Interview Questions for Freshers with Answers

Q1. What is the difference between JDK, JRE and JVM?

JVM executes bytecode; JRE = JVM + libraries (to run); JDK = JRE + compiler and tools (to develop). Follow-up trap: JVM itself is platform dependent — the bytecode is what's platform independent.

Q2. Why is Java platform independent?

Source compiles to bytecode, not machine code. The same .class file runs on any OS via that OS's JVM — "write once, run anywhere".

Q3. Why is String immutable?

String pool sharing, security (paths, URLs), thread safety, and stable hashcodes for HashMap keys. Any "change" creates a new String object.

Q4. Difference between == and equals()?

== compares references; equals() compares content when overridden. Literals may pass == due to the String pool, but runtime Strings won't — always use equals().

Q5. String vs StringBuffer vs StringBuilder?

String immutable; StringBuffer mutable + synchronized (thread-safe, slower); StringBuilder mutable, fastest — the default for string building in loops.

Q6. What are the 4 pillars of OOPs?

Encapsulation (data + methods bundled, private fields with getters/setters), Inheritance (extends for reuse), Polymorphism (overloading + overriding), Abstraction (hide implementation via abstract classes/interfaces). Give one example each.

Q7. Overloading vs overriding?

Overloading: same name, different parameters, same class, compile time. Overriding: identical signature, child class, runtime. Static methods cannot be overridden (only hidden).

Q8. Abstract class vs interface?

Abstract class: state + constructors + partial implementation, single inheritance. Interface: contract, multiple inheritance, no instance state; since Java 8 default/static methods can have bodies.

Q9. Why doesn't Java support multiple inheritance of classes?

The diamond problem — ambiguity when two parents define the same method. Interfaces solve it because they carry no state.

Q10. What is a constructor? What is the default constructor trap?

Special method, class name, no return type, runs with new. Java provides an invisible no-arg constructor only until you write any constructor — then it disappears.

Q11. Why is main() public static void?

public: JVM must reach it from outside; static: callable before any object exists; void: JVM expects no return value; String[] args: command-line arguments.

Q12. final vs finally vs finalize?

final locks (variable/method/class); finally always runs after try-catch for cleanup; finalize() was a GC callback, deprecated since Java 9.

Q13. What does static mean and why can't static methods use this?

static members belong to the class with one shared copy. Static methods run without an object, so there is no this — hence "non-static variable cannot be referenced from a static context".

Q14. Predict the output:

Integer a = 127, b = 127;
Integer x = 128, y = 128;
System.out.println(a == b);
System.out.println(x == y);

true then false — Integer caches -128 to 127 (like Python's small-int cache). Beyond the range, new objects are created. Moral: compare wrappers with equals().

Q15. Predict the output:

String s = "a";
s.concat("b");
System.out.println(s);

a — not "ab"! concat() returns a NEW String (immutability); the result was discarded because it wasn't assigned. Correct: s = s.concat("b");