Wrapper Class in Java — Java Technical Interview Questions & Answers

Wrapper Class in Java

Wrapper Class in Java

  • Wrapper classes box primitives into objects: int → Integer, double → Double, boolean → Boolean. ArrayList and HashMap need objects, not raw int.
  • Autoboxing: Integer n = 5; Unboxing: int x = n;. If n is null, unboxing throws NullPointerException — a favourite trap.
  • Use wrappers in collections. Use primitives for simple maths — cheaper.
  • Let's take this on the board with one tiny Main class — no extra files. Wrapper Class in Java — 10 is boxed into Integer, then unboxed back to int for math. Start from main, go line by line, and stop at each print. That output is the proof for Wrapper Class.

Technical Interview Tip: Why wrappers? Lists need objects. Then mention null unboxing NPE.

Code Examples for Wrapper Class in Java

Example

// Wrapper / autoboxing
public class Main {
  public static void main(String[] args) {
    Integer a = 10; // autobox
    int b = a;       // unbox
    System.out.println(a + b);
  }
}

Wrapper Class in Java — 10 is boxed into Integer, then unboxed back to int for math.

How to Explain Wrapper Class in Java to an Interviewer

What is Integer?

Wrapper for int

Why wrappers?

Collections need objects

Null Integer unboxed?

NullPointerException