Java · Theory

Wrapper Class in Java

Theory

62/270

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.

Exam tip

Why wrappers? Lists need objects. Then mention null unboxing NPE.

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.

Short notes

DefWrapper = object version of a primitive (int → Integer).
RuleCollections store Integer, not int.
RememberAutoboxing int → Integer. Unboxing Integer → int.
TrapUnboxing null Integer → NullPointerException.

Java · Theory

Wrapper Class in Java

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Theory

62/270

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.

Exam tip

Why wrappers? Lists need objects. Then mention null unboxing NPE.

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.

Short notes

DefWrapper = object version of a primitive (int → Integer).
RuleCollections store Integer, not int.
RememberAutoboxing int → Integer. Unboxing Integer → int.
TrapUnboxing null Integer → NullPointerException.

Wrapper Class in Java

Example

Short notes

Questions

Wrapper Class in Java

Example

Short notes

Questions