Autoboxing

Autoboxing is Java's automatic conversion between primitive types (int, double, boolean) and their wrapper classes (Integer, Double, Boolean). Added in Java 5 to make generics and collections usable with primitive values.

Autoboxing

Integer boxed = 42;       // autobox: int → Integer
List<Integer> list = new ArrayList<>();
list.add(10);              // autobox: int → Integer
list.add(20);

Unboxing

int n = boxed;              // unbox: Integer → int
int first = list.get(0);    // unbox: Integer → int

The null trap

Integer maybe = null;
int n = maybe;              // NullPointerException!

Unboxing a null wrapper throws NPE. This trips up many developers — use Optional or null checks when a wrapper might be null.

Integer cache gotcha

Integer a = 127;
Integer b = 127;
System.out.println(a == b); // true — cached!

Integer c = 200;
Integer d = 200;
System.out.println(c == d); // false — new object each time

Java caches boxed Integers in the range −128 to 127, so == coincidentally works for small values. Never rely on this — always use .equals().

Performance

Autoboxing has a real runtime cost (allocation, dereferencing). For hot loops with primitives, prefer primitive types directly or use the primitive-specialised streams (IntStream, LongStream, DoubleStream).