Wrapper class

Wrapper classes are object equivalents of the 8 primitive types in java.lang. They let primitives be used where an object is required — generics, collections, reflection.

The wrappers

Autoboxing and unboxing

List<Integer> nums = new ArrayList<>();
nums.add(42);       // autobox: int → Integer
int first = nums.get(0);  // auto-unbox: Integer → int

Utility methods

int n = Integer.parseInt("42");
String s = Integer.toString(255, 16);  // "ff"
int max = Integer.MAX_VALUE;           // 2147483647
boolean odd = n % 2 != 0;
int compare = Integer.compare(a, b);

Watch out: == on wrappers

Integer a = 1000;
Integer b = 1000;
System.out.println(a == b);       // false — reference comparison
System.out.println(a.equals(b));  // true  — value comparison

Always use .equals() for wrapper comparison. == works accidentally for small values (−128 to 127) due to Integer caching, but never rely on it.