Wrapper Classes

Wrapper classes are object versions of Java's primitive types. Every primitive type has a matching wrapper class, letting a primitive value be treated as a full-fledged object when that's what the situation calls for.

Primitive-to-Wrapper Reference

Why Wrapper Classes Are Needed

Java's collection classes — ArrayList, HashMap, and the rest of the Collections Framework — can only hold objects, not primitive values. A List<int> is not legal Java. Wrapper classes bridge this gap: a List<Integer> works because Integer is an object, even though it represents a plain int underneath.

List<Integer> scores = new ArrayList<>();
scores.add(95); // int 95 is autoboxed into an Integer
          

Autoboxing and Unboxing

Modern Java converts between primitives and their wrappers automatically. Converting a primitive into its wrapper is called autoboxing; converting a wrapper back into a primitive is called unboxing. Both happen behind the scenes, without any explicit cast.

public class BoxingDemo {
  public static void main(String[] args) {
    Integer i = 5;   // autoboxing: int 5 becomes an Integer object
    int n = i;         // unboxing: Integer object becomes a plain int

    System.out.println(i + n); // 10
  }
}

Common Wrapper Methods

Wrapper classes also come with handy static utility methods, most commonly for converting between Strings and their numeric or boolean equivalents.

int n = Integer.parseInt("42");       // String -> int
String s = Integer.toString(42);       // int -> String
double d = Double.parseDouble("3.14"); // String -> double