Java Wrapper Classes

Java has two families of types. Primitives like int, double, char, and boolean are fast and simple but are not objects, so they have no methods and cannot be stored in collections. Wrapper classes are object versions of each primitive. Every primitive has a matching wrapper, and the wrapper adds useful methods and lets the value be treated as an object.

Notice that most wrapper names are just the capitalised primitive, but int becomes Integer and char becomes Character. These small differences trip up beginners, so it is worth memorising the two odd ones.

Autoboxing and unboxing

In modern Java you rarely convert between a primitive and its wrapper by hand. The compiler does it automatically. Turning a primitive into a wrapper is called autoboxing, and turning a wrapper back into a primitive is called unboxing. This is why you can write list.add(5) even though the list holds Integer objects.

public class Main {
  public static void main(String[] args) {
    Integer boxed = 42;    // autoboxing: int 42 becomes Integer
    int unboxed = boxed;   // unboxing: Integer becomes int

    System.out.println(boxed + unboxed); // 84

    // Wrapper methods that a plain int does not have
    System.out.println(Integer.parseInt("100") + 1); // 101
    System.out.println(Integer.MAX_VALUE);           // 2147483647
  }
}

Wrapper classes also carry handy static tools. Integer.parseInt turns a String into an int, which is essential when reading numbers typed by a user. Constants like Integer.MAX_VALUE tell you the largest value the type can hold.

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    // ArrayList<int> is NOT allowed; use the wrapper
    ArrayList<Integer> scores = new ArrayList<>();
    scores.add(90);  // int 90 is autoboxed to Integer
    scores.add(75);

    int first = scores.get(0); // auto-unboxed back to int
    System.out.println("First score: " + first);
  }
}

• Each primitive has a matching wrapper class; int to Integer, char to Character.
• Wrappers are objects, so they work with collections and have useful methods.
• Autoboxing and unboxing convert between primitives and wrappers automatically.
• Use .equals() to compare wrappers, and watch out for null unboxing.