Wrapper classes and autoboxing

Key points

• Every primitive has an object wrapper: int/Integer, double/Double, boolean/Boolean.
• Generics only hold objects, so it is List<Integer> — List<int> does not exist.
• Autoboxing converts both ways automatically. Convenient, but every box is a real object allocation.
• Unboxing a null wrapper throws NullPointerException. This is the number one autoboxing bug.
• Compare wrappers with equals(). == appears to work for -128..127 only because those come from a shared cache.
• Wrappers carry the useful statics: Integer.parseInt, Integer.MAX_VALUE, Double.compare.

Example

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        list.add(7);
        int back = list.get(0);
        System.out.println("boxed into the list, unboxed back out: " + back);

        Integer x = 127, y = 127;
        Integer p = 128, q = 128;
        System.out.println("127 == 127  : " + (x == y) + "  (same cached object)");
        System.out.println("128 == 128  : " + (p == q) + " (two different objects)");
        System.out.println("p.equals(q) : " + p.equals(q));

        System.out.println("parseInt: " + (Integer.parseInt("21") * 2));
        System.out.println("binary of 10: " + Integer.toBinaryString(10));
        System.out.println("Double.compare(1.5, 2.5): " + Double.compare(1.5, 2.5));

        Integer missing = null;
        try {
            int boom = missing;
            System.out.println(boom);
        } catch (NullPointerException e) {
            System.out.println("unboxing null throws NullPointerException");
        }
    }
}

Boxing is automatic; the NullPointerException it hides is not.