What are autoboxing and unboxing in Java?

Each primitive has a wrapper class: int – Integer, double – Double, boolean – Boolean. Java converts between them by itself:

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

public class Main {
    public static void main(String[] args) {
        Integer boxed = 5;                // autoboxing
        int plain = boxed;                // unboxing

        List<Integer> list = new ArrayList<>();
        list.add(10);                     // autoboxing
        int first = list.get(0);          // unboxing
        System.out.println(boxed + plain + first);

        Integer a = 127, b = 127, c = 1000, d = 1000;
        System.out.println(a == b);       // true: small values are cached
        System.out.println(c == d);       // false: different objects
        System.out.println(c.equals(d));
    }
}

Compare wrapper objects with equals(), not with ==. Unboxing a null gives a NullPointerException.

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