ไวลด์การ์ดขอบเขตล่าง

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

public class Main {
    public static void main(String[] args) {
        List<? super Integer> a = new ArrayList<Number>();
        List<? super Integer> b = new ArrayList<Object>();
        a.add(1);
        b.add(2);
        System.out.println(a + " " + b);
    }
}

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

public class Main {
    static void addNumbers(List<? super Integer> dest) {
        for (int i = 1; i <= 3; i++) dest.add(i);
    }
    public static void main(String[] args) {
        List<Number> nums = new ArrayList<>();
        addNumbers(nums);
        System.out.println(nums);
    }
}

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

public class Main {
    public static void main(String[] args) {
        List<? super Integer> list = new ArrayList<Number>();
        list.add(10);
        Object o = list.get(0); // only Object is guaranteed
        // Integer i = list.get(0); // would NOT compile
        System.out.println(o);
    }
}

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

public class Main {
    public static void main(String[] args) {
        List<Object> raw = new ArrayList<>();
        raw.add("a string");
        List<? super Integer> view = raw; // legal: Object is a supertype of Integer
        view.add(5);
        System.out.println(view.get(0).getClass().getSimpleName());
    }
}

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

public class Main {
    static void fill(List<? super String> dest, int count) {
        for (int i = 0; i < count; i++) dest.add("item" + i);
    }
    public static void main(String[] args) {
        List<Object> objs = new ArrayList<>();
        fill(objs, 3);
        System.out.println(objs);
    }
}

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

public class Main {
    public static void main(String[] args) {
        List<? super Number> list = new ArrayList<Object>();
        list.add(1);      // Integer
        list.add(2.5);    // Double
        list.add(3L);     // Long
        System.out.println(list);
    }
}

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

public class Main {
    @SafeVarargs
    static <T> void addAll(List<? super T> dest, T... items) {
        for (T item : items) dest.add(item);
    }
    public static void main(String[] args) {
        List<Object> sink = new ArrayList<>();
        addAll(sink, "a", "b", "c");
        System.out.println(sink);
    }
}

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

public class Main {
    public static void main(String[] args) {
        Comparator<Object> byString = Comparator.comparing(Object::toString);
        List<String> names = new ArrayList<>(List.of("Charlie", "Alice", "Bob"));
        names.sort(byString); // List<String>.sort accepts Comparator<? super String>
        System.out.println(names);
    }
}

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

public class Main {
    public static void main(String[] args) {
        List<? super Integer> consumer = new ArrayList<Number>();
        consumer.add(1);
        System.out.println("consumer can write: " + consumer);
    }
}

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

public class Main {
    static <T> void copy(List<? extends T> src, List<? super T> dest) {
        for (T t : src) dest.add(t);
    }
    public static void main(String[] args) {
        List<Integer> src = List.of(1, 2, 3);
        List<Number> dest = new ArrayList<>();
        copy(src, dest);
        System.out.println(dest);
    }
}

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

public class Main {
    static <T> void swapFirstTwo(List<T> list) {
        T tmp = list.get(0);
        list.set(0, list.get(1));
        list.set(1, tmp);
    }
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(List.of("a", "b"));
        swapFirstTwo(list);
        System.out.println(list);
    }
}

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

public class Main {
    public static void main(String[] args) {
        List<? super Integer> list = new ArrayList<Number>();
        list.add(42);
        System.out.println("Lower bound recap: " + list);
    }
}