PECS( Producer Extends, Consumer Super)

• Consumer:

向上转型不适用：运行时 grandFather是一个Father 数组

public class CovariantArrays {
    public static void main(String... args) {
        GrandFather[] grandFather = new Father[10];
        grandFather[0] = new Son();
        grandFather[1] = new Father();
        try {
            // Runtime Error
            grandFather[2] = new GrandFather();
        } catch (Exception e) {
            System.out.println(e);  //java.lang.ArrayStoreException: GrandFather
        }
    }
}

class GrandFather {
    int gf;
}

class Father extends GrandFather{
    int f;
}

class Son extends Father{
    int s;
}

使用<? extends T>

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

public class ExtendsTest {
    public static void main(String... args) {
        //error: incompatible types: ArrayList<Father> cannot be converted to List<GrandFather>
        //List<GrandFather> grandFather = new ArrayList<Father>();
        List<? extends Father> father = new ArrayList<Father>();
        try {
            // compile error: no suitable method found for add(Son)
            father.add(new Son());
            // compile error: no suitable method found for add(Father)
            father.add(new Father());
            // compile error: no suitable method found for add(GrandFather)
            father.add(new GrandFather());
        } catch (Exception e) {
            System.out.println(e);  //java.lang.ArrayStoreException: GrandFather
        }
    }
}

class GrandFather {
    int gf;
}

class Father extends GrandFather{
    int f;
}

class Son extends Father{
    int s;
}

<? super T>

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

public class SuperTest {
    public static void main(String... args) {
        //error: incompatible types: ArrayList<Father> cannot be converted to List<GrandFather>
        //List<GrandFather> grandFather = new ArrayList<Father>();
        List<? super Father> father = new ArrayList<GrandFather>();
        try {
            father.add(new Son());
            father.add(new Father());
            // compile error: no suitable method found for add(GrandFather)
            father.add(new GrandFather());
        } catch (Exception e) {
            System.out.println(e);  //java.lang.ArrayStoreException: GrandFather
        }
    }
}

class GrandFather {
    int gf;
}

class Father extends GrandFather{
    int f;
}

class Son extends Father{
    int s;
}

• Producer

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

public class ExtendsTest {
    public static void main(String... args) {
        List<? extends Father> father = Arrays.asList(new Son());
        try {
            Son s = (Son)father.get(0);
            Father f = (Father)father.get(0);
            GrandFather gf = (GrandFather)father.get(0);
            father.contains(new Son());
            father.indexOf(new Son());

        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

class GrandFather {
    int gf;
}

class Father extends GrandFather{
    int f;
}

class Son extends Father{
    int s;
}

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

public class SuperTest {
    public static void main(String... args) {
        List<? super Father> father = new ArrayList<GrandFather>();
        try {
            Son son = new Son();
            System.out.print(son.toString()); //Son@4aa298b7
            father.add(son);
            Father f = new Father();
            father.add(f);
            System.out.println(father.contains(son)); //true
            System.out.println(father.indexOf(f)); //1
            Son s = (Son)father.get(0);
            System.out.print(s.toString());  //Son@4aa298b7
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

class GrandFather {
    int gf;
}

class Father extends GrandFather{
    int f;
}

class Son extends Father{
    int s;
}