泛型

泛型通配符

• <? extends T>：是指 “上界通配符（Upper Bounds Wildcards）”

Plate<? extends Fruit> p=new Plate<Apple>(new Apple());

• <? super T>：是指 “下界通配符（Lower Bounds Wildcards）”

Plate<? super Fruit> p=new Plate<Fruit>(new Fruit());

PECS（Producer Extends Consumer Super）原则：

• 频繁往外读取内容的，适合用上界Extends。
• 经常往里插入的，适合用下界Super。

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Practice

**Test1 - 代码减肥
借助? extends， 把代码减肥到只是用一种方法

package generic;
import java.util.ArrayList;
import charactor.ADHero;
import charactor.APHero;
import charactor.Hero;
public class TestGeneric {
    public static void iterate(ArrayList<Hero> list) {
        for (Hero hero : list) {
            System.out.println(hero.name);
        }
    }
    public static void iterateAP(ArrayList<APHero> list) {
        for (Hero hero : list) {
            System.out.println(hero.name);
        }
    }
    public static void iterateAD(ArrayList<ADHero> list) {
        for (Hero hero : list) {
            System.out.println(hero.name);
        }
    }
    public static void main(String[] args) {
        ArrayList<Hero> hs = new ArrayList<>();
        ArrayList<APHero> aphs = new ArrayList<>();
        ArrayList<ADHero> adhs = new ArrayList<>();
        iterate(hs);
        iterateAP(aphs);
        iterateAD(adhs);
    }
}

Ans:

package generic;
import java.util.ArrayList;
import charactor.ADHero;
import charactor.APHero;
import charactor.Hero;
public class TestGeneric {
    public static void iterate(ArrayList<? extends Hero> list) {
        for (Hero hero : list) {
            System.out.println(hero.name);
        }
    }
    public static void main(String[] args) {
        ArrayList<Hero> hs = new ArrayList<>();
        ArrayList<APHero> aphs = new ArrayList<>();
        ArrayList<ADHero> adhs = new ArrayList<>();
        iterate(hs);
        iterate(aphs);
        iterate(adhs);
    }
}