Java泛型，使用类层次结构时如何避免未经检查的分配警告？

Question

我想使用一种方法，该方法使用通用参数并在类层次结构上返回通用结果。

编辑:否 SupressWarnings("unchecked") 允许回答:-)

这是一个示例代码来说明我的问题:

import java.util.*;

public class GenericQuestion {

    interface Function<F, R> {R apply(F data);}
    static class Fruit {int id; String name; Fruit(int id, String name) {
        this.id = id; this.name = name;}
    }
    static class Apple extends Fruit { 
        Apple(int id, String type) { super(id, type); }
    }
    static class Pear extends Fruit { 
        Pear(int id, String type) { super(id, type); }
    }

    public static void main(String[] args) {

        List<Apple> apples = Arrays.asList(
                new Apple(1,"Green"), new Apple(2,"Red")
        );
        List<Pear> pears = Arrays.asList(
                new Pear(1,"Green"), new Pear(2,"Red")
        );

        Function fruitID = new Function<Fruit, Integer>() {
            public Integer apply(Fruit data) {return data.id;}
        };

        Map<Integer, Apple> appleMap = mapValues(apples, fruitID);
        Map<Integer, Pear> pearMap = mapValues(pears, fruitID);
    }

      public static <K,V> Map<K,V> mapValues(
              List<V> values, Function<V,K> function) {

        Map<K,V> map = new HashMap<K,V>();
        for (V v : values) {
            map.put(function.apply(v), v);
        }
        return map;
    }
}

如何从这些调用中删除通用异常:

Map<Integer, Apple> appleMap = mapValues(apples, fruitID);
Map<Integer, Pear> pearMap = mapValues(pears, fruitID);

奖励问题:如果我以这种方式声明 fruitId 函数，如何消除编译错误:

Function<Fruit, Integer> fruitID = new Function<Fruit, Integer>() {public Integer apply(Fruit data) {return data.id;}};

在处理层次结构时，我对泛型非常困惑。任何指向关于 和 的使用的好资源的指针都将不胜感激。

Answer 1

2 个小变化:

public static void main(final String[] args){

    // ... snip

    // change nr 1: use a generic declaration
    final Function<Fruit, Integer> fruitID =
        new Function<Fruit, Integer>(){

            @Override
            public Integer apply(final Fruit data){
                return data.id;
            }
        };

    // ... snip
}

public static <K, V> Map<K, V> mapValues(final List<V> values,

    // change nr. 2: use <? super V> instead of <V>
    final Function<? super V, K> function){

    // ... snip
}

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作为引用，请阅读:

The get-put principle