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问题描述

我想使用一个使用泛型参数的方法,并在类层次结构上返回泛型结果。

编辑: SupressWarnings (unchecked)答案允许: - )



下面是一个演示我的问题的示例代码:

  import java.util。*; 

public class GenericQuestion {

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


public static void main(String [] args){

List< Apple>苹果= Arrays.asList(
新苹果(1,绿色),新苹果(2,红)
);
列表< Pear> pears = Arrays.asList(
新梨(1,绿),新梨(2,红)
);

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

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

public static< K,V>地图< K,V> mapValues(
List< V>值,Function< V,K>函数){

Map< K,V> map = new HashMap< K,V>(); (v v:values)

map.put(function.apply(v),v);
}
返回地图;






$ b

如何从这些调用中删除泛型异常: / p>

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

额外问题:如果我以这种方式声明fruitId函数,如何消除编译错误:

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

当泛型处理层次结构时,我非常困惑。任何指向关于使用情况的好资源的指针都将不胜感激。

解决方案

小改动:

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

// ... snip

//更改nr 1:使用通用声明
final函数< Fruit,Integer> fruitID =
新功能< Fruit,Integer>(){

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

// ...剪切
}

公共静态< K,V>地图< K,V> mapValues(final List< V>值,

//改变nr.2:使用< ;? super V>而不是< V>
final Function< super V,K>函数){

// ...剪切
}






仅供参考,请阅读:


I want to use a method using generic parameters and returning generic result on a class hierarchy.

edit: no SupressWarnings("unchecked") answer allowed :-)

Here is a sample code illustrating my problem:

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;
    }
}

How to remove the generic exception from these calls:

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

Bonus question: how to remove the compilation error if I declare the fruitId Function this way:

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

I'm very confused about generics when it is dealing with hierarchy. Any pointer to a good resource about the usage of and will be greatly appreciated.

解决方案

2 small changes:

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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09-03 05:02