java Java泛型，如何在使用类层次结构时避免未经检查的赋值警告？
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Java Generics, how to avoid unchecked assignment warning when using class hierarchy?
提问by Guillaume
I want to use a method using generic parameters and returning generic result on a class hierarchy.
我想使用一种使用泛型参数并在类层次结构上返回泛型结果的方法。
edit: noSupressWarnings("unchecked")answer allowed :-)
编辑：不允许SupressWarnings("unchecked")回答:-)
Here is a sample code illustrating my problem:
这是说明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; 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:
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);
Bonus question: how to remove the compilation error if I declare the fruitId Function this way:
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：
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.
在处理层次结构时，我对泛型感到非常困惑。任何指向有关 和 使用的良好资源的指针将不胜感激。
回答by Sean Patrick Floyd
2 small changes:
2个小改动：
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
}
For reference, read this:
作为参考，请阅读以下内容：
