不兼容的通用通配符捕获 [英] Incompatible generic wildcard captures

问题描述

在下面的片段中：

  package test; 
 
 import java.util.Collection; 
 import java.util.Iterator; 
 import java.util.Map; 
 
 public class WildcardsTest< K，V> {
 private Iterator< Map.Entry< K，？扩展Collection< V>>>迭代器; 
 public WildcardsTest（Map< K，？extends Collection< V> map）{
 iterator = map.entrySet（）。iterator（）; 
 / *类型不匹配：无法从
转换Iterator< Map.Entry< K，capture＃1-of？扩展Collection< V>>>到
迭代器< Map.Entry< K ,?扩展Collection< V>>> * / 
} 
} 
  

分配不正确，完全匹配。

通过将Collection的类型指定为另一个泛型参数，我设计了一个肮脏的解决方法，如下所示：

  public class WildcardsTest< K，V，C扩展Collection< V>> {
 private Iterator< Map.Entry< K，C>>迭代器; 
 public WildcardsTest（Map< K，C> map）{
 iterator = map.entrySet（）。iterator（）; 
 
 
  

但是 C 参数实际上是一种不关心类型，只会使API复杂化，是否有任何方法可以在维护类型安全性的同时摆脱它？

解决方案

像这样做，它会工作：

  private final Iterator <？>延伸
 Map.Entry< K，？扩展Collection< V>> 
>迭代器; 
  

您仍然可以像这样使用迭代器：

（$ iterator.hasNext（））{
Entry< K，？扩展Collection< V>> entry = iterator.next（）;
Collection< V> value = entry.getValue（）;


$ / code $ / pre
$ b

仅供参考，请阅读获取和放置原则（最初来自 Java泛型和集合）

in the following snippet:

package test;

import java.util.Collection;
import java.util.Iterator;
import java.util.Map;

public class WildcardsTest<K, V> {
    private Iterator<Map.Entry<K, ? extends Collection<V>>> iterator;
    public WildcardsTest(Map<K, ? extends Collection<V>> map) {
        iterator = map.entrySet().iterator();
        /* Type mismatch: cannot convert from
           Iterator<Map.Entry<K,capture#1-of ? extends Collection<V>>> to
           Iterator<Map.Entry<K,? extends Collection<V>>> */
    }
}

The assignment is incorrect, although the types seem to match exactly.

I've devised a dirty workaround by specifying the type of the Collection as another generic parameter, like this:

public class WildcardsTest<K, V, C extends Collection<V>> {
    private Iterator<Map.Entry<K, C>> iterator;
    public WildcardsTest(Map<K, C> map) {
        iterator = map.entrySet().iterator();
    }
}

But that C parameter is really a "don't care" type that only complicates the API, is there any way to get rid of it while maintaining type safety?

Thanks.

解决方案

Do it like this and it will work:

private final Iterator<? extends
    Map.Entry<K, ? extends Collection<V>>
> iterator;

You can still use the iterator like this:

public void foo(){
    while(iterator.hasNext()){
        Entry<K, ? extends Collection<V>> entry = iterator.next();
        Collection<V> value = entry.getValue();
    }
}

For reference, read the get and put principle (originally from Java Generics and Collections)

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