入门集上的泛型迭代器 [英] Generic Iterator on Entry Set

问题描述

  static< V> void myMethod（Map<？，V> map）
 {
 Iterator< Entry<？，V>> it = map.entrySet（）。iterator（）; 
} 
  

我在下面看到编译错误：

类型不匹配：无法从 Iterator< Map.Entry< capture＃5-of？，V>> 转换为 Iterator< Map.Entry< ，V>>

解决方案

 迭代< ;?扩展Entry<？，V>> it = map.entrySet（）。iterator（）; 
  

您的尝试失败的原因有点难以理解，特别是因为 Iterator< T> 不会消耗任何 T （即它没有一个方法需要 T 作为参数）。

您不能指定 Iterator< Entry< V>> 到 Iterator< Entry<？，V>> ，原因与您无法指定 Iterator< Entry Iterator< Entry< / code>。 capture＃5 只是一个名称，用于区分方法参数中的特定？和其他不同的通配符实例。它可以很容易成为一个具体的类型。

如果不是 Iterator 你认为像 List 这样的类。

  List< Entry< ;整数，字符串>> entries = new ArrayList<>（）; 
 
 //这是一个编译错误，但假设它是可能的
 List< Entry<？，String>> wildcardEntries =条目; 
 
 //然后，因为这已经是可能的
 wilcardEntries.add（new Entry< String，String>（a，b））; 
条目<整数，字符串> entry1 = entries.get（0）; 
 
 //这将导致类型错误（ClassCastException）
 Integer i = entry1.getKey（）; 
  

通过使用？扩展Entry<？，V> 你并没有将自己暴露给这个，因为你并没有声称知道任何有关 Entry 类型的东西 Iterator 可以消耗，只能使用它。



编辑

虽然Jiman删除了他的答案，但他有一个很好的观点，那就是使用for-each循环是一种更简洁的方法（假设您只是试图遍历条目），这将完全避免这个问题。这应该可以工作： $（b

pre $ for（Entry<？，V> entry：map.entrySet（））{
/ / ...
}


static <V> void myMethod(Map<?, V> map)
{
 Iterator<Entry<?, V>> it = map.entrySet().iterator();
}

I'm seeing below compilation error:
Type mismatch: cannot convert from Iterator<Map.Entry<capture#5-of ?,V>> to Iterator<Map.Entry<?,V>>

解决方案

Try

Iterator<? extends Entry<?, V>> it = map.entrySet().iterator();

The reason your attempt doesn't work is a bit hard to see, particularly because Iterator<T> does not consume any T (i.e. it doesn't have a method which takes a T as a parameter).

You can't assign an Iterator<Entry<capture#5-of ?,V>> to an Iterator<Entry<?, V>> for the same reason that you can't assign an Iterator<Entry<Integer, String>> to an Iterator<Entry<?, V>>. The capture#5 is just a name used to differentiate the specific ? found in your method's parameter from other distinct wildcard instances. It could just as easily be a concrete type.

The reason this doesn't work is more clear if instead of Iterator you think of a class like List.

List<Entry<Integer, String>> entries = new ArrayList<>();

//this is a compile error, but assume it is possible
List<Entry<?, String>> wildcardEntries = entries; 

//then since this is already possible
wilcardEntries.add(new Entry<String, String>("a", "b"));
Entry<Integer, String> entry1 = entries.get(0);

//this would result in a type error (ClassCastException)
Integer i = entry1.getKey();

By using ? extends Entry<?, V> you're not exposing yourself to this, as you do not claim to know anything about the type of Entry your Iterator can consume, only what it can produce.

Edit

Though Jiman deleted his answer, he had a good point that using the for-each loop is a lot cleaner approach (assuming you're just trying to iterate over the entries) that would avoid this issue entirely. This should work:

for ( Entry<?, V> entry : map.entrySet() ) {
   //...
}

这篇关于入门集上的泛型迭代器的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！