is List< List< String>> Collection< Collection< T>>的实例 [英] Is List<List<String>> an instance of Collection<Collection<T>>?

问题描述

我写了这个方便的泛型函数，用于将集合集合转换为单个集合：

  public static< T& ;设置< T> makeSet（Collection< Collection< T> a_collection）{
 Iterator< Collection< T> it = a_collection.iterator（）; 
设置< T> result = new HashSet< T>（）; 
 while（it.hasNext（））{
 result.addAll（it.next（））; 
} 
 return result; 
} 
  

然后我试图调用它：

  List< List< String>> resultLists = ...; 
 Set< String> labelsSet = CollectionsHelper.makeSet（resultLists）; 
  

，我收到以下错误：

<$ CollectionsHelper
中的p $ p> < T> makeSet（java.util.Collection< java.util.Collection< T>>）不能应用于（java.util.List< java.util.List< java.lang.String>>）


c> List 是 集合和 String 是 T 。那么为什么这不工作，如何解决呢？

解决方案

  < T>设置< T> makeSet（Collection< ;? extends Collection< T>> a_collection）{
 Iterator< ;? extends Collection< T>> it = a_collection.iterator（）; 
设置< T> result = new HashSet< T>（）; 
 while（it.hasNext（））{
 result.addAll（it.next（））; 
} 
 return result; 
} 
  

I wrote this handy, generic function for converting a collection of collections into a single set:

public static <T> Set<T> makeSet(Collection<Collection<T>> a_collection) {
    Iterator<Collection<T>> it = a_collection.iterator();
    Set<T> result = new HashSet<T>();
    while (it.hasNext()) {
        result.addAll(it.next());
    }
    return result;
}

Then I tried to call it:

    List<List<String>> resultLists = ... ;
    Set<String> labelsSet = CollectionsHelper.makeSet(resultLists);

and I received the following error:

<T>makeSet(java.util.Collection<java.util.Collection<T>>) in CollectionsHelper 
cannot be applied to (java.util.List<java.util.List<java.lang.String>>)

Now a List is a Collection, and a String is a T. So why doesn't this work and how do I fix it?

解决方案

public static <T> Set<T> makeSet(Collection<? extends Collection<T>> a_collection) {
    Iterator<? extends Collection<T>> it = a_collection.iterator();
    Set<T> result = new HashSet<T>();
    while (it.hasNext()) {
            result.addAll(it.next());
    }
    return result;
}

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