是否需要在直通方法中使用有界通配符泛型？ [英] Is there a need to use bounded wildcard generics in a passthrough method?

问题描述

我知道在集合< E> 的以下方法中：

  public void addAll（Collection<？extends E> subcollection）; 
  

我们使用集合<？例如：



  List< / code>那里允许一个仅存在子元素的集合。 ;驱动> drivables = new ArrayList<>（）; 
列表< Car> cars = new ArrayList<>（）; 
 //我们在这里需要通配符，因为以下行：
 drivables.addAll（cars）; 
  

然而，在我的后续方法中是否需要这样的有界通配符？

  public static< E>收集和LT E  - 代替; requireNonEmpty（final Collection< E> collection）抛出NoSuchElementException {
 if（collection.isEmpty（））{
抛出new NoSuchElementException（collection必须为非空）; 
} 
返回集合; 
 
 
 
 $ b $ p 
 $ b这与 Objects.requireNonNull（ T对象），它实际上返回 T对象。
 
 
 
任何好处（或者它甚至是错误的？）写我的方法如下？
  public static< E>收集和LT ;? super E> 
 requireNonEmpty（final Collection< ;? extends E> collection）; 
  
 
 
解决方案
使用集合< ？扩展E> subcollection 使传递的参数成为通用的捐助者  - 即它可以自由地读取。 
 
 
需要从传递的集合中读取对象，因此不需要任何通配符。
 
 
 
顺便使用集合<？ super E> subcollection 使其成为通用收件人 - 即可以添加到它。
 
 
 
请参阅何时使用extends和super 进行清晰的介绍。
 
 
 
此泛型较低的未绑定vs较高有界通配符。
 
I know that in the following method in Collection<E>:
public void addAll(Collection<? extends E> subcollection);
We use Collection<? super E> there to allow a collection that solely exists of sub-elements, example:
List<Drivable> drivables = new ArrayList<>();
List<Car> cars = new ArrayList<>();
//we need the wildcard here, because of the following line:
drivables.addAll(cars);
However, are such bounded wildcards needed in my following method?
public static <E> Collection<E> requireNonEmpty(final Collection<E> collection) throws NoSuchElementException {
    if (collection.isEmpty()) {
        throw new NoSuchElementException("collection must be non-empty");
    }
    return collection;
}
This uses a similar idiom as Objects.requireNonNull(T object), which actually returns the T object.

So, is there any benefit (or is it even wrong?) to write my method as the following?
public static <E> Collection<? super E> 
    requireNonEmpty(final Collection<? extends E> collection);

解决方案
The use of Collection<? extends E> subcollection makes the passed parameter a universal donor - i.e. it can be read from with freedom.

You do not need to read objects from the passed collection so you do not need any wildcards.

Incidentally using Collection<? super E> subcollection makes it a universal recipient - i.e. you can add to it.

See When to use extends and super for a clear introduction.

Also a good answer covering this Generic lower unbound vs upper bounded wildcards.

                        
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