Java泛型放在Map< String，？ extends List< String>> [英] Java Generics putting on Map<String, ? extends List<String>>

问题描述

有没有办法以类型安全的方式使以下实现？

Is there a way to make the following implementation in a type safe manner?

public void myMethod( Map<String, ? extends List<String>> map )
{
   map.put("foo", Collections.singletonList("bar");
}

上面的实现不起作用，它需要一个 Map< String，？super List< String> / code>正确地编译方法 map.put（）但是myMethod不会接受List的任何子类型因此，我必须使用 Map< String，？extends List< String>> 而不是如何以类型安全的方式解决这个问题？

The above implementation doesn't work. It requires a Map<String, ? super List<String>> to compile the method map.put() correctly. But myMethod won't accept any subtype of List this way. So, I have to use Map<String, ? extends List<String>> instead. How can I solve this problem in a type safe manner?

推荐答案

public void myMethod( Map<String, List<String>> map ) {
    map.put("foo", Collections.singletonList("bar") );
}

列表（ Collection.singletonList（）的返回类型转换为映射 ？ extends List ，因为实际类型可以是 List 的任何实现。例如，将通用列表放入 Map< String，LinkedList> ，因为 List 可能不是 LinkedList 。然而，我们可以很容易地将 LinkedList 放入 。

You can't put a List (the return type of Collections.singletonList() into a Map of ? extends List since the actual type could be any implementation of List. For example, it is not safe to put a generic List into a Map<String,LinkedList> since the List might not be a LinkedList. However, we can easily put a LinkedList into a Map of <String,List>.

我想你是在想你的泛型。您不必使用？ extends 用于非泛型类。例如， List< Object> 将保存任何 Object ，？ extends 不需要添加对象。 列表< List< Foo>> 将只需要 List< Foo> 对象，而不是 List< FooSubclass> objects [通用类不根据参数继承]。这是？ extends 开始运行。将列表< Foo> 和列表< FooSubclass> 添加到列表，类型必须 List< List< ;?扩展Foo>> 。

I think you were over thinking your generics. You do not have to use ? extends for a non-generic class. For example, List<Object> will hold any Object, the ? extends is not needed to add an object. A List<List<Foo>> will only take List<Foo> objects and not List<FooSubclass> objects [Generic classes don't inherit based on their parameters]. This is where ? extends comes into play. To add both List<Foo> and List<FooSubclass> to a List, the type must be List<List<? extends Foo>>.

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