java - 下面的代码使用 (? extends aClass) 让我感到困惑

Question

这个问题已经有答案了:

What is PECS (Producer Extends Consumer Super)? (16 个回答)

已关闭 3 年前。

根据我的理解(显然需要更正)， map 应该采用 Integer 类及其所有子类。与 Location 类相同。

Map<? extends Integer, ? extends Location> test2 = new HashMap<>();
test2.put(new Integer(5), new Location(1, "Test2", exits));

我观看并阅读了许多不同的资源。但我仍然无法理解这个问题。我不是专业程序员。

Answer 1

引用Oracle Docs for generics

You can use an upper bounded wildcard to relax the restrictions on a variable. For example, say you want to write a method that works on List<Integer>, List<Double>, and List<Number>; you can achieve this by using an upper bounded wildcard.

和

The upper bounded wildcard, <? extends Foo>, where Foo is any type, matches Foo and any subtype of Foo. The process method can access the list elements as type Foo:

所以基本上test2是 Map它将键视为 Integer 或任何扩展了 Integer 的类(即 Integer 的子类)，并将值设为 Location或其子类