当我不提供泛型类型时，为什么这个类的行为不同？

问题描述

我不明白为什么这会让编译器感到困惑。我使用通用类型 T 来存放与放置和获取方法。我一直认为 GenericClass 和 GenericClass< Object> 在功能上是相同的，但我必须弄错。当编译 DoesntWork 类时，我得到不兼容的类型 - required：String - found：Object 。 Works 类完成我所期望的。这里发生了什么？

I don't understand why this confuses the compiler. I'm using the generic type T to hold an object that's not related to the put and get methods. I always thought GenericClass and GenericClass<Object> were functionally identical, but I must be mistaken. When compiling the DoesntWork class I get incompatible types - required: String - found: Object. The Works class does what I expect. What's going on here?

public class GenericClass<T> {
    public <V> void put(Class<V> key, V value) {
        // put into map
    }

    public <V> V get(Class<V> key) {
        // get from map
        return null;
    }

    public static class DoesntWork {
        public DoesntWork() {
            GenericClass genericClass = new GenericClass();
            String s = genericClass.get(String.class);
        }
    }

    public static class Works {
        public Works() {
            GenericClass<Object> genericClass = new GenericClass<Object>();
            String s = genericClass.get(String.class);
        }
    }
}

推荐答案

关于原始类型是如何工作的 - 你遗漏了参数的泛型类型 - 它们的所有泛型和它们的方法也被删除。因此，对于原始 GenericClass ， get 和放入方法 也会失去他们的泛型。

The thing about how raw types work -- generic types that you've left out the arguments for -- is that all generics for them and their methods are erased as well. So for a raw GenericClass, the get and put methods also lose their generics.

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