泛型-为什么类类型变量在静态上下文中无效? [英] Generics - Why are Class type variables not valid in static contexts?

问题描述

我正在从一本书中学习Java泛型.该书说类类型变量在静态上下文中无效"，并通过以下示例进行解释.

I am learning Java generics from a book. The book says that "Class Type Variables Are Not Valid in Static Contexts" and explains it with the following example.

考虑具有类型变量(例如Entry)的泛型类.您不能将类型变量K和V与静态变量或方法一起使用.例如，以下方法不起作用:

Consider a generic class with type variables, such as Entry. You cannot use the type variables K and V with static variables or methods. For example, the following does not work:

public class Entry<K, V> {
    // Compiler error - V in static context ("Entry.this' cannot be referenced from a static context")
    private static V defaultValue;

    // Compiler error - V in static context ("Entry.this' cannot be referenced from a static context")
    public static void setDefault(V value) {
        defaultValue = value;
    }
}

毕竟，类型擦除意味着在擦除的Entry类中只有一个这样的变量或方法，而对于每个K和V都没有一个.

After all, type erasure means there is only one such variable or method in the erased Entry class, and not one for each K and V.

我不理解上面的解释.我也尝试为K创建相同的代码，并且遇到相同的编译错误.为什么上面的代码是非法的?

I don't understand the above explanation. I tried to create the same code for K also and I got the same compile errors. Why is the above code illegal ?

推荐答案

Java泛型设计者选择使用称为类型擦除"的机制来实现它.这意味着像 Entry< String，Integer> 和 Entry< Integer，String> 之类的通用专业并不作为单独的类存在.类型参数被删除.

Designers of Java generics chose to implement it using a mechanism called "type erasure". It means that generic specializations like Entry<String,Integer> and Entry<Integer,String> do not exist as separate classes. The type parameters are erased.

从 Entry< String，Integer> 和 Entry< Integer，String> 中删除类型参数后，只剩下 Entry 类.

After you erase the type parameters from Entry<String,Integer> and Entry<Integer,String> you're left with just the Entry class.

如果可能有一个像 defaultValue 这样的静态变量，则您希望 Entry< String，Integer> .defaultValue 是一个整数.并且您希望 Entry< Integer，String> .defaultValue 是一个字符串.但是在类型擦除之后，只有一个 Entry 类只有一个 defaultValue 变量，该变量现在必须是Integer和String.这不可能.这就是为什么您不能拥有泛型类型的静态变量.

If it were possible to have a static variable like defaultValue you would expect Entry<String,Integer>.defaultValue to be a Integer. And you would expect Entry<Integer,String>.defaultValue to be a String. But after type erasure only one Entry class with only one defaultValue variable, which now has to be both Integer and String. That's impossible. That's why you can't have a static variable of the generic type.

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