基于内部类中的类型参数的类型声明 [英] Type declaration based on type parameters in inner classes

问题描述

Java阴影类型参数吗?我发现很难自我测试，因为Java泛型无法在运行时得到验证.

Does Java shadow type parameters? I am finding it hard to test for myself because Java generics do not get reified at run time.

例如，给出以下代码:

public class NestedGeneric<T> {
    private InnerGeneric<T> innerGenericInstance;

    private static class InnerGeneric<T> {
        public T innerGenericField;
    }

    NestedGeneric() {
    innerGenericInstance = new InnerGeneric<T>();
    }
}

下面的语句都可以很好地编译:

Both the below statements compile fine:

NestedGeneric<Integer> test1 = new NestedGeneric<Integer>();
NestedGeneric.InnerGeneric<String> test2  = new NestedGeneric.InnerGeneric<String>();

当将 NestedGeneric 传递给类型参数并调用其构造函数时， T 是什么?它总是与传递给 nestedGeneric 的type参数相同吗?

When the NestedGeneric is passed a type parameter and its constructor called, what is T? Is it always going to be the same as the type parameter passed to nestedGeneric?

换句话说，可以将外部类的类型参数传递给内部类的泛型类型声明吗?

In other words, can an outer classes type parameters be passed to an inner classes generic type declarations?

推荐答案

换句话说，我想问题是，外部类可以输入吗参数传递给内部类的泛型类型声明?

In other words, I suppose the question is, can an outer classes type parameters be passed to an inner classes generic type declarations?

不.内部和外部 static 类之间没有任何关系(如继承或作为字段).您可以创建内部静态类的对象，而无需像在您的示例中那样对外部类进行任何依赖:

No. There is no relationship (like inheritance or as a field) between the outer and the inner static class. You can create an object of the inner static class without any dependency on the outer class like in your example:

NestedGeneric.InnerGeneric<String> test2  = new NestedGeneric.InnerGeneric<String>();

但是，当您使用内部类的实例作为字段时，泛型类型是从外部类派生的:

However when you use an instance of the inner class as a field the generic type is derived from the outer class:

private InnerGeneric<T> innerGenericInstance;

innerGenericInstance = new InnerGeneric<T>();

第三个变体是将内部类定义为字段(非静态):

private class InnerGeneric<T> {
    public T innerGenericField;
}

由于它是成员变量，因此现在将从外部类获取类型.

which will now get the type from the outer class since its a member variable.

如注释中所指出的，它同时定义了内部静态&具有该类型的外部类只会使读者(以及您自己在稍后的时间)感到困惑.应该使用不同的泛型来声明它，例如

As pointed out in the comment defining both inner static & outer class with the type will just confuse the reader (and yourself at a later point in time). It should be declared with a different generic like

public class NestedGeneric<T> {
    private InnerGeneric<T> innerGenericInstance;

    private static class InnerGeneric<U> {
        private U innerGenericField;
    }

    NestedGeneric() {
        innerGenericInstance = new InnerGeneric<T>();
    }
}

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