泛型、类型参数和通配符 [英] Generics, Type Parameters and Wildcards

问题描述

我正在尝试理解 Java 泛型，但它们似乎非常难以理解.例如，这很好...

I am trying to understand java generics and they seem extremely difficult to understand. For example, this is fine...

public class Main {

    public static void main(String[] args) {
        List<?> list = null;
        method(list);
    }

    public static <T> void method(List<T> list) { }
}

...原来如此...

public class Main {

    public static void main(String[] args) {
        List<List<?>> list = null;
        method(list);
    }

    public static <T> void method(List<T> list) { }
}

...还有这个...

public class Main {

    public static void main(String[] args) {
        List<List<List<?>>> list = null;
        method(list);
    }

    public static <T> void method(List<List<T>> list) { }
}

...但这不能编译:

public class Main {

    public static void main(String[] args) {
        List<List<?>> list = null;
        method(list);
    }

    public static <T> void method(List<List<T>> list) { }
}

有人可以用简单的语言解释发生了什么吗?

Can someone explain what is going on in simple language?

推荐答案

要理解泛型类型的主要内容是它们不是协变的.

The main thing to understand with generic types, is that they aren't covariant.

所以虽然你可以这样做:

So whilst you can do this:

final String string = "string";
final Object object = string;

以下不会编译:

final List<String> strings = ...
final List<Object> objects = strings;

这是为了避免绕过泛型类型的情况:

This is to avoid the situations where you circumvent the generic types:

final List<String> strings = ...
final List<Object> objects = strings;
objects.add(1);
final String string = strings.get(0); <-- oops

所以，一个一个地浏览你的例子

So, going through your examples one by one

你的泛型方法需要一个List，你传入一个List；它(本质上)是一个 List