无法使用泛型转换为不特定的嵌套类型 [英] Can't cast to to unspecific nested type with generics

问题描述

我有两个带有嵌套泛型的类.有没有办法摆脱

I have two classes with nested generics. Is there a way to get rid of the

类型不匹配:无法从 Msg> 转换为 Msg> 错误?在上次作业中

Type mismatch: cannot convert from Msg<Value<String>> to Msg<Value<?>> error ? In the last assignment

public class Value<V> {
    V   val;

    public Value(V val) {
        this.val = val;
    }
    @Override
    public String toString() {
        return "" + val;
    }
}

public class Msg<T> {

    T holder;

    public Msg( T holder) {
        this.holder = holder ;
    }
    public String toString() {
        return "" + holder;
    }

    public static void main(String[] args) {
        Msg<Value<String>>strMsg = new Msg(new Value<String>("abc"));
        // This is OK
        Msg<?>objMsg = strMsg;
        // Type mismatch: cannot convert from Msg<Value<String>> to Msg<Value<?>>   
        Msg<Value<?>>objMsg = strMsg;
    }
}

推荐答案

使用以下内容:

Msg<? extends Value<?>> someMsg = strMsg;

问题是Msg>中的?objMsg 不 能够捕获转换.它不是some类型的Value的Msg.它是Value的Msg任何类型的代码>".

The problem is that the ? in Msg<Value<?>> objMsg is NOT capable of capture conversion. It's not "a Msg of Value of some type. It's "a Msg of Value of ANY type".

这也解释了为什么随着声明的更改，我还将变量重命名为 someMsg.Value 不能只是任何 Object.它必须属于 some 类型(在本例中为 String).

This also explains why along with the declaration change, I've also renamed the variable to someMsg. The Value can't just be any Object. It must belong to some type (String in this example).

让我们考虑一个更通用的 List> 示例.与原始场景类似，List 可以NOT捕获转换List>.

Let's consider a more generic example of a List<List<?>>. Analogously to the original scenario, a List<List<?>> can NOT capture-convert a List<List<Integer>>.

    List<List<Integer>> lolInt = null;

    List<List<?>> lolAnything = lolInt;         // DOES NOT COMPILE!!!
    // a list of "lists of anything"

    List<? extends List<?>> lolSomething = lolInt;   // compiles fine!
    // a list of "lists of something"

这是另一种看待它的方式:

Here's another way to look at it:

• Java 泛型是类型不变的
• 有一个从 Integer 到 Number 的转换strong>，但是 List<Integer> 不是 列表<数字>
  • 类似地，List 可以通过 List，但是一个List<List> 不是 List<List>
  • Java generics is type invariant
  • There's a conversion from Integer to Number, but a List<Integer> is not a List<Number>
    • Similarly, a List<Integer> can be capture-converted by a List<?>, but a List<List<Integer>> is not a List<List<?>>
    • 同样，ListList> 可以捕获转换一个 List<List>
    • Similarly, a List<? extends List<?>> can capture-convert a List<List<Integer>>

    某些 ? 可以捕获而其他人不能捕获的事实也解释了以下代码段:

    The fact that some ? can capture and others can't also explains the following snippet:

        List<List<?>> lolAnything = new ArrayList<List<?>>(); // compiles fine!
    
        List<?> listSomething = new ArrayList<?>(); // DOES NOT COMPILE!!!
            // cannot instantiate wildcard type with new!
    

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