Java泛型：类型不匹配：无法从整数转换为K. [英] Java generics : Type mismatch: cannot convert from Integer to K

问题描述

以下代码引发编译时异常：
$ b

类型不匹配：无法从整数转换为K

我的理解是 K 应处理任何延伸 Number 。

  public< K extends Number> K getValue（）{
 Integer a = new Integer（1）; 
 return a; //类型不匹配：无法从整数转换为K 
} 
  



< （注意：这只是一个测试代码，可以提供我的问题，而不会给出我实际场景的不相关细节。）

这里的问题是 K 是一个延伸 Number 的类型，但它是未知因为 Integer 只是一个特定的子类 code> Number ，所以它不能匹配任何潜在的目标类型。

假设你想明确地强制转换它使用

  public< K extends Number> K getValue（）{
 Integer a = new Integer（1）; 
 return（K）a; 
} 
  

然后，如果您打电话给期望 Number 除 Integer 以外，您将得到 ClassCastException ，这正是编译器想要的避免：

  Double d = getValue（）; <  -  throws ClassCastException 
  

解决方法是使用 Number

  public Number getValue（）{
 return new Integer （1）; 
} 
  

Following code is throwing compile time exception

Type mismatch: cannot convert from Integer to K

What I understand is K should handle any value that extends Number.

public <K extends Number> K getValue(){
    Integer a = new Integer(1);
    return a;//Type mismatch: cannot convert from Integer to K
}

(Note: This is just a test code to put my question without giving irrelevant details of my actual scenario.)

解决方案

The problem here is the fact that K is a type that extends Number but it is an unknown sub class of Number that is why the compiler raises this error because Integer is only one specific sub class of Number so it cannot match with any potential target types.

Let's say that you want to cast it explicitly to make it compile with

public <K extends Number> K getValue(){
    Integer a = new Integer(1);
    return (K) a;
}

Then if you call it expecting any sub class of Number except Integer you will get a ClassCastException, that is exactly what the compiler wants to avoid:

Double d = getValue(); <-- throws ClassCastException

As workaround you could define your method using Number as returned type as next:

public Number getValue() {
    return new Integer(1);
}

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