无法从java.lang.Integer转换为R. [英] Cannot convert from java.lang.Integer to R

问题描述

我有以下代码：

  class inner {
 Integer i; 
 public Integer getValue（）{
 return i; 
} 
 public void setValue（Integer i）{
 this.i = i; 
} 
} 
 
 class outer {
 public static inner i1; 
 
 outer（整数i）{
 i1.setValue（i）; 
} 
} 
 
公共类MyClass {
 public void main（）{
 List< Integer> ll = Arrays.asList（new outer（2））。stream（）。map（outer.i1 :: getValue）.collect（Collectors.toList（））; 
 
} 
  

我收到以下错误：

 必填：功能<？超级对象，？扩展R>找到
：outer.i1 :: getValue 
原因：无法推断类型变量R 
（参数不匹配;无效方法引用
方法类内部的getValue方法无法应用给定类型
 required：无参数
 found：Object 
 reason：实际和形式参数列表的长度不同）
其中R，T是类型变量：
 R extends方法< R> map中声明的Object（函数<？super T，？extends R>）
  

我是溪流的新手，阅读文档并不能解决这个问题。任何帮助将不胜感激。

解决方案

getValue 是一种方法没有争论。
当您尝试将 getValue 的方法引用传递给 Stream 的 map 方法，您试图将 Stream 的元素传递给 getValue ，但是 getValue 不接受任何参数。

如果您愿意，可以用lambda表达式替换方法引用忽略流的外部元素：

  List< Integer> ll = Arrays.asList（new outer（2））。stream（）。map（o  - > outer.i1.getValue（））。collect（Collectors.toList（））; 
  

然而，这将导致 NullPointerException ，因为你没有在任何地方初始化 public static inner i1 ，所以调用外部构造函数会抛出该异常。 / p>

如果不知道你想要做什么，很难建议如何解决这个问题。

我不知道知道在外部类中有一个类型为 inner 的静态成员是否有意义，但如果有意义，你可能应该在静态初始化程序块（或其声明）中初始化它。

你可以，例如，改变

  public static inner i1; 
  

到

  public static inner i1 = new inner（）; 
  

这将消除异常。

I have the following code:

class inner {
    Integer i;
    public Integer getValue() {
        return i;
    }
    public void setValue(Integer i) {
        this.i = i;
    }
}

class outer {
    public static inner i1;

    outer(Integer i) {
        i1.setValue(i);
    }
}

public class MyClass{
public void main() {
    List<Integer> ll = Arrays.asList(new outer(2)).stream().map(outer.i1::getValue).collect(Collectors.toList());

}

I get the following error:

required: Function<? super Object,? extends R>
  found: outer.i1::getValue
  reason: cannot infer type-variable(s) R
    (argument mismatch; invalid method reference
      method getValue in class inner cannot be applied to given types
        required: no arguments
        found: Object
        reason: actual and formal argument lists differ in length)
  where R,T are type-variables:
    R extends Object declared in method <R>map(Function<? super T,? extends R>)

I am new to streams, and reading the docs doesn't clear this up for me. Any help will be appreciated.

解决方案

getValue is a method that takes no arguments. When you try to pass a method reference of getValue to Stream's map method, you are trying to pass the Stream's element to getValue, but getValue doesn't take any arguments.

You can replace the method reference with a lambda expression if you wish to ignore the outer element of the Stream:

List<Integer> ll = Arrays.asList(new outer(2)).stream().map(o -> outer.i1.getValue()).collect(Collectors.toList());

However, this will lead to a NullPointerException, since you don't initialize public static inner i1 anywhere, so invoking the outer constructor will throw that exception.

It's hard to suggest how to fix that without knowing what you are trying to do.

I don't know if it makes sense to have a static member of type inner in your outer class, but if it makes sense, you should probably initialize it in a static initializer block (or in its declaration).

You could, for example, change

public static inner i1;

to

public static inner i1 = new inner();

which will eliminate the exception.

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