Java，Google Collections Library; AbstractIterator有问题吗？ [英] Java, Google Collections Library; problem with AbstractIterator?

问题描述

我正在使用Google Collections库 AbstractIterator 实现生成器。这样做我遇到了一个问题;我把它缩小为更基本的类型并重现了这个问题。这种减少对于它的作用显然有点过分，通过Iterable从1计数到数字。

I am using the Google Collections library AbstractIterator to implement a generator. I ran across a problem while doing so; I've reduced it to a more basic type and reproduced the problem. This reduction is obviously overkill for what it does, counting from 1 to numelements via an Iterable.

基本上在以下代码中，未注释的版本可以工作，而注释的版本可以not（最后提供一个null元素，而不是以最后一个数字结尾）。

Essentially in the following code, the uncommented version works, and the commented one does not (provides a null element last, instead of ending on the last number).

我做错了什么，或者这是库的问题？

Am I doing something wrong, or is this a problem with the library?

private Iterable<Integer> elementGenerator(final int numelements) {
  return new Iterable<Integer>() {
    @Override public Iterator<Integer> iterator() {
      return new AbstractIterator<Integer>(){
        int localcount=0;
        @Override protected Integer computeNext() {
          if (localcount++ == numelements) return endOfData();
          return localcount;
          // return (localcount++ == numelements) ? endOfData() : localcount;
        }
      };
    }
  };
}

我还尝试摆弄？：安排（例如，在返回前加上前缀并与+1比较），无济于事。我捅了一下寻找关于这个的文档，但没有找到任何东西。显然，？：语法只是方便，不是必需，但仍然......

I also tried fiddling around with the ?: arrangement (e.g., prefixing the return and comparing to +1 instead), to no avail. I poked around a bit looking for documentation about this, but didn't find anything. Obviously, the ?: syntax is only a convenience, not a necessity, but still...

推荐答案

由于使用了具有不同数值类型的三元运算符条件表达式，因此得到 NullPointerException 。在三元表达式中混合不同类型的数值时，Java有复杂的规则： JLS第15.25节 。

You get a NullPointerException because of the use of the ternary operator, conditional expression, with different numerical types. Java has complex rules when mixing numerical values of different types in ternary expression: JLS Section 15.25.

鉴于 endOfData（）之前返回 Integer ，而 localcount 是 int ，Java取消装箱 endOfData（）的值。但是，假设 endOfData（）返回null，则取消装箱操作会导致空指针异常。

Given that endOfData() is preceived to return Integer, while localcount is an int, Java unboxes the value of endOfData(). However, given that endOfData() returns a null, the unboxing operation results into a null pointer exception.

你可以继续使用if语句，也可以将localcount声明为 Integer 。

You can either continue using the if statement, or declare localcount as Integer.

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