泛型在 Eclipse 中编译和运行，但不在 javac 中编译 [英] Generics compiles and runs in Eclipse, but doesn't compile in javac

问题描述

注意:这是从 关于 null 的 Comparable 和 Comparator 契约

此代码在 Eclipse 中编译并运行良好 (20090920-1017)

This code compiles and runs fine in Eclipse (20090920-1017)

import java.util.*;
public class SortNull {
   static <T extends Comparable<? super T>>
   Comparator<T> nullComparableComparator() {
      return new Comparator<T>() {
         @Override public int compare(T el1, T el2) {
         return
            el1 == null ? -1 :
            el2 == null ? +1 :
            el1.compareTo(el2);
         }
      };
   }
   public static void main(String[] args) {
      List<Integer> numbers = new ArrayList<Integer>(
         Arrays.asList(3, 2, 1, null, null, 0)
      );
      Comparator<Integer> numbersComp = nullComparableComparator();
      Collections.sort(numbers, numbersComp);
      System.out.println(numbers);
      // "[null, null, 0, 1, 2, 3]"

      List<String> names = new ArrayList<String>(
         Arrays.asList("Bob", null, "Alice", "Carol")
      );
      Comparator<String> namesComp = nullComparableComparator();
      Collections.sort(names, namesComp);
      System.out.println(names);
      // "[null, Alice, Bob, Carol]"
   }
}

但它不能在 javac 1.6.0_17 上编译.这是错误消息:

And yet it doesn't compile on javac 1.6.0_17. This is the error message:

SortNull.java:17: incompatible types; no instance(s) of type variable(s) T exist
 so that java.util.Comparator<T> conforms
 to java.util.Comparator<java.lang.Integer>
found   : <T>java.util.Comparator<T>
required: java.util.Comparator<java.lang.Integer>
     Comparator<Integer> numbersComp = nullComparableComparator();

SortNull.java:25: incompatible types; no instance(s) of type variable(s) T exist
 so that java.util.Comparator<T> conforms
 to java.util.Comparator<java.lang.String>
found   : <T>java.util.Comparator<T>
required: java.util.Comparator<java.lang.String>
     Comparator<String> namesComp = nullComparableComparator();

2 errors

有人可以解释为什么会出现这种差异吗?这是一个错误吗?如果是这样，谁有错误?

Can someone explain why the discrepancy? Is this a bug? If so, who has the bug?

推荐答案

这是一个已确认的错误:错误 ID 6468354.以下是相关性的摘录:

This is a confirmed bug: Bug ID 6468354. Here's an extract of relevance:

这个问题是由于有时javac的实现JLS3 15.12.2.8 忽略递归边界，有时不会(如本例).当递归边界包含通配符时，在计算非推断类型变量时会包含此类边界.这使得后续的子类型 test (Integer <: Comparable<? super T> 其中 T 是要推断的类型变量).

This problem is caused by the fact that sometimes javac's implementation of JLS3 15.12.2.8 ignores recursive bounds, sometimes not (as in this case). When recursive bounds contains wildcards, such bounds are included when computing uninferred type variables. This makes subsequent subtyping test (Integer <: Comparable<? super T> where T is a type-variable to be inferred).

将在 6369605 之后修复

Will be fixed after 6369605

我在 WinXP 上也遇到过 1.6.0_13.嗯，我会坚持使用 Eclipse :)

Occured to me on WinXP with 1.6.0_13 as well. Ah well, I'll just stick using Eclipse :)

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