java - JLS如何对应Sun javac/为什么不匹配

Question

在 Java 中给出这个:

String a = "str";
CharSequence b = "charseq";

你可以写

b = b + a;

但无法写入(给出编译器错误)

b += a;

错误是

incompatible types
found   : java.lang.CharSequence
required: java.lang.String

现在在 JLS 第二版中，这可以通过 15.26.2 Compound Assignment Operators 中的这一行来解释:

所有复合赋值运算符都要求两个操作数都是基本类型，但 += 除外，它允许如果左侧操作数是字符串类型，则右侧操作数可以是任何类型。

但是在 JLS 第三版中这个注释消失了，关于复合运算符的唯一说法是在 15.26.2 Compound Assignment Operators :

E1 op= E2 形式的复合赋值表达式等同于 E1 = (T)((E1) op (E2))，其中 T 是 E1 的类型，只是 E1 仅计算一次.

这似乎不起作用(见上文)。

所以我的问题是 - javac 和 JLS 之间到底是什么关系，这个特定示例是 javac 中的错误还是 JLS 中的错误？

Answer 1

编译器错误 是您的 javac 版本中的错误。作为pointed in prior answer此错误已在 Java 7 中修复。

参见例如 Bug ID 7058838在 Sun 错误数据库:

• 描述:

  A following function cannot be compiled in java 1.6 or less,. but it can be compiled in java 1.7.
  

  public static void main(String[] args) {
         Object x = "x";
         String y = "y";
         x += i;
  }
  

• 状态:
  不是缺陷
• 评价:

  For an Object x and a String y, x+=y is just x=(Object)(x+y). Since y is a String, x undergoes string conversion to produce a string which is concatenated with y, before the no-op cast to Object. The JLS has not changed in this area between SE 6 and SE 7; the program should have been legal for many years.

---

有关背景，另请参见旧版 Bug Id 4741726

• 描述:

  javac used to allow expressions of the form o += s where o is a variable of type Object and s is an expression of type String. We fixed that recently (4642850) and this caused a build failure (4741702). Perhaps this is common enough that we should relax the spec instead of fixing the compiler?

• 类别:
  java:编译器
• 固定发布:
  7(b25) - 据我了解，这意味着已在 Java 7 的 build 25 中修复
• 评价:

  I'm inclined to relax the spec, though we'd have to know what other implementations do before making a final call on this.
  2002-09-04
  JLS3 permits Object+=String because the '+' means string concatenation and that is able to concatenate an Object with a String as easily as a String with an Object.
  2008-01-31