JLS如何与Sun javac对应/为什么它们不匹配

在Java中给出了这个：

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

你可以写

b = b + a;

但无法写入(给出编译错误)

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：

除= =之外,所有复合赋值运算符都要求两个操作数都是基本类型,如果左操作数是String类型,则允许右手操作数为任何类型.

但是在JLS第三版中,这个评论消失了,关于复合运算符的唯一说法是在15.26.2 Compound Assignment Operators：

形式E1 op = E2的复合赋值表达式等效于E1 =(T)((E1)op(E2)),其中T是E1的类型,除了E1仅被评估一次.

这似乎不起作用(见上文).

所以我的问题是 – javac和JLS之间究竟是什么关系,这个特殊的例子是javac中的错误还是JLS中的错误？

解决方法:

编译器错误是您的javac版本中的错误.作为pointed in prior answer,这个错误在Java 7中得到修复.

请参阅例如Bug ID 7058838在Sun bug数据库：

>说明：

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;
}

>州：
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.

有关背景信息,请参阅旧版Bug Id 4741726

>说明：

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的：编译
>发布修复：
7(b25) – 据我所知,这意味着修复了Java 7的构建25
>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

标签：java,javac,jls
来源： https://codeday.me/bug/20190530/1186014.html