java - 为什么 =+ 不会导致编译错误？

Question

遇到有人在他们的代码中错误地使用 =+ 而不是 += 并且它没有显示为编译错误。

这是因为

int a =+ 2;

与

相同

int a = 0 + 2;

?

Answer 1

没有编译错误，因为 + 是一个有效的(尽管相当无用)一元运算符，就像 - 一样:

int x = +1;
int y = -1;

Java 语言规范中的相关部分是Unary Plus Operator + (§15.15.3 ) .它指定调用一元 + 操作会导致 Unary Numeric Promotion (§5.6.1)的操作数。这意味着:

• If the operand is of compile-time type Byte, Short, Character, or Integer, it is subjected to unboxing conversion (§5.1.8). The result is then promoted to a value of type int by a widening primitive conversion (§5.1.2) or an identity conversion (§5.1.1).

• Otherwise, if the operand is of compile-time type Long, Float, or Double, it is subjected to unboxing conversion (§5.1.8).

• Otherwise, if the operand is of compile-time type byte, short, or char, it is promoted to a value of type int by a widening primitive conversion (§5.1.2).

• Otherwise, a unary numeric operand remains as is and is not converted.

In any case, value set conversion (§5.1.13) is then applied.

简而言之，这意味着

1. 数字原始包装类型未装箱，并且；
2. 小于 int 的整数类型加宽为 int。