java - Findbugs 警告 : Integer shift by 32 -- what does it mean?

Question

我正在使用 Findbugs 扫描第三方源代码(只是在集成到我的代码之前要小心)，并发现以下警告:

long a = b << 32 | c

Bug: Integer shift by 32 Pattern id: ICAST_BAD_SHIFT_AMOUNT, type: BSHIFT, category: CORRECTNESS

The code performs an integer shift by a constant amount outside the range 0..31. The effect of this is to use the lower 5 bits of the integer value to decide how much to shift by. This probably isn't want was expected, and it at least confusing.

谁能解释一下上面的意思吗？

谢谢! (我是Java编程的新手)

Answer 1

来自Java Language Specification :

If the promoted type of the left-hand operand is int, only the five lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x1f. The shift distance actually used is therefore always in the range 0 to 31, inclusive.

因此，如果 b 是 int，则表达式等同于

long a = b | c;

我非常怀疑这是什么意思。应该是这样的

long a = ((long) b << 32) | c;

(如果 b 已经是 long，则代码是正确的，FindBugs 对该 bug 的判断是错误的)。