[英]Differing behaviour when shifting bits in java
I'm working with bit shifting in Java and have the following piece of code that works as expected: 我正在使用Java中的位移,并使下面的代码按预期工作:
final byte value = 1;
final int shift = 1;
byte result = value << shift;
This produces the value 2
as expected. 这会产生预期的值
2
。 If however I attempt to extract this into a method like so: 但是,如果我尝试将其提取到如下方法中:
private void shiftAndCheck(final byte value, final int shift) {
byte result = value << shift;
}
This results in a compilation error: 这会导致编译错误:
java: incompatible types: possible lossy conversion from int to byte
The question is what is it about the method that causes this to fail? 问题是导致失败的方法是什么?
Since the value
and shift
are compile-time constants in this snippet: 由于
value
和shift
是此代码段中的编译时常量:
final byte value = 1;
final int shift = 1;
byte result = value << shift;
then the compiler inlines their values (replaces all the occurrences of value
and shift
with their actual values of 1
) and can verify before Runtime that the result of value << shift
won't cause any loss of precision. 然后编译器内联它们的值(替换所有出现的
value
并使用它们的实际值1
shift
)并且可以在运行时之前验证value << shift
的结果不会导致任何精度损失。
Meanwhile, for the second snippet: 同时,对于第二个片段:
private void shiftAndCheck(final byte value, final int shift) {
byte result = value << shift;
}
the compiler has no evidence that shift
will represent such value, which wouldn't cause loss of precision and that's why raises a compilation error. 编译器没有证据表明
shift
代表这样的值,这不会导致精度损失,这就是为什么会引发编译错误。
If compilation was possible in this case, then you'd have been able to do shiftAndCheck(1, 32);
如果在这种情况下可以编译,那么你就可以做
shiftAndCheck(1, 32);
which would result in overflowing the byte
type. 这会导致
byte
类型溢出。
默认情况下,对非浮点数的算术运算会产生一个int
结果
private void shiftAndCheck(final byte value, final int shift) {
byte result = (byte) (value << shift);
}
As @Konstantin already said it returns int
, so either cast it into byte or use int result
. 由于@Konstantin已经说它返回
int
,所以要么将其转换为byte,要么使用int result
。
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