Can someone explain why the following bitwise expressions return different results:
System.out.println((-1<<31)<<1); // it prints 0
System.out.println(-1<<32); // it prints -1
Asked
Active
Viewed 196 times
13
nenito
- 1,134
- 6
- 17
- 32
-
1Also http://stackoverflow.com/q/34193787/1743880 and http://stackoverflow.com/q/3170412/1743880 – Tunaki Mar 30 '16 at 09:50
-
@Tunaki: Yes, it is similar. – nenito Mar 30 '16 at 09:56
2 Answers
14
-1<<32 is equivalent to -1<<0, i.e. a no-op. The reason is that the shift distance (32) is AND-ed with 0x1f and 32 & 0x1f is 0.
This is defined in the JLS #15.19 (emphasis mine):
If the promoted type of the left-hand operand is int, then 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 (0b11111). The shift distance actually used is therefore always in the range 0 to 31, inclusive.
assylias
- 310,138
- 72
- 642
- 762
3
The shift count value is used modulo 32. So the second example is actually the same as shifting by 0.
Henry
- 41,816
- 6
- 58
- 77