Shift amounts outside the valid range may produce unexpected resultsJAVA-E0399

The code performs a shift of an int or long by a constant amount outside the acceptable range. This could potentially cause overflow or other similar errors and is at best very confusing.

The Java Language Specification, Section 15.19 has the following to say on this matter:

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.

If the promoted type of the left-hand operand is long, then only the six 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 0x3f (0b111111). The shift distance actually used is therefore always in the range 0 to 63, inclusive.

Bad Practice

Consider the following shift operations on the int value a, and the long value b.

int a = 2;

a << 40 == a << 8 // true, 40 % 32 = 8

a << 32 == a // true, 32 % 32 = 0

long b = 2;

b << 72 == b << 8 // true, 72 % 64 = 8

b << 64 == b // true, 64 % 64 = 0

Recommended

The absolute shift amounts for int and long values must always be one less than the number of bits in their representation. That is, 64-bit values can only have shift amounts in the range -63, 63 while 32-bit values can only have shift amounts in the range of -31, 31.

References

• SpotBugs - ICAST_BAD_SHIFT_AMOUNT
• Java SE 11 JLS Section 15.19 - Shift Operators