15.18 Shift Operators

What, I say, is to become of those wretches?
. . . What more can you say to them than "shift for yourselves?"
--Thomas Paine, The American Crisis (1780)

The shift operators include left shift <<, signed right shift >>, and unsigned right shift >>>; they are syntactically left-associative (they group left-to-right). The left- hand operand of a shift operator is the value to be shifted; the right-hand operand specifies the shift distance.

ShiftExpression:

	AdditiveExpression

	ShiftExpression << AdditiveExpression

	ShiftExpression >> AdditiveExpression

	ShiftExpression >>> AdditiveExpression

The type of each of the operands of a shift operator must be a primitive integral type, or a compile-time error occurs. Binary numeric promotion (§5.6.2) is not performed on the operands; rather, unary numeric promotion (§5.6.1) is performed on each operand separately. The type of the shift expression is the promoted type of the left-hand operand.

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 & (§15.21.1) with the mask value 0x1f. 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 & (§15.21.1) with the mask value 0x3f. The shift distance actually used is therefore always in the range 0 to 63, inclusive.

At run time, shift operations are performed on the two's complement integer representation of the value of the left operand.

The value of n<<s is n left-shifted s bit positions; this is equivalent (even if overflow occurs) to multiplication by two to the power s.

The value of n>>s is n right-shifted s bit positions with sign-extension. The resulting value is . For nonnegative values of n, this is equivalent to truncating integer division, as computed by the integer division operator /, by two to the power s.

The value of n>>>s is n right-shifted s bit positions with zero-extension. If n is positive, then the result is the same as that of n>>s; if n is negative, the result is equal to that of the expression (n>>s)+(2<<~s) if the type of the left-hand operand is int, and to the result of the expression (n>>s)+(2L<<~s) if the type of the left-hand operand is long. The added term (2<<~s) or (2L<<~s) cancels out the propagated sign bit. (Note that, because of the implicit masking of the right-hand operand of a shift operator, ~s as a shift distance is equivalent to 31-s when shifting an int value and to 63-s when shifting a long value.)