Left Shift of byte

[John Ryan]

Hi all,

When a left shift is performed on a byte it is first promoted to an int. When you want the result of the shifted byte value is it true that you must discard the top three bytes of the int result?. Is this because the top three bytes are used only for representing the sign of a byte or a short vaue?

This statement is confusing me because I thought the sign of a byte/short/int was only represented by the right most bit?

Cheers,
John

[Stuart Gray]

The sign bit is the left-most bit, not the right-most.

[John Ryan]

Originally posted by John Ryan:
Hi all,

When a left shift is performed on a byte it is first promoted to an int. When you want the result of the shifted byte value is it true that you must discard the top three bytes of the int result?. Is this because the top three bytes are used only for representing the sign of a byte or a short vaue?

This statement is confusing me because I thought the sign of a byte/short/int was only represented by the right most bit?

Cheers,
John

Thanks Stuart, I meant to say left most bit. Is the statement regarding the top three bytes true??

[Edwin Dalorzo]

Well, you know bytes just have 8 bits. When you shift left the right operand of the shift operator [<<] is promoted to an integer (if it is not a integer already). The right operand is not taken into account for promotion, so it could be a long and would not affect the result.

Now, if the shifted value is assigned back to a byte the upper 16 bits will be discarded.

So, in the next example, when you move the 1 bit to the left, up to the 7th position, then you turn the byte into a negative number.

byte one = 1;
one <<= 7;
System.out.println(one); //output -255

Hope it helps.
[ June 18, 2005: Message edited by: Edwin Dalorzo ]

[John Ryan]

Originally posted by Edwin Dalorzo:


Now, if the shifted value is assigned back to a byte the upper 16 bits will be discarded.

byte one = 1;
one <<= 7;
System.out.println(one); //output -255

Hope it helps.

[ June 18, 2005: Message edited by: Edwin Dalorzo ]

Do you not mean that the upper 24 bytes will be discarded - given that a byte has only 8 bits (32-8 = 24) ??

[Edwin Dalorzo]

You are right, Mr. Ryan. I did the math wrong. Thanks for the correction.