Question

I am having trouble understanding converting to and from bytes, or bytes in general

byte b = (byte)170;
System.out.println("Byte to int" + (int)b);

why is it that when I cast 170 to a byte then cast that byte to an int I get -86 and what is the proper way to do this?

EDIT: Okay so the answer to my question was really simple (byte ranges from -128 to 127)…
Where I was getting confused is that I really want is binary 10101010 to be in the byte so I figured represent that number in int and put that in the byte. Can someone please tell me how I can get that binary number into a byte?

Answer 1

The binary representation of 170 as a 32-bit integer is

0000 0000 0000 0000 0000 0000 1010 1010 

When you cast that to a byte, it becomes 1010 1010. Since all integers in Java are signed, it sees 1010 1010 as a 2’s complement 8-bit integer. That’s -86.

When you convert this byte back to integer using (int) b, it gets signed extended to:

1111 1111 1111 1111 1111 1111 1010 1010 

Which is also -86. You want to apply a bit mask to it, so it becomes 170 again.

(int) (0xff & b) will give you 170 back:

1111 1111 1111 1111 1111 1111 1010 1010 
0000 0000 0000 0000 0000 0000 1111 1111 & (bitwise and)
---------------------------------------
0000 0000 0000 0000 0000 0000 1010 1010