Convert unsigned byte to signed byte

Question

Is there an easy and elegant way to convert an unsigned byte value to a signed byte value in java? For example, if all I have is the int value 240 (in binary (24 bits + 1111

Answer 1

Here's how to store and convert an unsigned byte value:

byte b = (byte) 144; // binary value = 10010000

If you cast b to an int here's what happens:

int i = b;
System.out.println("value: "+i);  
System.out.println("binary: " + Integer.toBinaryString(i));

Output:

value: -112 <- Incorrect value
binary: 11111111111111111111111110010000 

Why does this happen? Bytes are signed in Java, and b is negative so the cast operation fills the resulting int with 1s from the left. More on java data formats.

So how do we get back from -112 to the correct value of 144? We need a 32-bit bitmask, which we can create with an int:

int mask = 0x000000FF; // This is often shortened to just 0xFF

Now we can use the bitwise & operator to "zero out" the left most 24 bits, leaving only our original 8 bits.

int unsignedValue = b & mask; // value = 144

Our original value of 144 has been restored and can be safely cast back to a byte:

byte original = (byte) unsignedValue;