How to Convert Int to Unsigned Byte and Back

Question

I need to convert a number into an unsigned byte. The number is always less than or equal to 255, and so it will fit in one byte.

I also need to convert that byte back into that number. How would I do that in Java? I've tried several ways and none work. Here's what I'm trying to do now:

int size = 5; // Convert size int to binary String sizeStr = Integer.toString(size); byte binaryByte = Byte.valueOf(sizeStr); 

and now to convert that byte back into the number:

Byte test = new Byte(binaryByte); int msgSize = test.intValue(); 

Clearly, this does not work. For some reason, it always converts the number into 65. Any suggestions?

Answer 1

A byte is always signed in Java. You may get its unsigned value by binary-anding it with 0xFF, though:

int i = 234; byte b = (byte) i; System.out.println(b); // -22 int i2 = b & 0xFF; System.out.println(i2); // 234 

Answer 2

Java 8 provides Byte.toUnsignedInt to convert byte to int by unsigned conversion. In Oracle's JDK this is simply implemented as return ((int) x) & 0xff; because HotSpot already understands how to optimize this pattern, but it could be intrinsified on other VMs. More importantly, no prior knowledge is needed to understand what a call to toUnsignedInt(foo) does.

In total, Java 8 provides methods to convert byte and short to unsigned int and long, and int to unsigned long. A method to convert byte to unsigned short was deliberately omitted because the JVM only provides arithmetic on int and long anyway.

To convert an int back to a byte, just use a cast: (byte)someInt. The resulting narrowing primitive conversion will discard all but the last 8 bits.