How to Convert Int to Unsigned Byte and Back

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

Except char, every other numerical data type in Java are signed.

As said in a previous answer, you can get the unsigned value by performing an and operation with 0xFF. In this answer, I'm going to explain how it happens.

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

int i2 = b & 0xFF;      
// This is like casting b to int and perform and operation with 0xFF

System.out.println(i2); // 234

If your machine is 32-bit, then the int data type needs 32-bits to store values. byte needs only 8-bits.

The int variable i is represented in the memory as follows (as a 32-bit integer).

0{24}11101010

Then the byte variable b is represented as:

11101010

As bytes are signed, this value represent -22. (Search for 2's complement to learn more about how to represent negative integers in memory)

Then if you cast is to int it will still be -22 because casting preserves the sign of a number.

1{24}11101010

The the casted 32-bit value of b perform and operation with 0xFF.

 1{24}11101010 & 0{24}11111111
=0{24}11101010

Then you get 234 as the answer.

If you just need to convert an expected 8-bit value from a signed int to an unsigned value, you can use simple bit shifting:

int signed = -119;  // 11111111 11111111 11111111 10001001

/**
 * Use unsigned right shift operator to drop unset bits in positions 8-31
 */
int psuedoUnsigned = (signed << 24) >>> 24;  // 00000000 00000000 00000000 10001001 -> 137 base 10

/** 
 * Convert back to signed by using the sign-extension properties of the right shift operator
 */
int backToSigned = (psuedoUnsigned << 24) >> 24; // back to original bit pattern

http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op3.html

If using something other than int as the base type, you'll obviously need to adjust the shift amount: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

Also, bear in mind that you can't use byte type, doing so will result in a signed value as mentioned by other answerers. The smallest primitive type you could use to represent an 8-bit unsigned value would be a short.

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.