Reputation: 197

Converting to and from bytes in java

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?

Upvotes: 2

Answers (6)

Aillyn

Reputation: 23793

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

Upvotes: 3

SuperTron

Reputation: 4233

To convert integer number into a byte:

int original = 170;
byte b = (byte)(0xFF & original);

This will put 10101010 into the byteNum by "cutting off" anything outside the first byte. If you print that number, you will get a -86. To convert back do this:

int num = (int)b & 0xFF;

This converts the byte into an integer, and then masks off the signature bit, restoring the value of 170.

Upvotes: 1

Bhesh Gurung

Reputation: 51030

The byte primitive type has a minimum value of -128 and a maximum value of 127 (inclusive).

You are trying to assign 170 (int) which is out of range of byte by explicit casting, which results in the loss of information about the overall magnitude of a numeric value.

See here: Primitive Data Types

Also check this JLS section: 5.1.3 Narrowing Primitive Conversions

Upvotes: 1