Java 字节数组中的无符号字节

Question

This may seem like an easy question, but I'm so confused.

byte[] bArray = new byte[]{(byte) (0x80 & 0xff)};
System.out.println(bArray[0]);

and my output is -128. Why? How can I rewrite this so that the byte array contains 128? I thought that the 0xff made it unsigned. Thanks for your help!

Answer 1

I thought that the 0xff made it unsigned.

Well, it does, sort of - it promotes it to an int , and keeps just the last 8 bits.

However, you're then casting the result of that back to a byte , and a byte in Java is always signed. (It's annoying, but it's the way it is.) So you might as well just have written:

byte[] bArray = { (byte) 0x80 };

The result would be exactly the same.

You're storing the bit pattern you want to store (10000000) so if you're transmitting this elsewhere, you don't need to worry... it's just when you're viewing it as a byte in Java that it's annoying.

Answer 2

Value range of byte according Java Language Specification 4.2.1

For byte, from -128 to 127, inclusive

so there is no way (in Java) that a byte will hold (byte) 128.

You can cast the byte to an int and apply & 0xff to get the unsigned representation of the byte (as an int). But if you cast it back to an byte it will again be interpreted as being a signed value between -128 and 127...

If you are only concerned with printing:

System.out.println(bArray[0] & 0xff);

or, for hexadecimal

System.out.printf("0x%02x\n", bArray[0]);

Answer 3

My answer is attached in the big picture