Question

So I am sending some stuff over bluetooth, for this I am using byte arrays, one of the byte arrays I am using I need to change one of the values before I send it, using this…

private void sendIntensity(int I){
    intensity[3] = (byte) i;
    sendIntensity();
}

The original byte array is made up like this:

private byte[] intensity = new byte[]{58,0,42,0,10,13}

I am getting the intensity from an edit text, getting the text and using regex to find the intensity they want to use, I use

int option = Integer.parseInt(text);

To do that. Basically, any number under 128 works perfectly, I can see this because I log the values in the array, setting it to 127 will log 58, 0, 42, 127, 10, 13 but when I do 128 or over I get a weird set of data when I log it.

The max number I will be putting in there is 255.

When I log what is in the array when use intensity[3] = (byte) 128; I get back 50, 0, 42, -17, -66, -128, 10, 13

When I log what happens when I use 129 I get back 50, 0, 42, -17, -66, -127, 10, 13

What is happening? Why am I not just getting back 50, 0, 42, 128, 10, 13 like I should be?

Answer 1

That’s because byte range is [-128, 127], if you try to set a value larger than 127 in a byte, it will do a round trip to -128, -127 and so on.

From the SCJP 6 book:

byte a = (byte) 128;

(…) what’s the result? When you narrow a primitive, Java simply truncates
the higher-order bits that won’t fit. In other words, it loses all the bits to the left of
the bits you’re narrowing to.

More info:

• Java Primitive Data Types
• Java in a Nutshell: Primitive Data Types, check the Type Conversions section, the narrowing conversion part.