CODEWITHSUNDEEP
java
Lisa Anne
Lisa Anne

Reputation: 4595

Java read/write: what am I doing wrong here?

This is my code snippet:

byte value = (byte) rand.nextInt();
TX.write(value);                    
int read = RX.read() & 0xFF;

The hardware I am connected gives me back on RX what I write on TX. That works OK as long as I am writing positive numbers, BUT if I write a negative number what i get back does not match with what I wrote....

Please what am I missing?

EDIT: example output

Write: 66
Read: 66
OK AS EXPECTED

Write: -44
Read: 212
???

Write: -121
Read: 135

Write: -51
Read: 205
???

Write: -4
Read: 252
???

Upvotes: 3

Views: 119

Answers (4)

logee
logee

Reputation: 5077

It's because 0xFF is by default an int which is 32 bits. Take -51 for example. In binary it is represented using 2's complement:

You are doing this:

                             11001101
& 00000000 00000000 00000000 FFFFFFFF

which gives 

00000000 00000000 00000000 11001101

= 205

What you want is

  11001101
& FFFFFFFF

so you should do 

((byte) RX.read()) & ((byte) 0xFF)

See example:

public static void main(String[] args) {
    int negative = -51;
    System.out.println((int) (negative & (byte) 0xFF)); // = -51
    System.out.println((int) (negative & 0xFF)); // = 205
}

Upvotes: 1

M. Shaw
M. Shaw

Reputation: 1742

It seems that casting int to byte will represent the int as a signed int using two's complement (for one byte the range would be [10000000] -128 to 127 [01111111]; note that -1 is 11111111 and 0 is 00000000). However RX.read() treats the byte as an unsigned int (range from 0 [00000000] to 255 [11111111]).

If you only use one byte you can use:

int r = RX.read() & 0xFF
int read = -(255 - r) if r > 128 else r

Upvotes: 1

Harish
Harish

Reputation: 269

The BYTE is of 8bit out of which 1 is reserved for sign so max its range is from -128 to 127 so when you to the & 0xFF with the negative sign you will get the unsigned value of the range. hence when u r doing -4 it is giving 252 if u do the -1 then it will give 255 and so on.

Also in your code you can simple use int casting as it works at my end.. example:-

   byte b = -14;
    int i = (int)b;

Upvotes: 1

Eran
Eran

Reputation: 393996

If you write a negative byte and then you read it and assign it into an int using RX.read() & 0xFF, you will get a positive number, since the sign bit of the int will be 0.

Try 

int read = RX.read();

Upvotes: 2

Related Questions