Java convert string hex values to byte[], recreati

Question

I am making an app that communicates with a specific Bluetooth Low Energy device. It requires a specific handshake and this is all working perfectly in Objective-C for iOS, however, I am having trouble recreating this functionality in Java

Any thoughts greatly appreciated!

WORKING Objective-C code:

uint8_t bytes[] = {0x04,0x08,0x0F,0x66,0x99,0x41,0x52,0x43,0x55,0xAA};
NSData *data = [NSData dataWithBytes:bytes length:sizeof(bytes)];
[_btDevice writeValue:data forCharacteristic:_dataCommsCharacteristic type:CBCharacteristicWriteWithResponse];

So far for android I have the following as an equivalent:

byte[] handshake = {0x04,0x08,0x0F,0x66,(byte)0x99,0x41,0x52,0x43,0x55,(byte)0xAA};

characteristic.setValue(handshake);
boolean writeStatus = gatt.writeCharacteristic(characteristic);
Log.d(TAG,"Handshake sent: " + writeStatus);

As mentioned, iOS works great, but the equivalent in Java is getting no response from the device, leading me to think that the data being sent is wrong/not recognised

UPDATE So, after plenty of wrestling with this I have a little more insight into what is going on 'I think!'

As Scary Wombat mentioned below the maximum value of an int is 127 so the 2 values in the array of 0x99 and 0xAA are of course out of this range

The below is where I am at with the values:

byte bytes[] = {0x04,0x08,0x0F,0x66,(byte)0x99,0x41,0x52,0x43,0x55,(byte)0xAA};
Log.d(TAG, Arrays.toString(bytes));

Produces

[4, 8, 15, 102, -103, 65, 82, 67, 85, -86]

However the expected values need to be

[4, 8, 15, 102, 153, 65, 82, 67, 85, 170]

I have tried casting these troublesome bytes implicitly and also tried the below below:

byte bytes[] = {0x04,0x08,0x0F,0x66,(byte)(0x99 & 0xFF),0x41,0x52,0x43,0x55,(byte)(0xAA & 0xFF)};

However the resulting values in the array are always the same.

Please help!!! :)

UPDATE 2

After a day of digging it appears that although the values are logging incorrectly the values perceived by the Bluetooth device SHOULD still be correct, so I have modified this question and continuing over here

Answer 1

String handshakeString = "0x04,0x08,0x0F,0x66,0x99,0x41,0x52,0x43,0x55,0xAA";
byte[] value = handshakeString.getBytes();

will also account the , and so creates to much bytesand will not geht the same bytes AS in your c code.

Try to use a byte[] directly.

byte[] value=new byte[]{0x04,0x08,0x0F,0x66,0x99,0x41,0x52,0x43,0x55,0xAA};

Answer 2

Why are you not doing it the same way as for C

In this code

String handshakeString = "0x04,0x08,0x0F,0x66,0x99,0x41,0x52,0x43,0x55,0xAA";
byte[] value = handshakeString.getBytes();

this is just making a text String where the first char is 0 and the second is x etc

try

byte arr[] = {0x04,0x08,0x0F,0x66,0x99,0x41,0x52,0x43,0x55,0xAA};

edit

You maybe need to reconsider values such as 0x99 as in java the byte values are as per javadocs

It has a minimum value of -128 and a maximum value of 127 (inclusive).

See Can we make unsigned byte in Java