Java MSB negative value

Question

If I understand it right, in java you cannot define a value as unsigned int. So the MSB of a Byte will decide, if the value is positive or negative.

But I have to use all 8 Bits of the Byte for the value of the variable.

Does anyone have experience with this, and how is it possible still to send 8 Bit variables?

With the MCP everything is working fine. But it should work the same trough a mobile device.

Is it defined by the BLE protocol, that I must send bytes?

Could I also setup a characteristic to send and recieve floats or double?

What other opportunities do I have?

I tried it to mask with '& 0xFF'

links[1] = (byte) (i & (0xFF));

links[2] = (byte) ((i >> 8) & (0xFF));

links[3] = (byte) ((i >> 16) & (0xFF));

As example I take i = 201600

What I get:

links[1] = -128 (should be 0x80) <---Result is false

links[2] = 19 (should be 0x13) <---Result is right

links[3] = 3 (should be 0x03) <---Result is right

Did I wrong mask, or why its still negative and wrong value?

Eric Stutzenberger · Accepted Answer

I'm not sure I fully understand your question so I'll point out a few things I've learned in dealing with BLE characteristic data for Android: 
 
 
 Java, as you said, does not have the concept of unsigned anything which, for us embedded guys, is extremely annoying. However, from a BLE perspective, you simply have to consider the fact that a negative value interpreted by Java requires 8 bits. It is simply the MSb that makes it negative or positive. As long as all of the bits are there, then the BLE APIs will transfer the correct value. Nothing in the Android BLE API strips off this 'negative marker bit'. 
 
 
 If you need to convert BLE characteristic values of say, 4 bytes, to some other value, then you'll need to do some casting to make sure the value ends up being correct. Here is a code snippet for how I handle this: 
 int mTotalBytesSent = ((value[1] & 0xFF) + ((value[2] & 0xFF) << 8) + ((value[3] & 0xFF) << 16) + ((value[4] & 0xFF) << 24)); 
 
 
 The important part in this is the '& 0xFF' mask of the value. I initially did this operation without the mask and received some very strange values. 
 
 If you don't want to do it the manual way, Android provides some conversion functions to convert a characteristic's bytes into certain types of values. There are functions such as getIntValue, getFloatValue, and getStringValue. All of these take an offset parameter and the number methods also required a type where you can specify types such as FORMAT_UINT8, FORMAT_UINT32, etc. 
 
 For example: 
 int val = someChar.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT32, 0);
 
 Hope this helps, 
 Eric

Java MSB negative value

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