Last days , I am trying to work on blockchain and some mining things but resources are limited. Blockchain and mining are not too much the same things as you know.
What a mining things does is request a work request from a mining pool or a wallet with rpc call , receive json resposne for the work , and do that work in users computer and submit the found shares to pool ( or try to find the whole solution to mine a block with wallet ) .
To be able to use GPU power ( which is a necessity ) , nvidia cuda or amd programming is also needed as I know.
For example to calculate the solution of an equihash algorithm ( which is the best algo for nvidia and gpu ) blake2s lib is used.
For example for lite coin mining there is a miner for java (
https://github.com/pooler/JMiner) but it is not valuable as it is not possible to mine scrypt algo with gpu.
People has to concentrate on equihash algo which is based on blake2s if they want to create a valuable miner.
Do you have a specific Bitcoin Java library you would like to use in B4J?
In the meantime, you might look at this thread:
https://b4x.com/android/forum/threads/bitcoin.87783/