I am a merchant that accepts bitcoins, and I'm building my own desktop client for receiving them in Java.
When I receive a transaction, I would like to be able to see in my client how much bitcoin is stored in the address that I am receiving from.
I have the full blockchain downloaded on my computer.
How can I access the balance of an address from the blockchain? I would like to be able to do this offline by accessing the blockchain stored on my hard drive.
Thanks in advance
The concept of balance in Bitcoin does not exist. It is based on the number of outputs you can redeem AND that have not been reedimed yet. Bitcoind provides RPC APIs to get this information provided that it has indexed the transactions in a way that it is easily accessible.
If you have the full blockchain, I suppose you are using bitcoind. If so, you might add watch-only addresses to bitcoind and then reindex the blockchain.
You can add a watch-only address using 'importAddress' RPC.