BitCoinJ is a Java implementation of the native Bitcoin P2P protocol, which allows it to maintain a wallet and send/receive transactions without needing a local copy of the official implementation. It comes with full documentation and some example apps showing how to use the library.

The project aims to be easier to understand than the C++ implementation, and be suitable for usage on constrained devices such as mobile phones. There is a C# port of the library, BitCoinSharp, which is suitable for usage on .NET based platforms.

BitCoinJ implements the "simplified payment verification" mode of Satoshis paper. It does not store a full copy of the block chain, rather, it stores what it needs in order to verify transactions with the aid of an untrusted peer node.

Download the library and then read:

• Getting started
• Depending on BitCoinJ with Maven
• How to make your wallet app start faster for new users
• The JavaDocs

There are also some more in depth articles covering various topics:

• Understanding the BitCoinJ security model
• Working with BitCoinJ - basic patterns and things you should know
• Working with transactions
• Working with the wallet

Got questions? Comments? Patches? Talk to us on the mailing list. We have a continuous Jenkins build running at ci.bitcoinj.org. You can quickly reach this website via bitcoinj.org. You can read the release notes for each release starting from 0.3.

Be warned: This software is at an early development stage. It is not safe to use with any serious quantity of money on the production network. You risk losing your coins, or creating spends that will never confirm.