Ta4j is an open-source Javalibrary for technical analysis. It provides the basic components for the creation, evaluation, and execution of trading strategies. Ta4j is available on Maven Central. You can also download example code from the maven central repository. The wiki is the best place to start learning about ta4j. For more detailed questions, please use the issues tracker.
Lightweight Java and Android library for integration with Ethereum
Web3j is a lightweight, highly modular, reactive, type safe Java and Android library for working with Smart Contracts and integrating with Ethereum blockchains. This allows you to work with Ethereum blockchains, without the additional overhead of having to write your own integration code for the platform. Web3j integrates seamlessly with the Epirus Blockchain Explorer. Web3j provides full type-safe access to your blockchain applications.
bitcoinj is a library for working with the Bitcoin protocol. It can maintain a wallet, send/receive transactions without needing a local copy of Bitcoin Core and has many other advanced features. It's implemented in Java but can be used from any JVM compatible language, examples in Python and JavaScript are included. Highly optimized lightweight simplified payment verification (SPV) mode.