Writing asynchronous code with CompletableFuture

1. https://stackoverflow.com/questions/27723546/completablefuture-supplyasync-and-thenapply
2. http://www.deadcoderising.com/java8-writing-asynchronous-code-with-completablefuture/
3. https://blog.krecan.net/2013/12/25/completablefutures-why-to-use-async-methods/