CompletableFutureExample.java

//fetch something from A to make a lookup on B
ADao adao = new ADao();
CompletableFuture<A> aFutureLoaded = adao.findByIdAsync(123);
BDao bdao = new BDao();
aFutureLoaded.thenCompose(
  a->bdao.findBySomeAValueAsync(a.getSomeAValue());
).whenComplete((bResult,ex)->{
  if(ex==null){
    //do something with the result
  }else{
    //oops something went wrong
  }
});