promise_old.java

String output = "value";
ListenableFuture<String> future = Futures.immediateFuture(output);

ListenableFuture<String> firstTransformedFuture = Futures.transform(
    future, (Function<String, String>) value -> value + "_a"
);

ListenableFuture<String> secondTransformedFuture = Futures.transform(
    firstTransformedFuture, (Function<String, String>)  value -> value + "_b"
);

String transformedOutput = secondTransformedFuture.get();