Promises for Android?

There is something pretty similar already available as part of the Java language, and supported by Android: java.util.concurrent.Future. Perhaps it is good enough for your needs.

By the way, Java 8, which Android does not yet support, has a variant called CompletableFuture that is even closer to a Promise.

As of 2020, android has support for CompletableFuture, Java's answer to Javascript promises: https://developer.android.com/reference/java/util/concurrent/CompletableFuture

If that is not possible for your app's android api level, then see https://github.com/retrostreams/android-retrofuture .

Example:

CompletableFuture.supplyAsync(()->{
            String result = somebackgroundFunction();
            return result;
        }).thenAcceptAsync(theResult->{
            //process the result
        }).exceptionallyCompose(error->{
            ///process the error
            return CompletableFuture.failedFuture(error);
        });

To handle the result and update the UI, you need to specify main thread executor:

        CompletableFuture.supplyAsync(()->{
            String result = somebackgroundFunction();
            return result;
        }).thenAcceptAsync(theResult->{
            //process the result
        }, ContextCompat.getMainExecutor(context))
                .exceptionallyComposeAsync(error->{
            ///process the error
            return CompletableFuture.failedFuture(error);
        }, ContextCompat.getMainExecutor(context));