CompletableFuture chaining results

Question

I am trying to chain the calls/results of the methods to the next call. I get compile time error methodE because if am not able to get the reference of objB from the previou

Answer 1

You should use thenCompose, which is an asynchronous mapping, as opposed to thenApply, which is synchronous. Here's an example that chains two future-returning functions:

public CompletableFuture<String> getStringAsync() {
    return this.getIntegerAsync().thenCompose(intValue -> {
        return this.getStringAsync(intValue);
    });
}

public CompletableFuture<Integer> getIntegerAsync() {
    return CompletableFuture.completedFuture(Integer.valueOf(1));
}

public CompletableFuture<String> getStringAsync(Integer intValue) {
    return CompletableFuture.completedFuture(String.valueOf(intValue));
}

With thenApply you don't return a future. With thenCompose, you do.

Answer 2

You could store intermediate CompletableFuture in a variable and then use thenCombine:

CompletableFuture<ClassA> futureA = CompletableFuture.supplyAsync(...)
    .thenApply(...)
    .thenApply(...);

CompletableFuture<ClassB> futureB = futureA.thenApply(...);

CompletableFuture<ClassC> futureC = futureA.thenCombine(futureB, service::methodE);

objC = futureC.join();