ConditionalFuture.java

package com.benjiweber;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;

public class ConditionalFuture {

    public static void main(String... args) throws ExecutionException, InterruptedException {
        var i = new AtomicInteger(0);
        var future =
            run(() -> System.out.println("Running... " + i.get()))
            .when(() -> i.incrementAndGet() < 10) ;

        future.get();

    }

    private static <T> Until<T> run(Runnable op) {
        return condition -> CompletableFuture
            .runAsync(() -> {
                if (!condition.get()) {
                    throw new Done();
                }
            })
            .thenRun(op)
            .thenCompose(__ -> ConditionalFuture.<T>run(op).when(condition))
            .exceptionally(e -> null);
    }

    static class Done extends RuntimeException {
        public synchronized Throwable fillInStackTrace() {
            return null;
        }
    }

    interface Until<T> {
        CompletableFuture<T> when(Supplier<Boolean> condition);
    }

}