Best way to use Future in java

I have a set of jobs which I am submitting using executor framework and Future. Let's say that I have 100 futures. As of now, I am using Future.get and using the output for subsequent processing. However for further tuning, I want to change the flow as below:

1. iterate through the set of future tasks and start consuming the result as soon as a future task is complete. I am reading the API doc to understand what might be a good way to accomplish this but reaching out to see if there is a better way to accomplish what I am looking for.

Here is the sample code:

public class ImplCallable implements Callable<String> {
    	
    	int timeOut;
    	ImplCallable(int timeOut) {
    		this.timeOut=timeOut;
    	}
    	
    	public String call() throws Exception {
    		Thread.sleep(timeOut);
    		return Thread.currentThread().getName();
    	}
    
    }

and the main class:

public class MainProg {
    	public static void main(String...args) throws Exception {
    		long startTimeInMillis = System.currentTimeMillis();
    		ImplCallable callable1 = new ImplCallable(1000);
    		ImplCallable callable2 = new ImplCallable(2000);
    		
    		
    		ExecutorService service = Executors.newFixedThreadPool(4);
    		Future<String> task1 = service.submit(callable1);
    		Future<String> task2 = service.submit(callable2);
    		
    		List<Future<String>> futureList = new ArrayList();
    		futureList.add(task1);
    		futureList.add(task2);
    		
    		
    		String retVal;
    		for(Future<String> task:futureList) {
    			retVal = task.get();
    			//do something with the retVal
    		}
    		long endTimeInMillis = System.currentTimeMillis();
    		
    		System.out.println("time taken by code - " + (endTimeInMillis-startTimeInMillis) + "-ms");
    	}
    } 

Basically I don't want to use Future.get() and wait for its completion. I want to know if either of the task is complete and use the result as soon as its done.

There are many ways do this so, without a concrete example, you won't get a concrete answer. Likely want to look at CompletableFuture which has many methods for defining follow-on work, combining work, splitting work etc.

Future<String> f = CompletableFuture.supplyAsync(() -> "INITIAL WORK")
  .thenApply(String::toLowerCase) // Do some more work
  .thenAccept(queue::add); // put results onto a queue something is reading from

f.join();
// Batch complete

I hope you are using Java 8 or later version.
Whenever you mention "as soon as a future task is complete", you want to use CompletableFuture and its .thenApply() method, as @drekbour suggests.

Then you have multiple threads running different tasks in non-determenistic sequence. But at the end you want to get all the results in the single (Main) thread. To achieve it, you can use CompletableFuture.allOf method, .join() it - and then iterate over all the (already completed) future results without waiting.