How to run multiple methods parallely and get outputs from each of them in java

I want to run three different methods in parallel to improve the performance in Java. Also I need to get the outputs from all the three of them. Below is the sample which I have tried. here, I'm not sure how to retrieve the returned string values. Please help me to add(concatenate all the three strings to total).

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class test {
public static void main(String[] args) {
String total = "";
Callable<String> callable1 = new Callable<String>()
{
@Override
public String call() throws Exception
{
String t1 = "";
t1 = method1();
return t1;
}
};
Callable<String> callable2 = new Callable<String>()
{
@Override
public String call() throws Exception
{
String t2 = method2();
return t2;
}
};
Callable<String> callable3 = new Callable<String>()
{
@Override
public String call() throws Exception
{
String t3 = method3();
return t3;
}
};
List<Callable<String>> taskList = new ArrayList<Callable<String>>();
taskList.add(callable1);
taskList.add(callable2);
taskList.add(callable3);
ExecutorService executor = Executors.newFixedThreadPool(3);
try
{
executor.invokeAll(taskList);
//total = ;(want to concatenate all the strings here).
System.out.println(total);
}
catch (InterruptedException ie)
{
//do something if you care about interruption;
}
}
public static String method1()
{
System.out.println("method1");
return "1";
}
private static String method2()
{
System.out.println("method2");
return "2";
}
private static String method3()
{
System.out.println("method3");
return "3";
}
}

As taskList is a List<Callable<String>>, executor.invokeAll(taskList) returns a List<Future<String>> containing a Future<String> corresponding to each task in taskList. You need to save that List<Future<String>> so that you can later get at the results of your tasks. Something like this:

List<Future<String>> futureList = executor.invokeAll(tasklist);
String result = futureList.get(0).get() +
futureList.get(1).get() +
futureList.get(2).get();

In addition to InterruptedException, Future.get() can also throw CancellationException and ExecutionException so you need to be prepared to deal with these in your try block.

As you have very few number of task, you can create 3 CompletableFutures and stream over it and join it.

CompletableFuture<String> task1 = CompletableFuture.supplyAsync(() -> method1());
CompletableFuture<String> task2 = CompletableFuture.supplyAsync(() -> method2());
CompletableFuture<String> task3 = CompletableFuture.supplyAsync(() -> method3());
String concate = Stream.of(task1, task2, task3)
.map(CompletableFuture::join)
.reduce("", (s1, s2) -> s1 + s2);
System.out.println(concate);

Adding on top of @Govinda answer -
You can create Stream of CompletableFutures by using the supplyAsync factory method and complete them by calling CompletableFuture::join and concat by calling Collectors.joining().

CompletableFuture<String> task1 = CompletableFuture.supplyAsync(Test::method1);      
CompletableFuture<String> task2 = CompletableFuture.supplyAsync(Test::method2);      
CompletableFuture<String> task3 = CompletableFuture.supplyAsync(Test::method3);      
String concat =                                                                               
Stream.of(task1, task2, task3).map(CompletableFuture::join).collect(Collectors.joining());
System.out.println(concat);