Java CompletableFuture - main class not terminated - java-8

Question

I am trying to implment CompletableFuture which invokes a dummy callback method when completed.
However, after adding CompletableFuture.get() method my main class doesn't terminate.
I tried replacing CompletableFuture.get() with Thread.sleep(5000) but it doesn't seem to be right approach.
Please suggest what is causing CompletableFuture.get() to keep blocking even if the thread is complete.
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.stream.IntStream;
public class CallableAsyncWithCallBack {
public static void main(String[] args) throws InterruptedException {
CompletableFuture<String> compFuture=new CompletableFuture<String>();
compFuture.supplyAsync(()->{
//Compute total
long count=IntStream.range(Integer.MIN_VALUE, Integer.MAX_VALUE).count();
return ""+count;
}).thenApply(retVal->{
try {
return new CallBackAsynchClass(retVal).toString();
} catch (InterruptedException | ExecutionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return "";
}
);
System.out.println("Main Thread 1");
try {
compFuture.get();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("Lock cleared");
}
}
class CallBackAsynchClass
{
String returnVal="";
public CallBackAsynchClass(String ret) throws InterruptedException, ExecutionException {
System.out.println("Callback invoked:"+ret);
returnVal=ret;
}
#Override
public String toString() {
return "CallBackAsynchClass [returnVal=" + returnVal + "]";
}
}
I am expecting "Lock cleared" to be outputted but .get() seems to be holding up the lock.

Answer 1

.thenApply function returns a new instance of CompletableFuture, and it's this instance that you need to use, try using this way instead :
public class CallableAsyncWithCallBack {
public static void main(String[] args) throws InterruptedException {
CompletableFuture<String> compFuture = CompletableFuture.supplyAsync(() -> {
//Compute total
long count = IntStream.range(Integer.MIN_VALUE, Integer.MAX_VALUE).count();
return "" + count;
});
CompletableFuture<String> future = compFuture.thenApply(retVal -> {
try {
return new CallBackAsynchClass(retVal).toString();
} catch (InterruptedException | ExecutionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return ""; });
System.out.println("Main Thread 1");
try {
future.get();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("Lock cleared");
}
}
Hope this helps