给出以下使用多线程的java示例：

import java.util.concurrent.*;

 public class SquareCalculator {
     private ExecutorService executor = Executors.newSingleThreadExecutor();

     public Future<Integer> calculate(Integer input) {
         return executor.submit( () -> {
             Thread.sleep(1000);
             return input * input;
         });
     }

     public static void main(String[] args) {
         try {
             Future<Integer> future = new SquareCalculator().calculate(10);
             while (!future.isDone()){
                 System.out.println("Calculating...");
                 Thread.sleep(300);
             }

             Integer result = future.get();
             System.out.println("we got: " + result);
         } catch(InterruptedException | ExecutionException e) {
             System.out.println("had exception");
         }
     }
 }

它产生：

java SquareCalculator
Calculating...
Calculating...
Calculating...
Calculating...
we got: 100

但应用程序永远不会终止。
我是不是该把线接起来？