The difference between the Runnable and Callable interfaces in Java..

• A Callable needs to implement call() method while a Runnable needs to implement run() method.
• A Callable can return a value but a Runnable cannot.
• A Callable can throw checked exception but a Runnable cannot.
• A Callable can be used with ExecutorService#invokeXXX methods but a Runnable cannot be.
  

  public interface Runnable {
      void run();
  }
  
  public interface Callable<V> {
      V call() throws Exception;
  }

  NOTE : Inside your class you can make the calls to Callable and Runnable
   on a single thread executor, making this mechanism similar to a serial 
  dispatch queue. So as long as the caller calls your Runnable wrapped 
  methods the calling thread will execute really fast without blocking. As
   soon as it calls a Callable wrapped in Future method it will have to 
  block till all the other queued items are executed. Only then the method
   will return with values. This is a synchronization mechanism.