Asynchronous and Synchronous Callbacks in Java

https://knaidu.gitbooks.io/problem-solving/parallel_processing/implement_asynchronous_callbacks.html
https://www.geeksforgeeks.org/asynchronous-synchronous-callbacks-java/

Synchronous Callback : Any process having multiple tasks where the tasks must be executed in sequence and doesn’t occupy much time should use synchronous Callbacks.
For example : You’re in a movie queue for ticket you can’t get one until everyone in front of you gets one.

Asynchronous Callback : When the tasks are not dependent on each other and may take some time for execution we should use Asynchronous callbacks.
For example : When you order your food other people can also order their food in the restaurant. They don’t have to wait for your order to finish, If you’re downloading a song from internet, Getting an API response.

Asynchronous Callback

interface OnGeekEventListener {        // this can be any type of method     void onGeekEvent(); }    class B {        private OnGeekEventListener mListener; // listener field        // setting the listener     public void registerOnGeekEventListener(OnGeekEventListener mListener)     {         this.mListener = mListener;     }        // My Asynchronous task     public void doGeekStuff()     {            // An Async task always executes in new thread         new Thread(new Runnable() {             public void run()             {                    // perform any operation                 System.out.println("Performing operation in Asynchronous Task");                    // check if listener is registered.                 if (mListener != null) {                        // invoke the callback method of class A                     mListener.onGeekEvent();                 }             }         }).start();     }        // Driver Program     public static void main(String[] args)     {            B obj = new B();         OnGeekEventListener mListener = new A();         obj.registerOnGeekEventListener(mListener);         obj.doGeekStuff();     } }    class A implements OnGeekEventListener {        @Override     public void onGeekEvent()     {         System.out.println("Performing callback after Asynchronous Task");         // perform some routine operation     }     // some class A methods }

Synchronous Callback

interface OnGeekEventListener {        // this can be any type of method     void onGeekEvent(); }    class B {        private OnGeekEventListener mListener; // listener field        // setting the listener     public void registerOnGeekEventListener(OnGeekEventListener mListener)     {         this.mListener = mListener;     }        // my synchronous task     public void doGeekStuff()     {            // perform any operation         System.out.println("Performing callback before synchronous Task");            // check if listener is registered.         if (this.mListener != null) {                // invoke the callback method of class A             mListener.onGeekEvent();         }     }        // Driver Function     public static void main(String[] args)     {         B obj = new B();         OnGeekEventListener mListener = new A();         obj.registerOnGeekEventListener(mListener);         obj.doGeekStuff();     } }    class A implements OnGeekEventListener {        @Override     public void onGeekEvent()     {         System.out.println("Performing callback after synchronous Task");         // perform some routine operation     }     // some class A methods }