Tackling the Circular Dependency in Java...

Let me first define what we mean by circular dependency in OOAD terms vis-a-vis Java.

Suppose we have a class called A which has class B’s Object. (in UML terms A HAS B). at the same time we class B is also composed of Object of class A (in UML terms B HAS A). obviously this represents circular dependency because while creating the object of A, the compiler must know the size of B... on the other hand while creating object of B, the compiler must know the size of A. this is something like egg vs. chicken problem...

this may be possible in real life situation as well. for example suppose a multi storied building has a lift. so in the UML terms, the building HAS lift... but at the same time, suppose, while constructing the lift object, we need to give it the information about the building object to access various functionalities of the Building class... for example, suppose the speed of the lift is set depending on the number of floors of the Building... hence while constructing the Lift object it must access the functionalities of the Building object which will give the number of floors the building has got...hence in UML terms the lift HAS building... so this is a sure case of circular dependency...

in real java code it will look something as follows:


public class Building {
private Lift lift;
private int floor;
public Building(){
lift = new Lift();
setFloor(15);
}
public int getFloor(){
return floor;
}

public void setFloor(int floor){
this.floor = floor;
}
}//end of class building

//class Lift
public class Lift {
private Building building;
private int Speed;
public Lift(){
building = new Building();
setSpeed();
}

public void setSpeed(){
if (building.getFloor()>20){
//one set of functionalities
//may be the the speed of the lift will be more
this.Speed = 10;
}
else {
//different set of functionalities
//may be the speed of the lift will be less
this.Speed = 5;
}
}

public int getSpeed(){
return Speed;
}
}//end of class Lift


As it becomes clear from the above code, that while creating the Building object it will create the Lift object, and while creating the Lift object, it will try to create a Building object to access some of its functionalities... So, ultimately it will go out of memory and we get a StackOverflow runtime exception...

So how do we handle this problem in Java? We actually tackle this problem by declaring an IBuildingProxy interface and by deriving our Building class from that... the lift class, instead of Having Building object, it Has IBuildingProxy...

the source code of the solution looks like the following...


//IBuildingProxy
public interface IBuildingProxy {

int getFloor();
void setFloor(int floor);
}//IBuildingProxy


//class Building
public class Building implements IBuildingProxy{
private Lift lift;
private int floor;
public Building(){
lift = new Lift(this);
setFloor(15);
}
public int getFloor(){
return floor;
}

public void setFloor(int floor){
this.floor = floor;
}
}

//class Lift
public class Lift {
private IBuildingProxy building;
private int Speed;
public Lift(Building b){
this.building = b;
setSpeed();
}
public void setSpeed(){
if (building.getFloor()>20){
//one set of functionalities
//may be the the speed of the lift will be more
this.Speed = 10;
}
else {
//different set of functionalities
//may be the speed of the lift will be less
this.Speed = 5;
}
}

public int getSpeed(){
return Speed;
}
}

and the code of the main class is

public class CircularDependencyTest {
public static void main(String[] args){
Building b = new Building();
Lift l = new Lift(b);
}
}

So whats the principle behind such work around... It will be clear soon...

As it becomes clear from the code that Building HAS Lift... That is not a problem... Now when it comes to solve the part that Lift HAS Building, instead of the Building object, we have created an IBuildingProxy interface and we pass it to the Lift class... what it essentially means, that the building class knows the memory requirement to initialize the Lift object, and as the Lift class HAS just a proxy interface of the Building, it does not have to care for the Building's memory requirement... and that solves the problem...

Hope this discussion becomes helpful for Java learners...

My Android AsyncTask implementation vis-a-vis Command Pattern...

Note: If someone wants to learn about the Asynctask internal, please have a look at this document.

As i was developing a sample Android App to showcase the Android AsyncTask, i found it very much similar to the Command design pattern. i would like to share this with you...

Let me first state about the Command design pattern. The purpose of the command pattern is to associate a command object with its receiver where the receiver knows what to do when this command is called... We have an interface called command, from where we deduce different concrete commend classes... The client creates the concrete command objects passing the information of the receiver for that command... And when the invoker ( which is generally a framework object) calls the execute method of the command, the concrete command actually executes it with the help of the Receiver’s functionalities...

The class diagram of the command pattern is as follows:

And the sequence diagram is as follows:

Now let me decipher my App to show how it implements the command pattern... the class diagram of this application looks like the following:

As it becomes clear from the code, the client (which is the Activity class) creates a concrete command class, the Callback interface through the concept of Anonymous inner class (see the code c = new CallBack() {.....}; it then stores this command in the invoker (the MyAsyncTask class) by passing the reference to the invoker (see the line of code aTask = new MyAsyncTask(c)).. . the invoker (the MyAsyncTask class here) stores a reference to this command object... then if something interesting happens, the invoker calls the functionality from the callback interface (the command)... the functionality of the callback interface have been implemented in the Client (the Activity class)...

hence i think that my implementation of the Asynctask is very similar to the Command design pattern implementation...

Hope this discussion helps the android learners...

Here is my AsyncTask implementation.. Here you can find the callback interface... And you can find the main Activity class here.

And here goes the apk of the sample app... You can clone the whole source from here...

SMS Scheduler App for Android with Source Code...

I have developed a simple SMSScheduler App for android devices. This has been open sourced and the source code can be cloned from github.

This can be found at Google Play. This app will let the users schedule his/her sms, view them and cancel them if he/she wants it to. Hope you enjoy it. And i am pretty sure that it will come handy for those people who want to learn how to develop an android app.

Here is a screenshot of this app.

I have also developed another rudimentary SMS Scheduler App with the help of Buzzbox. Here is the link to download it... hope you enjoy it...
please find here the full source code.. it is pretty simple with BuzzBox.

Here are the screenshots of the App

hope this discussion becomes useful for Android learners...

My good days with Ubuntu...

i have now almost completely switched over from Windows to Ubuntu... when the google page first comes out in the chrome browser, or the test page prints Ubuntu, the feeling is of sheer joy... i am loving it... no fear of spyware, no fear of virus... no headache for installing spybot or spyware doctor... no worry about how to install malware bytes... hopefully my days with Ubuntu will pass by peacefully...

CS != Programming

While teaching the Computer Science students in an engineering college, i had this strange feelings that CS != Programming... The theoretical aspects of computer science which the students usually learn in the engineering college will never help them become proficient programmers... many people would shun this point of view... some times back i interacted with one of my colleagues who insisted that programming is only  small part of computer science world... however i feel that it is the most important part of the computer science... with the wake of Test Driven Development and other Agile processes, when the long quality process is loosing its significance, its only good efficient programming that remains the most significant factor of the software project development... i believe that one can get a CS degree without even  having a computer... but one cannot develop and design software that runs without getting his hands dirty with debugger, breakpoint, issues with memory leaks and so on...

Patent and Copyright - from my perspective

today during our breakfast my wife asked me a question about the difference between patent and copyright... while answering her queries, i reminded her how the way we create patents of software these days has become an hindrance to innovation... if one is not convinced see this or this... think about the situation had all the basic scientific theories (e.g. newton’s law of motion and gravity, Archimedes's principles, Einstein's theory of relativity, the knowledge of periodic table and in-numerous chemical reactions or the basic ideas of zero and decimals) being patented and copyrighted, where would our scientific exploration have gone so far? from another angle had the Almighty patented air, water, earth, fire or so many of His living creatures on whom we do scientific experimentation, where would our mankind and civilization stand today?or even had we have to pay royalty to the community of cows for their milk or to the community of plants and vegetables for their own products, where would we go for the money?

Eye opener for educators...

“Intellect has been cultured with the result that hundreds of sciences have been discovered, and their effect has been that the few have made slaves of the many – that is all the good that has been done. Artificial wants have been created; and every poor man, whether he has money or not, desires to have those wants satisfied, and when he cannot, he struggles, and dies in the struggle” - Swami Vivekananda.

Is the mad rush towards consumerism of the modern day a product of such faulty education system?