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Introduction to State Design Pattern(Java, C# Example)

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In this article we are going to learn about State Design Pattern. What is State pattern? What are the basic principles? How to implement and where we can use this.
I have used Java with Eclipse and C# with VS2010.

What is State Pattern?

In OO design, when we need to change behavior of an object depending on its state we use State pattern. Lets think about payment transaction domain. The main object responsible for a payment transaction used to notify a set a set of other objects depend on his state. Suppose, there is a successful transaction, it will notify customer, balance object. If, there were communication problem, it will notify client, then notify bank manager type objects or it there is any insecure activity then it also notify bank's security notifying object.

State pattern allows an object to change its behavior depend on the internal state change. So, now, what is state. A state of an object refers to a.values of its properties b.effects of functions(may be void or any valuated output). So, if the state change, it will work differently.

Let's consider a sales order , a sales order can be new order, registered in system, granted order, shipped order, invoicing or even cancelled. These are the state of the same order state and its inner states will be change along with the functionality of the whole sales order object.

Typically in state design pattern contains
1. Context :Context is the main object. It is the container of different states.
2. States : It should be derived among same grouped classes

Basic Principle :
- Allowing changing behavior
- Let an object decide it behavior based on its state

UML :

How to implement?

As we know, we will have context , states and client who actually calls the context. So, lets think about how to implement. It is for sure, a context will contain all of its state. And, a context will have the functions which will be changed as soon as its state has been changed. So, we can understand this context is actually a composition of all state.
And, an individual state should have function defining for each variety of state. That means either a method taking parameter to vary from state to state or, methods for each state behavior.
This are the simple ideas, let see more detail with example.

Example : Let's consider an android mobile connecting to PC. We will represent an Android mobile as Context object(we may apply interface also that will be called from client). We have mainly three states( three way to connect a device to PC)
1. Connect to PC for charge only : An android device will be connected. Device will be charging only in this state.
2. Connect to PC with ADB Enabled : An android device can be connected to PC with debug mode. In this mode ADB(Android Debug Bridge) will be activated and we can control the android mobile from PC
3. Connect to PC with USB Storage Enabled : An Android device can be connected to PC with USB storage mode. We can see the mobile's storage as drive in this mode.

And, every time the android mobile is connected , depending on state its behavior will be changed. So, that means , each state will represent different behavior. That means, we have to define at least one method to represent the function of a particular behavior. We may use more than one method or even describe in a single method, but I guess it is better to use separate methods to represent separate state behaviors(easy to understand).
And, as I told in the beginning, Android will be our context. We may use a context Interface IMobile which should be implemented by the context object Android so that client can initiate Android using IMobile. This context will have facility to change its own state and providing method to access its state.
And, lastly a Client. A client calls the context and may change behavior to get the context see functions are changing. So,

Step -1 :Defining States : So, Start with IState interface that contains all state related changing method.

   1:  public interface IState {

   2:      public void charge();

   3:      public void usbMount();

   4:      public void adbConnect();

   5:  }

Note: We could make a common method and use parameter to define functionality. But, I use separate methods to understand properly as well as I have use those methods in side separate states to change the states.

And, implementing states ChargingState, AdbState ,USBMountingState

   1:  public class ChargingState implements IState{

   2:      private IMobile myMobile;

   3:      public ChargingState(IMobile aMobile){

   4:          myMobile = aMobile;

   5:      }

   6:      public void charge() {

   7:          System.out.println("Mobile is connected to PC for charging only");

   8:      }

   9:      public void usbMount() {

  10:          System.out.println("While Charging Only mode device can not mount as USB storage, please select USB mount from Device");

  11:          myMobile.setMobileState(myMobile.getUsb());

  12:      }

  13:      public void adbConnect() {

  14:          System.out.println("While Charging Only mode we can not access ADB , please select Android Debugging from Device settings");

  15:          myMobile.setMobileState(myMobile.getAdb());

  16:      }

  17:  }

   1:  public class AdbState implements IState{

   2:      private IMobile myMobile;

   3:      public AdbState(IMobile aMobile){

   4:          myMobile = aMobile;

   5:      }

   6:      public void charge() {

   7:          System.out.println("While ADB mode, your device is by default charging");

   8:          myMobile.setMobileState(myMobile.getCharging());

   9:      }

  10:      public void usbMount() {

  11:          System.out.println("While ADB mode , device can not mount as USB storage, please select USB mount from Device");

  12:          myMobile.setMobileState(myMobile.getUsb());

  13:      }

  14:      public void adbConnect() {

  15:          System.out.println("Your device is connected in Android Debugging Mode, to cancel ADB mode please dis-select Android Debugging from Device settings");

  16:      }

  17:  }

   1:  public class USBMountingState implements IState{

   2:      private IMobile myMobile;

   3:      public USBMountingState(IMobile aMobile){

   4:          myMobile = aMobile;

   5:      }

   6:      public void charge() {

   7:          System.out.println("While USB Only mode, your device is by default charging");

   8:          myMobile.setMobileState(myMobile.getCharging());

   9:      }

  10:      public void usbMount() {

  11:          System.out.println("Deviec is in USB mode, access mobile storage from file explorer");

  12:      }

  13:      public void adbConnect() {

  14:          System.out.println("While USB Only mode we can not access ADB , please select Android Debugging from Device settings");

  15:          myMobile.setMobileState(myMobile.getAdb());

  16:      }

  17:  }

Step -2 : Creating Context : Now, Lets make a context interface IMobile that will contain all state changing behavior as well as behavior calling functions.

   1:  //Context Interface

   2:  public interface IMobile {

   3:      public void setMobileState(IState aState);

   4:      public void ChargeMobile();

   5:      public void USBConnect();

   6:      public void AdbAccess();

   7:      public IState getCharging() ;

   8:      public IState getUsb() ;

   9:      public IState getAdb() ;

  10:  }

And, implementing class Android

   1:  //Context Class

   2:  public class Android implements IMobile{

   3:      private IState charging;

   4:      private IState usb;

   5:      private IState adb;

6:

   7:      private IState mobileState;

   8:      public void setMobileState(IState aState) {

   9:          mobileState= aState;

  10:      }

  11:      public Android(){

  12:          charging = new ChargingState(this);

  13:          usb = new USBMountingState(this);

  14:          adb = new AdbState(this);

  15:          mobileState = charging;

  16:      }

  17:      //next three are for accessing functions in one state

  18:      public void ChargeMobile(){

  19:          mobileState.charge();

  20:      }

  21:      public void USBConnect(){

  22:          mobileState.usbMount();

  23:      }

  24:      public void  AdbAccess() {

  25:          mobileState.adbConnect();

  26:      }

  27:      // next three are getting states

  28:      public IState getCharging() {

  29:          return charging;

  30:      }

  31:      public IState getUsb() {

  32:          return usb;

  33:      }

  34:      public IState getAdb() {

  35:          return adb;

  36:      }

  37:  }

Step -3 : Calling From Client : So, now we call context from client and we will see when the context changed, the same method of a same context provides different results.

   1:  public class Program {

   2:      public static void main(String[] args) {

   3:          IMobile samsungGalaxy = new Android();//initial state will be charging

   4:          samsungGalaxy.ChargeMobile();

   5:          samsungGalaxy.AdbAccess();//mode will be change to ADB

   6:          samsungGalaxy.AdbAccess();

   7:          samsungGalaxy.USBConnect();//mode will change to USB

   8:          samsungGalaxy.USBConnect();

   9:      }

  10:  }

Java Example :
1. To keep simple, I have used only command print to define behaviors.

C# Example :
1. Some method names are changed due to different implementation.

Uses :
1. When we need an individual object's behavior specified by its state.(not decided by other objects)
2. When are design stand alone system object( like as device representing objects).
3. Very good to represent a data table or database state or record state with this pattern.
And sometimes it is handy to use with other patterns.

Thanks...:)

Introduction to Command Design Pattern (Java, C# Example)

By 0x000216

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In this article we are going to learn about Command Design Pattern. What is Command pattern? What are the basic principles? How to implement and where we can use this.
I have used Java with Eclipse and C# with VS2010.

What is Command Design Pattern?
In OO design, some time we need to encapsulate method detail info from client as well as decoupling function detail. Command design pattern allows us to complete decoupling between sender and receiver. Where sender refers to function invoker and receiver refers to function receiver(who gets) and executor.
Command pattern encapsulate how method will be execute, what is the exact method name, what are the method details. It does encapsulation as object and allow clients to parameterize different requests, queues or big requests. Because of command pattern's working process, we can easily implement undo and redo operation. Command pattern has mainly 3 Components :
a. Invoker
b. Receiver
c. The Command
Invoker links between commands and receiver. It also stores the command that it passes.
The Receiver receives the request and act to perform the request. By functionality we can understand it is a behavioral pattern.

The command object actually encapsulate the request. In this way we get flexibility as well as extensibility.(spatial use, when we are designing event functionality on keyword based selenium framework for testing, command patterns helps to create a scenario by providing step by step execution without knowing specific function details).

Command pattern turns a request into an object which can be passed like as other objects. Lets consider a small example(function testing domain).

Suppose we are testing a web site by selenium functional testing. Testing client will instruct test runner to perform step by step without knowing inner detail of the steps, and test runner send the steps by individual command object(as invoker). And , a keyword driven framework object get the command and understand with current webpage's context and execute the specific function that satisfied the command. And, in here if we have a layer architecture and the selenium framework is in server and test runner in client machine, by command patter Test Client can easily execute the test suit without knowing the inner details. This is one of the good use of Command pattern that I have used.

[I got some difficulties to explain in words, i hope my example may cover those]

Basic Principles :
- Each request will be encapsulated in an object
- Request can be saved in a queue. (allowing undo/redo)
- Clients can generate

UML :

How to implement?
Implementation of command pattern is very simple. Let's think about how to implement. As we need single or or set of command, so there will be an interface ICommand representing command and each command will implement that.
And, there will be a invoker which may maintain a list of commands. The stored command may be used to undo/redo operation.
Receiver, who will get the command perform the function. We may implement receiver interface for applying more than one group of receiver .
And, a client who creates command to be performed and sets to receiver.
That means, Client -> Command -> Invoker -> Receiver , lets see more from example.

Example : Lets think about Android phone. Typically a android phone contains 6 buttons (considering touch & non touch). Usually buttons are, power button( long press = on/off, short press = screen on/off), Volume +, Volume -, Home key(shows home screen), menu (shows options), back(undo previous option/go to back where I came from). And globally, touch pad, where we touch and out app starts.
Now, see the basic functions of those buttons. Power, volumes, home screen these three have fixed command but, menu and back buttons functions used to change (depend on which app you are currently working). So, from client, we only press those button and ask for the command that is applicable for the specific running application. Let's consider a client who will do all operation(power on,, off, volume Up & Down, back and home button press) and some time undo operation. I will explain this example applying Command pattern step by step.

Step 1 : Receiver Constriction: I have use an IReceiver interface and implementing class MobileReceiver which have the method detail of all tasks. It might be a single class or even a group of class.

public interface IReceiver {

    public void home();

    public void on();

    public void off();

    public void volumeUp();

    public void volumeDowne();

    public void back();

public class MobileReceiver implements IReceiver{

    private int vol = 0;

    public void home() {

        System.out.println("You are now at Home Screen");

    public void on() {

        System.out.println("Mobile is turning on");

    public void off() {

        System.out.println("Mobile is turning off");

    public void volumeUp() {

        vol++;

        System.out.println("The volume is up to "+vol);

    public void volumeDowne() {

        vol--;

        System.out.println("The volume is down to "+vol);

    public void back() {

        System.out.println("Going Back...");

Step 2 : Command Generation: We need a command interface that has all method which will be implement by all commands where each command represents each method of receiver class(or group of receivers) . So, the ICommand interface

public interface ICommand {

    public void execute();

    public void undo();//in android mobile, back button refers to undo if any command do not have inverse function(like volume has + & - or power button has On & off

Now, the command objects. Each object will represent each function.

public class PressHome implements ICommand{

    private IReceiver myReceiver  = null;

    public PressHome(IReceiver aReceiver ){

        myReceiver= aReceiver ;

    public void execute() {

        myReceiver.home();

    public void undo() {

        myReceiver.back();

public class PressBack implements ICommand{

    private IReceiver myReceiver  = null;

    public PressBack(IReceiver aReceiver ){

        myReceiver= aReceiver ;

    public void execute() {

        myReceiver.back();

    public void undo()// Usually back is the undo it self.

        myReceiver.back();

public class TurnMobileOff implements ICommand{

    private IReceiver myReceiver  = null;

    public TurnMobileOff(IReceiver aReceiver ){

        myReceiver= aReceiver ;

    public void execute() {

        myReceiver.off();

    public void undo() {

        myReceiver.on();

public class TurnMobileOn implements ICommand{

    private IReceiver myReceiver  = null;

    public TurnMobileOn(IReceiver aReceiver ){

        myReceiver= aReceiver ;

    public void execute() {

        myReceiver.on();

    public void undo() {

        myReceiver.off();

public class VolumeUp implements ICommand{

    private IReceiver myReceiver  = null;

    public VolumeUp(IReceiver aReceiver ){

        myReceiver= aReceiver ;

    public void execute() {

        myReceiver.volumeUp();;

    public void undo() {

        myReceiver.volumeDowne();;

public class VolumeDown  implements ICommand{

    private IReceiver myReceiver  = null;

    public VolumeDown(IReceiver aReceiver ){

        myReceiver= aReceiver ;

    public void execute() {

        myReceiver.volumeDowne();

    public void undo() {

        myReceiver.volumeUp();

Step 3 : Invoker Generation: We have commands and receiver. Now, need invoker. As we know invoker allow client call any command, so it will contain a command object(as element, ICommand). And, we can apply Undo operation by keeping a operation list. When ever we want we can call back the function from the list. I have applied differently. I use a separate method undo(mentioned in ICommand as not all button have same undo procedure, example Volume+ undo will be Volume- but, Home button press undo will be pressing back). This portion is commented(general undo implementation) . But, you can apply that way. I have kept a separate method pressBack(), that calls undo from ICommand. So, the invoker is

public class MobileButtonInvoker {

    private ICommand aCommand;

    //private ArrayList commandQueue = new ArrayList();

    //If we want to apply stack based undo operation, we can keep an array list of ICommands and perform undo by invoker, but I apply that undo in ICommand.

    public MobileButtonInvoker(ICommand newConnand){

        aCommand=newConnand;

    public void press() {

        aCommand.execute();

        //commandQueue.add(aCommand);

    public void pressBack() {

        aCommand.undo();

For getting a receiver(mobile), I have added an extra class with static method output. The purpose is only to get us the functional receiver.

public class Mobile {

    public static MobileReceiver getAMobile(){

        return new MobileReceiver();

Step 4 : Finally the client who have main method and calls command through invoker.

public class Program {

    public static void main(String[] args) {

        MobileReceiver aMobile= Mobile.getAMobile();

        ICommand command = new TurnMobileOn(aMobile);

        MobileButtonInvoker invoker = new MobileButtonInvoker(command);

        invoker.press();

        command = new PressHome(aMobile);

        invoker = new MobileButtonInvoker(command);

        invoker.press();

        command = new PressBack(aMobile);

        invoker = new MobileButtonInvoker(command);

        invoker.press();

        command = new VolumeUp(aMobile);

        invoker = new MobileButtonInvoker(command);

        invoker.press();

        invoker.press();

        invoker.pressBack();

        command = new VolumeDown(aMobile);

        invoker = new MobileButtonInvoker(command);

        invoker.press();

        command = new TurnMobileOff(aMobile);

        invoker = new MobileButtonInvoker(command);

        invoker.press();

        invoker.pressBack();

Java Example :
1. Comment : In android mobile, back button refers to undo if any command do not have inverse function(like volume has + & - or power button has On & off
2. Comment : If we want to apply stack based undo operation, we can keep an array list of ICommands and perform undo by invoker, but I apply that undo in ICommand.
3. I have kept command as generic in client as interface sharing among client is a best practice.
4. All functions are applied with command line out put to keep it simple.

C# Example :
1. I have change method names as Dot Net works differently than Java.

Uses :
1. By Command pattern, it is easy to make components that needs delegate/sequence /execute method calls at any time without knowing the class of the method or the method parameter.
2. Macro recording and playback, where user actions are kept as command and program records the action as sequence.
3. Remote code execution. If we implement the sequence, we can call method remotely (code will come to client) , similar action to java RMI.
4. UNDO/REDO operation can be done, even if in multiple levels.
5. GUI components that can keep the method in Action command. (spatially helpful in java swing)
6. Progress bars can be implemented by showing number of command performed and associated time
7. Parallel processing, multiple threading, thread pools , in all cases commands are abstracted from thread where a queue of commands are kept for sequence of execution. (undo/redo stacks).

Thanks...:)

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NAVISION - Design Patterns Video Series Available.

Introduction to State Design Pattern(Java, C# Example)

Introduction to Command Design Pattern (Java, C# Example)

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