Object Oriented Design Pattern: Command

Introduction

The Command design pattern is a behavioral software design pattern. One of the most common examples of explaining the Command design pattern is that of a Graphical User Interface where its user interaction (e.g. a button click) is decoupled from the implementation of the button click. So instead of directly addressing the receiver of the button click, a Command object will be send to it. The receiver is then able to do something with this command. One of the benefits of this approach is that the User Interface can be developed apart from the receiving part of the application.

For example, when you create a word processing application, you can attach an action to the File -> New button click via a Command object.

Use cases

Decoupling GUI and Business logic
Implementing undo/redo
Keeping history of actions
Macro commands, by adding more than one command to a GUI element

Pitfalls

Over-complication when an application is small and does not require (that much) flexibility
A lot of commands can become hard to manage

Java Example

Remote Control

In this example I use Java with SWT. We will create a Remote Control which can control a Device. The RemoteControl class can create buttons. Each button has a label and has a Command assigned. At line 22 we add a Mouse Click listener to the button, which calls execute() on a Command.

package com.chosengambit.designpatterns.command;

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Event;
import org.eclipse.swt.widgets.Shell;

public class RemoteControl {
	
	private Shell shell;
	
	public RemoteControl(Shell shell) {
		this.shell = shell;
	}
	
	public Button createButton(String label, Command command) {
		Button btn = new Button(this.shell, SWT.PUSH);
		btn.setText(label);
		GridData gridData = new GridData(SWT.CENTER, SWT.CENTER, true, false);
		btn.setLayoutData(gridData);
		btn.addListener(SWT.MouseDown, (Event event) -> command.execute());
		return btn;		
	}
}

Java

Command class

I made an abstract Command class. It holds a Device, so the command can do something with it. The execute() function needs to be overridden in a subclass.

package com.chosengambit.designpatterns.command;

public abstract class Command {
	
	protected Device device;
	
	public abstract void execute();	
	
	public Command(Device device) {
		this.device = device;
	}

}

Java

Turn On and Turn Off

The CommandTurnOff class will make a call to the turnOff method of a Device, regardless of the Device that it has been given to control.

package com.chosengambit.designpatterns.command;

public class CommandTurnOff extends Command {
	
	public CommandTurnOff(Device device) {
		super(device);
	}

	@Override
	public void execute() {
		device.turnOff();
	}
}

Java

package com.chosengambit.designpatterns.command;

public class CommandTurnOn extends Command {

	public CommandTurnOn(Device device) {
		super(device);
	}

	@Override
	public void execute() {
		device.turnOn();
	}
}

Java

Device

Now that we have a Remote Control class and two commands, let’s make the Device we want to control. I’ve made Device abstract as well. In this example we’ll just change a boolean value to indicate the device is either turned on or off. We’ll ignore volume.

package com.chosengambit.designpatterns.command;

public abstract class Device {
	
	protected int volume = 0;
	protected boolean isOn = false;
	
	public void turnOn() {
		this.isOn = true;	
	}

	public void turnOff() {
		this.isOn = false;	
	}

	abstract void increaseVolume();
	abstract void decreaseVolume();
	
}

Java

Television

Television is a subclass of Device.

package com.chosengambit.designpatterns.command;

public class Television extends Device {
	
	private static final int MAX_VOLUME = 7;

	@Override
	public void turnOn() {
		super.turnOn();
		System.out.println("Turn on television");		
	}

	@Override
	public void turnOff() {
		super.turnOff();
		System.out.println("Turn off television");		
	}

	@Override
	public void increaseVolume() {
		volume = (volume + 1 > MAX_VOLUME) ? MAX_VOLUME : volume + 1; 
		System.out.println("Television volume set to "+volume);		
	}

	@Override
	public void decreaseVolume() {
		volume = (volume - 1 < 0) ? 0 : volume - 1;
		System.out.println("Television volume set to "+volume);				
	}
}

Java

Main class

We can tie together our command design pattern at the entrance point of the application. The highlighted lines are part of the pattern. This will render two buttons and will send text messages to the console when pressing them.

package com.chosengambit.designpatterns.command;

import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.*;

public class App 
{
    public static void main( String[] args )
    {
    	  Display display = new Display();
        Shell shell = new Shell(display);
        shell.setText("Command pattern");
        shell.setSize(640, 480);        
        shell.setLayout(new GridLayout(1, false));
        
        Device television = new Television();                  
        RemoteControl rc = new RemoteControl(shell);
        Button buttonOn = rc.createButton("On", new CommandTurnOn(television));
        Button buttonOff = rc.createButton("Off", new CommandTurnOff(television));
                               
        shell.open();
        
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {
                display.sleep();
            }
        }
        display.dispose();
    }
}

Java

On line 16 we’ve created a Television, which is a Device. Then we’ve created a RemoteControl which can create buttons and attach a Command to each of them. Each type of Command class holds a reference to a Device. Implementing the volume controls would be a breeze, by just creating two more Command class extensions and adding two extra lines of code in the App class which would create the buttons.

Class Diagram

A slightly different approach

Redesign of RemoteControl

The following section is not the official way to implement or use the Command pattern.

When you think about a real life remote control, it’s a device which has a fixed number of buttons. Therefore I created an enum for each button. Special attention to line 58 where a device is now able to receive commands, instead of a Command holding a reference to a Device.

package com.chosengambit.designpatterns.command;

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Event;
import org.eclipse.swt.widgets.Listener;
import org.eclipse.swt.widgets.Shell;

public class RemoteControl {
	
	public enum ButtonType {
		ON, OFF, VOLUME_UP, VOLUME_DOWN, RESET_VOLUME
	}
	
	private Shell shell;
	private Device device;
	
	public RemoteControl(Shell shell, Device device) {
		this.shell = shell;
		this.device = device;
	}
	
	public void setDevice(Device device) {
		this.device = device;
	}
	
	public void createDefaultButtons() {
        createButton(RemoteControl.ButtonType.ON, new CommandTurnOn(), "Turn On");
        createButton(RemoteControl.ButtonType.OFF, new CommandTurnOff(), "Turn Off");
        createButton(RemoteControl.ButtonType.VOLUME_UP, new CommandIncreaseVolume(), "Increase volume");
        createButton(RemoteControl.ButtonType.VOLUME_DOWN, new CommandDecreaseVolume(), "Decrease Volume");
	}
	
	public Button createButton(ButtonType buttonType, Command command, String label) {

		Button btn = new Button(this.shell, SWT.PUSH);
		btn.addListener(SWT.MouseDown, (Event event) -> device.receiveCommand(command));
		
		btn.setText(label);
		GridData gridData = new GridData(SWT.CENTER, SWT.CENTER, true, false);
		btn.setLayoutData(gridData);
		
		return btn;		
	}
}

Java

Command classes

The abstract Command class does now have only one abstract method with Device as a parameter.

package com.chosengambit.designpatterns.command;

public abstract class Command {
	
	public abstract void execute(Device device);	
	
}

Java

Each subclass of Command should implement the execute method and call the corresponding method on device.

package com.chosengambit.designpatterns.command;

public class CommandIncreaseVolume extends Command {
	
	public void execute(Device device) {
		device.increaseVolume();
	}
}

Java

The other 3 subclasses; CommandDecreaseVolume, CommandTurnOn and CommandTurnOff can be implemented in the same fashion.

Device has an extra method

A Device is now able to receive a Command. The command will then execute on “this” class, which in our case is the Device set in RemoteControl.

package com.chosengambit.designpatterns.command;

public abstract class Device {
	
	protected int volume = 0;
	protected boolean isOn = false;
	
	public void turnOn() {
		this.isOn = true;	
	}

	public void turnOff() {
		this.isOn = false;	
	}

	abstract void increaseVolume();
	abstract void decreaseVolume();
	
	public void receiveCommand(Command command) {
		command.execute(this);
	}
	
}

Java

App main class changes

package com.chosengambit.designpatterns.command;

import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.*;

public class App 
{
    public static void main( String[] args )
    {
    	Display display = new Display();
        Shell shell = new Shell(display);
        shell.setText("Command pattern");
        shell.setSize(640, 480);        
        shell.setLayout(new GridLayout(1, false));
        
        Device television = new Television();                      
        RemoteControl rc = new RemoteControl(shell, television);
        rc.createDefaultButtons();
                               
        shell.open();
        
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {
                display.sleep();
            }
        }
        display.dispose();    
    }
}

Java

This approach gives the same output results.

Loose ends

According to the Single Responsibility Principle, the RemoteControl class now has a task too many. The button creation process could be moved to another class dedicated for that.

The createButton() method in RemoteControl depends on the fact that a device is set. With the setDevice() method a television could be easily replaced with another device, such as a Radio, but also with null which will give an error.

/*
 * You can hot swap a device on the RemoteControl instance
 */
rc.setDevice(new Radio());

Java

Command with Parameter

Let’s create a “reset volume” button for our Remote Control that only works for a Television. This button will reset the volume to a amount which can be given as a parameter.

RemoteControl gets an extra ButtonType.

	// RemoteControl.java: 
	// Extra ButtonType: RESET_VOLUME
	public enum ButtonType {
		ON, OFF, VOLUME_UP, VOLUME_DOWN, RESET_VOLUME
	}

Java

Television gets an extra method for resetting the volume:

	// Television.java:
	// Extra method with parameter
	public void resetVolume(int amount) {
		this.volume = amount;
		System.out.println("Television volume resetted to "+volume);	
	}

Java

An extra Command class is added: CommandResetVolume. This class Casts a device to Television, and gives an error when it fails. Otherwise it will call resetVolume with the given volume parameter.

package com.chosengambit.designpatterns.command;

public class CommandResetVolume extends Command {
	
	private int volume;
	
	public CommandResetVolume(int amount) {
		this.volume = amount;
	}

	@Override
	public void execute(Device device) {
		try {
			Television television = (Television) device;
			television.resetVolume(this.volume);
		}
		catch(Exception e) {
			System.out.println("Could not execute command");
		}
	}
}

Java

The code in App.java main method is altered:

// App.java
// The Reset button resets the volume of a Television to 3
Television television = new Television();                      
RemoteControl rc = new RemoteControl(shell, television);
rc.createButton(RemoteControl.ButtonType.RESET_VOLUME, new CommandResetVolume(3), "Reset Volume");
rc.createDefaultButtons();

Java

Conclusion

We’ve created a minimal structure of the command design pattern in Java. The command pattern passes on object for each Command which makes it possible to implement more advanced features, such as undo or a command queue.

References

Freeman, E., Bates, B., & Sierra, K. (2004). Head first design patterns.

Gamma, E., Helm, R., Johnson, R., & Vlissides, J. (1994). Design Patterns: Elements of Reusable Object-Oriented Software.

Wikipedia contributors. (2024). Software design pattern. Wikipedia. https://en.wikipedia.org/wiki/Software_design_pattern

Object Oriented Design Pattern: Command

Introduction

Use cases

Pitfalls

Java Example

Remote Control

Command class

Turn On and Turn Off

Device

Television

Main class

Class Diagram

A slightly different approach

Redesign of RemoteControl

Command classes

Device has an extra method

App main class changes

Loose ends

Command with Parameter

Conclusion

References

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