The command pattern is a type of behavioural design pattern that transforms a request into a stand-alone object containing all the details about the request. This pattern is a data-driven pattern because we make use of the information about the request by wrapping it as an object and is passed to the invoker object as a command. The invoker object checks for the object that can handle the command and passes it to that object to execute the command. The following diagram is the UML diagram that represents the command design pattern.
We have a client that calls the invoker to run a command. We have a Command interface that acts as an abstraction to the underlying concrete classes. Let us understand this with the help of an example of remote control that has only one button. Using this button, we will be controlling the behaviour of two objects tubelight and a radio. The command to control the objects will be implemented using the command design pattern.
Create Command interface:
// Command Interface
interface Command
{
public void execute();
}
Create Tubelight class and its command classes that extend the above interface to control the tubelight.
// Tubelight class
class TubeLight
{
public void lightOn(){
System.out.println("TubeLight on...");
}
public void lightOff(){
System.out.println("TubeLight off...");
}
}
// Command class to turn on the tubelight
class TubeLightOnCommand implements Command
{
TubeLight tubeLight;
// The constructor is passed the light it
// is going to control.
public TubeLightOnCommand(TubeLight tubeLight){
this.tubeLight = tubeLight;
}
public void execute(){
tubeLight.lightOn();
}
}
// Command class to turn off the tubelight
class TubeLightOffCommand implements Command
{
TubeLight tubeLight;
public TubeLightOffCommand(TubeLight tubeLight) {
this.tubeLight = tubeLight;
}
public void execute() {
tubeLight.lightOff();
}
}
Create a Radio class and its command classes that extend the above interface to control the radio.
// Radio class
class Radio
{
public void radioOn()
{
System.out.println("Radio on ...");
}
public void radioOff()
{
System.out.println("Radio off...");
}
public void setVolume(int volumeLevel)
{
// code to set the volume
System.out.println("Radio volume set to " + volumeLevel);
}
}
// Command class to turn on the radio
class RadioOnCommand implements Command
{
Radio radio;
public RadioOnCommand(Radio radio)
{
this.radio = radio;
}
public void execute()
{
radio.radioOn();
}
}
// Command class to set the volume of the radio
class RadioVolumeCommand implements Command
{
Radio radio;
int volumeLevel;
public RadioVolumeCommand(Radio radio, int volumeLevel)
{
this.radio = radio;
this.volumeLevel=volumeLevel;
}
public void execute()
{
radio.setVolume(volumeLevel);
}
}
Create a remote control class that has only one button and on click of the button, execute the command functionality:
// remote control with one button
class RemoteControl{
Command button; // only one button
public RemoteControl(){}
public void setCommand(Command command){
// set the command the remote will
// execute
button = command;
}
public void pressButton(){
// execute the command on click (call) of the button
button.execute();
}
}
Create a Driver class to implement the pattern. Here we will be first turning on the tubelight on the first click of the button, on next click, we will be turning on the radio, then we will be setting the volume of the radio to 4 and then we will be turning off the tubelight.
// Driver class
public class Driver
{
public static void main(String[] args)
{
RemoteControl remote = new RemoteControl();
TubeLight tubeLight = new TubeLight();
Radio radio = new Radio();
// Turn on Tubelight
remote.setCommand(new TubeLightOnCommand(tubeLight));
remote.pressButton();
//Turn on Radio
remote.setCommand(new RadioOnCommand(radio));
remote.pressButton();
//Turn off Radio
remote.setCommand(new RadioVolumeCommand(radio,4));
remote.pressButton();
// Turn off Tubelight
remote.setCommand(new TubeLightOffCommand(tubeLight));
remote.pressButton();
}
}
Validate the result of this pattern to see if it’s working fine. The result of this code would be:
TubeLight on...
Radio on ...
Radio volume set to 4
TubeLight off...
