Design Pattern: Command Pattern in Java

Command pattern is used to encapsulate/wrap a command in an object so that the object can be sent as a parameter and can be used in other places.

Check Details of Command Pattern

This article demonstrates Command pattern implementations in Java. Check the following examples.

Examples

Here are a few examples of command pattern in Java-

Example #1: UI Elements

Let’s take the example of printing UI elements. Let’s take a look at the class diagram first.

Command Interface

// UiCommand.java

package com.bigboxcode.designpattern.command.uielement;

public interface UiCommand {

    void print();

    void remove();

}

Button Concrete Class

// ButtonUi.java

package com.bigboxcode.designpattern.command.uielement;

public class ButtonUi implements UiCommand {

    private final String name;

    public ButtonUi(String name) {
        this.name = name;
    }

    @Override
    public void print() {
        System.out.println("Printing " + this.name + " Button");
    }

    @Override
    public void remove() {
        System.out.println("Removing " + this.name + "  Button");
    }
}

Input Element Concrete Class

// UiCommand.java

package com.bigboxcode.designpattern.command.uielement;

public class InputUi implements UiCommand {
    @Override
    public void print() {
        System.out.println("Printing Input");
    }

    @Override
    public void remove() {
        System.out.println("Removing Input");
    }
}

Table Element Concrete Class

// TableUi.java

package com.bigboxcode.designpattern.command.uielement;

public class TableUi implements UiCommand {
    @Override
    public void print() {
        System.out.println("Printing Table");
    }

    @Override
    public void remove() {
        System.out.println("Removing Table");
    }
}

UI Element Control Class

// UiControl.java

package com.bigboxcode.designpattern.command.uielement;

import java.util.ArrayList;
import java.util.List;

public class UiControl {
    private List<UiCommand> commandList = new ArrayList<>();

    public void addElement(UiCommand command) {
        // Execute command
        command.print();

        // Store command in list to have a history
        commandList.add(command);
    }

    public void removeElement(UiCommand command) {
        // Remove element
        command.remove();

        // Store command in list to have a history
        commandList.remove(command);
    }

    public void undo() {
        UiCommand lastCommand = commandList.get(commandList.size() - 1);
        removeElement(lastCommand);
    }
}

Demo

// Demo.java

package com.bigboxcode.designpattern.command.uielement;

public class Demo {
    public static void main(String[] args) {
        UiControl uiControl = new UiControl();

        UiCommand inputUi = new InputUi();
        UiCommand tableUi = new TableUi();
        UiCommand buttonUi = new ButtonUi("Submit");

        uiControl.addElement(inputUi);
        uiControl.addElement(tableUi);
        uiControl.addElement(buttonUi);

        // Remove specific element
        uiControl.removeElement(tableUi);

        // Add some new elements
        uiControl.addElement(new ButtonUi("Cancel"));
        uiControl.addElement(new TableUi());
        uiControl.addElement(new InputUi());
        uiControl.addElement(new ButtonUi("Wrong button"));

        // Undo last two command
        uiControl.undo();
        uiControl.undo();

    }
}

Output

Printing Input
Printing Table
Printing Submit Button


Removing Table


Printing Cancel Button
Printing Table
Printing Input
Printing Wrong button Button


Removing Wrong button  Button
Removing Input