Command Pattern

What

Encapsulates a request as an object, allowing parameterization of clients with queues, logs, and undoable operations.

When

• You want to decouple the sender of a request from the object that performs it.
• Support for operations like undo, logging, or queueing is needed. You want to encapsulate requests, e.g., for undo/redo or UI buttons.

class Light {
    public void turnOn() {
        System.out.println("Light on");
    }
}
 
class Remote {
    private Light light = new Light();
 
    public void buttonPressed() {
        light.turnOn(); // Remote is tightly coupled to specific action
    }
}
 

How

• Define a Command interface with an execute() method.
• Create concrete command classes that implement Command and call methods on a receiver.
• Invoker stores command objects and calls execute().

interface Command { void execute(); }
 
class Light {
    public void turnOn() { System.out.println("Light on"); }
}
 
class TurnOnCommand implements Command {
    private Light light;
    public TurnOnCommand(Light l) { this.light = l; }
    public void execute() { light.turnOn(); }
}
 
class Remote {
    public void press(Command c) { c.execute(); }
}
 

Examples

• GUI buttons triggering commands (e.g., copy, paste, save)
• Transaction queue in a banking app
• Undo functionality in a text editor

Why

Advantages

• Decouples sender and receiver
• Adds support for undo/redo, logging, and queuing
• Commands can be parameterized and reused

Disadvantages

• Can lead to many small classes
• Extra layer of indirection