實(shí)現(xiàn)命令設(shè)計模式需先明確其核心組成並逐步實(shí)施。 1. 理解命令模式的核心組件：包括命令接口、具體命令類、接收者、調(diào)用者和客戶端；2. 定義命令接口，通常包含execute()方法；3. 創(chuàng)建執(zhí)行實(shí)際操作的接收者類如Light；4. 實(shí)現(xiàn)具體命令類如LightOnCommand和LightOffCommand，封裝接收者的方法；5. 使用調(diào)用者如RemoteControl觸發(fā)命令；6. 在客戶端創(chuàng)建並綁定命令與接收者，通過調(diào)用者執(zhí)行操作。該模式實(shí)現(xiàn)了請求的封裝、操作的解耦及功能擴(kuò)展。

Implementing the Command design pattern in Java is a solid way to encapsulate requests as objects, allowing you to parameterize clients with operations, queue or log requests, and support undoable actions. It's especially useful when you want to decouple the object that invokes an operation from the one that knows how to perform it.

Let's walk through how to do this step by step.

1. Understand the core components of the Command pattern

Before writing any code, it helps to understand what parts make up the Command pattern:

• Command – an interface or abstract class that declares an execute() method.
• Concrete Command – implements the execute() method by calling the appropriate method on a Receiver.
• Receiver – the actual object that performs the action.
• Invoker – holds a command and calls its execute() method when needed.
• Client – creates the command and sets the receiver.

This structure allows for flexibility, such as queuing commands or adding undo functionality later.

2. Define the Command interface

Start by creating a simple interface that all concrete commands will implement. The most basic version includes just one method: execute() .

 public interface Command {
    void execute();
}

You can also extend this interface to include things like undo() , but keep it simple at first.

3. Create a Receiver class

The receiver does the real work. For example, imagine a light switch system. You might have a Light class like this:

 public class Light {
    public void turnOn() {
        System.out.println("The light is on");
    }

    public void turnOff() {
        System.out.println("The light is off");
    }
}

This is where the actual logic resides — the command will call these methods indirectly.

4. Implement Concrete Command classes

Now create a command that uses the receiver. For turning the light on:

 public class LightOnCommand implements Command {
    private Light light;

    public LightOnCommand(Light light) {
        this.light = light;
    }

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

And if you want to support turning it off:

 public class LightOffCommand implements Command {
    private Light light;

    public LightOffCommand(Light light) {
        this.light = light;
    }

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

These command objects act as wrappers around the receiver's methods.

5. Use an Invoker to trigger commands

An invoker doesn't know what the command does — it just triggers execution. Here's a simple one:

 public class RemoteControl {
    private Command command;

    public void setCommand(Command command) {
        this.command = command;
    }

    public void pressButton() {
        command.execute();
    }
}

You could expand this to handle multiple buttons or queues, but this gives you the idea.

6. Put it all together in the Client

Finally, wire everything up in your main class or client code:

 public class Client {
    public static void main(String[] args) {
        // Create the receiver
        Light light = new Light();

        // Create the command and pass the receiver
        Command lightOn = new LightOnCommand(light);

        // Create the invoker and set the command
        RemoteControl remote = new RemoteControl();
        remote.setCommand(lightOn);

        // Trigger the command
        remote.pressButton();
    }
}

If you run this, you'll see:

 The light is on

Change the command to LightOffCommand , and pressing the button turns it off instead.

This setup keeps your code modular and makes it easy to add new commands without modifying existing ones. Once you've got the basics down, you can easily extend it to support undo, logging, or macro-style batched commands.

基本上就這些。

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