设计模式之命令模式

命令模式：将一个请求封装成一个对象，从而可以使用不同的请求对客户端进行参数化，还可以记录请求日志和撤销请求。

当我们发出一个或者多个请求的时候，如果请求的接收者和执行者重合的话，会导致职责过重，难以对请求进行管理。如果将请求接收者和请求执行者分开，就可以比较好地对请求进行管理和完成。一个典型的例子就是餐厅点餐，服务员负责接受客人点餐，厨师负责完成炒菜。

基本实现

执行者：

public class Executor {    public void execute() {        System.out.println("Executor executes");    }}

命令类及其具体实现，与执行者相关联，命令的执行就是调用执行者去执行：

public abstract class Command {    protected Executor executor;    public Command(Executor executor) {        this.executor = executor;    }    public abstract void execute();}class ConcreteCommand extends Command {    public ConcreteCommand(Executor executor) {        super(executor);    }    @Override    public void execute() {        executor.execute();    }}

命令接收者，依赖于命令，接收者的执行方法就是执行命令：

public class Receiver {    private Command command;    public void setCommand(Command command) {        this.command = command;    }    public void executeCommand() {        command.execute();    }}

测试类：

public class Main {    public static void main(String[] args) {        final Executor executor = new Executor();        final Command command = new ConcreteCommand(executor);        final Receiver receiver = new Receiver();        receiver.setCommand(command);        receiver.executeCommand();    }}

输出：

Executor executes

示例实现：

厨师类：

public class Chef {    public void kungPowChicken() {        System.out.println("老师傅炒出宫保鸡丁这道菜");    }    public void seaweedSoup() {        System.out.println("老师傅熬制上等紫菜汤");    }    public void rice() {        System.out.println("老师傅准备特级香米饭");    }}

点菜命令类及其子类：

public abstract class Order {    protected Chef chef;    public Order(Chef chef) {        this.chef = chef;    }    public abstract void execute();}class KungPowChickenOrder extends Order {    public KungPowChickenOrder(Chef chef) {        super(chef);    }    @Override    public void execute() {        chef.kungPowChicken();    }    @Override    public String toString() {        return "宫保鸡丁";    }}class SeaweedSoupOrder extends Order {    public SeaweedSoupOrder(Chef chef) {        super(chef);    }    @Override    public void execute() {        chef.seaweedSoup();    }    @Override    public String toString() {        return "海带汤";    }}class RiceOrder extends Order {    public RiceOrder(Chef chef) {        super(chef);    }    @Override    public void execute() {        chef.rice();    }    @Override    public String toString() {        return "香米饭";    }}

服务员类,，可以对接收的命令进行记录，还可以进行判断命令是否有效，以及统一执行命令：

public class Waiter {    private List<Order> orders = new ArrayList<>();    public void addOrder(Order order) {        orders.add(order);        System.out.println("客人点了" + order);    }    public void deleteOrder(Order order) {        orders.remove(order);        System.out.println("客人不点" + order + "了");    }    public void finishOrder() {        for (Order order : orders) {            order.execute();        }    }}

测试类：

public class Main {    public static void main(String[] args) {        final Chef chef = new Chef();        final KungPowChickenOrder kungPowChickenOrder = new KungPowChickenOrder(chef);        final SeaweedSoupOrder seaweedOrder = new SeaweedSoupOrder(chef);        final RiceOrder riceOrder = new RiceOrder(chef);        final Waiter waiter = new Waiter();        waiter.addOrder(kungPowChickenOrder);        waiter.addOrder(seaweedOrder);        waiter.addOrder(riceOrder);        System.out.println();        waiter.deleteOrder(riceOrder);        System.out.println();        waiter.finishOrder();    }}

输出：

客人点了宫保鸡丁客人点了海带汤客人点了香米饭客人不点香米饭了老师傅炒出宫保鸡丁这道菜老师傅熬制上等紫菜汤