简介：

简单来说，就像我们的复制、删除、插入等等都是命令，我们将命令封装为一个对象，并且支持撤销，将一系列命令串成一条链或者与链表结合使用，可以实现一系列的do和undo

模式类图：

command: 抽象的命令类
X_Command、Y_Command、Z_Command: 具体的命令

使用场景：

我现在能想到的一般是在画图软件，办公软件，开发工具（IDE）当中比较常见的，毕竟作为一个码农，复制粘贴用的多了:)

模式实例：

比如我对一串字符串进行一系列的操作，我这个例子可能不太准确，角色划分不太明显，参考：https://www.cnblogs.com/meet/p/5116430.html
https://blog.csdn.net/qq_22764659/article/details/81335701

1、Content被操作的对象

package com.mashibing.command;
public class Content {String msg = "hello everybody ";;
}

2、Command抽象命令

package com.mashibing.command;
public abstract class Command {public abstract void doit();public abstract void undo();
}

3、InsertCommand插入命令：

package com.mashibing.command;
public class InsertCommand extends Command{Content c;String strToInsert="http://www.mashibing.com";public InsertCommand(Content c) {this.c = c;}@Overridepublic void doit() {c.msg = c.msg + strToInsert;}@Overridepublic void undo() {c.msg = c.msg.substring(0,c.msg.length()-strToInsert.length());}
}

4、CopyCommand 复制命令

package com.mashibing.command;
public class CopyCommand extends Command{Content c;public CopyCommand(Content c) {this.c = c;}@Overridepublic void doit() {c.msg = c.msg+c.msg;}@Overridepublic void undo() {c.msg = c.msg.substring(0,c.msg.length()/2);}
}

5、DeleteCommand删除命令

package com.mashibing.command;
public class DeleteCommand extends Command{Content c;String deleted;public DeleteCommand(Content c) {this.c = c;}@Overridepublic void doit() {deleted = c.msg.substring(0,5);c.msg = c.msg.substring(5,c.msg.length());}@Overridepublic void undo() {c.msg = deleted + c.msg;}
}

6、Main 测试

package com.mashibing.command;
import java.util.ArrayList;
import java.util.List;
/*** command模式一般跟责任链模式结合，实现一连串的undo* 也可以用双向链表的方式来做*/
public class Main {public static void main(String[] args) {Content c = new Content();//单个执行命令Command insertCommand = new InsertCommand(c);insertCommand.doit();insertCommand.undo();Command copyCommand = new CopyCommand(c);copyCommand.doit();copyCommand.undo();Command deleteCommand = new DeleteCommand(c);deleteCommand.doit();deleteCommand.undo();System.out.println(c.msg);//串在一起一连串执行List<Command> commands = new ArrayList<>();commands.add(new InsertCommand(c));commands.add(new CopyCommand(c));commands.add(new DeleteCommand(c));for(Command command : commands) {command.doit();}System.out.println(c.msg);//一连串撤回for (int i = commands.size()-1; i >=0; i--){commands.get(i).undo();}System.out.println(c.msg);}
}

测试结果：