14、Command封装命令

属于行为型模式（共11种）

目的：
结合 chain of responsibility 可以实现 undo的功能 （就是撤销ctrl+z）

类图：

部分代码实现

public abstract class Command {
    public abstract void doit(); //exec run
    public abstract void undo();
}
---------------------------------------------------------
public class CopyCommand extends Command {
    Content c;
    public CopyCommand(Content c) {
        this.c = c;
    }
    @Override
    public void doit() {
        c.msg = c.msg + c.msg;
    }
    @Override
    public void undo() {
        c.msg = c.msg.substring(0, c.msg.length()/2);
    }
}
---------------------------------------------------------
public class DeleteCommand extends Command {
    Content c;
    String deleted;
    public DeleteCommand(Content c) {
        this.c = c;
    }
    @Override
    public void doit() {
        deleted = c.msg.substring(0, 5);
        c.msg = c.msg.substring(5, c.msg.length());
    }
    @Override
    public void undo() {
        c.msg = deleted + c.msg;
    }
}
---------------------------------------------------------
public class Main {
    public static void main(String[] args) {
        Content c = new Content();
        Command insertCommand = new InsertCommand(c);
        insertCommand.doit();
        insertCommand.undo();
        Command copyCommand = new CopyCommand(c);
        insertCommand.doit();
        insertCommand.undo();
        Command deleteCommand = new DeleteCommand(c);
        deleteCommand.doit();
        deleteCommand.undo();
        List<Command> commands = new ArrayList<>();
        commands.add(new InsertCommand(c));
        commands.add(new CopyCommand(c));
        commands.add(new DeleteCommand(c));
        for(Command comm : commands) {
            comm.doit();
        }
        System.out.println(c.msg);
        for(int i= commands.size()-1; i>=0; i--) {
            commands.get(i).undo();
        }
        System.out.println(c.msg);
    }
}

command + memento 实现 transaction事务回滚
command + composite 实现 宏命令 多种命令组合