画一个“使用命令模式设计一个图形绘制系统，该系统可以绘制矩形、椭圆、三角形等图形，恢复和撤销功能。绘制类图并编程实现。”的uml类图

好的，下面是我画的UML类图：

     +-----------------+          +---------------------+
     |     Invoker     |          |      Command        |
     +-----------------+          +---------------------+
     | executeCommand()| <------  |     execute()        |
     | addCommand()    |          |     undo()           |
     | removeCommand() |          +---------------------+
     +-----------------+                     /\
             /\                              |
             |                               |
             |                               |
             |                               |
             |                               |
    +--------+-----------+          +--------+----------+
    |     Receiver      |          |      Concrete      |
    +-------------------+          |     CommandA       |
    | drawRectangle()   |          |     CommandB       |
    | drawEllipse()     |          |     CommandC       |
    | drawTriangle()    |          +--------------------+
    | undoLastDraw()    |          | + draw()           |
    | redoLastDraw()    |          | + undo()           |
    +-------------------+          +--------------------+

其中，Invoker为调用者，Command为抽象命令类，Receiver为接受者，Concrete Command为具体命令类。在该图形绘制系统中，Invoker负责调用具体的Command对象，而Command对象则负责调用Receiver中的方法进行具体的绘制操作。Receiver则负责实现具体的绘制方法，并提供撤销和恢复绘制操作的方法。

编程实现可以参考以下示例代码：

//命令接口
public interface Command {
    void execute();
    void undo();
}

//具体命令类-绘制矩形
public class DrawRectangleCommand implements Command {
    private Receiver receiver;

    public DrawRectangleCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    @Override
    public void execute() {
        receiver.drawRectangle();
    }

    @Override
    public void undo() {
        receiver.undoLastDraw();
    }
}

//具体命令类-绘制椭圆
public class DrawEllipseCommand implements Command {
    private Receiver receiver;

    public DrawEllipseCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    @Override
    public void execute() {
        receiver.drawEllipse();
    }

    @Override
    public void undo() {
        receiver.undoLastDraw();
    }
}

//具体命令类-绘制三角形
public class DrawTriangleCommand implements Command {
    private Receiver receiver;

    public DrawTriangleCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    @Override
    public void execute() {
        receiver.drawTriangle();
    }

    @Override
    public void undo() {
        receiver.undoLastDraw();
    }
}

//接受者
public class Receiver {
    private List<String> drawHistory = new ArrayList<>();

    public void drawRectangle() {
        //绘制矩形
        drawHistory.add("rectangle");
    }

    public void drawEllipse() {
        //绘制椭圆
        drawHistory.add("ellipse");
    }

    public void drawTriangle() {
        //绘制三角形
        drawHistory.add("triangle");
    }

    public void undoLastDraw() {
        //撤销上一次绘制
        if (drawHistory.size() > 0) {
            drawHistory.remove(drawHistory.size() - 1);
        }
    }

    public void redoLastDraw() {
        //恢复上一次绘制
    }
}

//调用者
public class Invoker {
    private List<Command> commandHistory = new ArrayList<>();
    private Receiver receiver;

    public Invoker(Receiver receiver) {
        this.receiver = receiver;
    }

    public void executeCommand(Command command) {
        command.execute();
        commandHistory.add(command);
    }

    public void undoLastCommand() {
        if (commandHistory.size() > 0) {
            Command lastCommand = commandHistory.remove(commandHistory.size() - 1);
            lastCommand.undo();
        }
    }

    public void redoLastCommand() {
        //恢复上一次操作
    }
}

//客户端代码
public class Client {
    public static void main(String[] args) {
        Receiver receiver = new Receiver();
        Invoker invoker = new Invoker(receiver);

        Command commandA = new DrawRectangleCommand(receiver);
        Command commandB = new DrawEllipseCommand(receiver);
        Command commandC = new DrawTriangleCommand(receiver);

        invoker.executeCommand(commandA);
        invoker.executeCommand(commandB);
        invoker.executeCommand(commandC);

        invoker.undoLastCommand();
        invoker.undoLastCommand();
    }
}