Java新手-汽车应用

Question

我是一名学生，是的，这是我的作业。上周我复习了笔记，阅读了这本书，并在网上研究了相关主题，但我就是不明白问题出在哪里。你能告诉我我做错了什么吗？任何帮助将不胜感激。 (我只使用记事本和命令提示符。)

我得到的指导方针:创建一个由两个类组成的 Java 应用程序。第一类将是您的应用程序类。第二类将是一个名为 Car 的类。您的应用程序将创建一个名为 nova 的 Car 实例并驱动它。

汽车规则:

• 如果汽车未启动，您就无法驾驶汽车(向控制台发送错误消息)。
• 如果汽车未启动，您就无法停止它(向控制台发送错误消息)。
• 如果汽车已经启动，您将无法启动汽车(向控制台发送错误消息)。
• 一旦您告诉汽车行驶，您唯一能做的就是停下来(向控制台发送消息)
• 一旦您调用停止，汽车将返回到初始状态，用户必须先启动汽车，然后才能尝试执行任何其他功能。 (向控制台发送消息)

showState方法的目的是提供一种检查汽车状态的方法。它应该构建一条消息，然后将其发送到控制台。

我的代码:

public class MyAppAssignment3
{
   public static void main (String[] args)
   {
       System.out.println("Scenario 1");
       Car nova1 = new Car();
       nova1.start();
       nova1.showState();
       nova1.drive();
       nova1.stop();
       nova1.showState();
       System.out.println("");

       System.out.println("Scenario 2");
       Car nova2 = new Car();
       nova2.showState();
       nova2.drive();  //needs to send error message - can't drive a car that's not started
       nova2.stop();
       nova2.showState();
       System.out.println("");

       System.out.println("Scenario 3");
       Car nova3 = new Car();
       nova3.showState();
       nova3.start();
       nova3.showState();
       nova3.stop();   //needs to send error message - can't stop a car that's not driving
       nova3.showState();
       nova3.drive();
       nova3.stop();
   }
}

class Car
{
   private boolean isStarted;
   private boolean isDriving;
   private boolean isStopped;
   private String showState;

   public Car()
   {
       this.showState = showState;
   }

   public void start()
   {
       isStarted = true;
       isDriving = false;
       isStopped = false;
       System.out.println("The car is " + this.showState);
   }

   public void drive()
   {
       isStarted = false;
       isStopped = false;
       isDriving = true;
       System.out.println("The car is " + this.showState);
   }

   public void stop()
   {
       isStopped = true;
       isStarted = false;
       isDriving = false;
       System.out.println("The car is " + this.showState);
   }

   public String showState()
   {
       if (isStarted)
       {
           showState = "started";
       }
       else if(isDriving)
       {
           showState = "driving";
       }
       else if(isStopped)
       {
           showState = "stopped";
       }
       System.out.println("The car is " + this.showState);
       return showState;
   }
}

我的输出(完全错误 - 值不正确):

Scenario 1
The car is null
The car is started
The car is started
The car is started
The car is stopped

Scenario 2
The car is null
The car is null
The car is null
The car is stopped

Scenario 3
The car is null
The car is null
The car is started
The car is started
The car is stopped
The car is stopped
The car is stopped

抱歉，如果这篇文章的内容很奇怪。我输入得很好，但预览看起来很奇怪。

Answer 1

这实际上并没有做任何事情......

public Car()
{
    this.showState = showState;
}

基本上，它只是将相同的值重新分配给自身。我会更改为初始状态，可能是stopped

我会使用 enum 来表示我的汽车状态，而不是依赖 boolean 状态，这可能会变得困惑......

public enum CarState {
    Stopped,
    Started,
    Driving
}

然后只需将其分配给单个state变量...

class Car
{
    private CarState state;

    public Car()
    {
       this.state= CarState.Stopped;
    }

    public void start()
    {
        if (state.equals(State.Stopped)) {
            state = CarState.Started;
            showState();
        } else {
            System.error.println("Car is not in a valid state to be started");
        }
    }

    public void drive()
    {
        if (state.equals(State.Started)) {
            state = CarState.Driving;
            showState();
        } else {
            System.error.println("Car is not in a valid state to be driven");
        }
    }

    public void stop()
    {
        if (state.equals(State.Driving)) {
            state = CarState.Stopped;
            showState();
        } else {
            System.error.println("Car is not in a valid state to be stopped");
        }
    }

    public String showState()
    {
        System.out.println("The car is " + state);
    }
}

您遇到的另一个问题是，当您更改状态时，未调用 showStatus，这不会将当前状态分配给 showState 变量...我已使用 enum

更正了这一点