Java对象和方法

Question

我试图了解对象是如何使用自动传输车辆程序在Java上工作的。我有一种方法可以提高汽车speedIncrease的速度，另一种方法可以获得当前对象currentSpeed的速度。在我的测试用例中，我在TransmissionBox对象renault上创建了一个初始速度为25的对象。speedUp方法将速度提高了2，所以新的速度应该是27。似乎在他第一次调用之后，这个对象就失去了所有的修改。怎么可能呢？

测试:设置

  @Before
  public void setUp(){
    private TransmissionBox renault;
    renault = new TransmissionBox(25,10,20,30,40,50);
  }

测试:速度= 27

  @Test
  public void speedIncrease(){
   assertEquals(27,renault.speedUp().currentSpeed());
  }

测试:速度= 25

  @Test
  public void speedIncrease(){
   renault.speedUp();
   assertEquals(25,renault.currentSpeed());
  }


**TransmissionBox class**

public class TransmissionBox {
private int speed;
private int gear;
private int threesholdOne;
private int threesholdTwo;
private int threesholdThree;
private int threesholdFour;
private int threesholdFive;



  public TransmissionBox(int iniSpeed, int iniThresholdOne, int iniThresholdTwo, int iniThresholdThree, int iniThresholdFour, int iniThresholdFive){
      this.speed = iniSpeed;
      this.threesholdOne = iniThresholdOne;
      this.threesholdTwo = iniThresholdTwo;
      this.threesholdThree = iniThresholdThree;
      this.threesholdFour = iniThresholdFour;
      this.threesholdFive = iniThresholdFive;

      if(speed == 0 && speed < threesholdOne){
        this.gear = 1;
      }

    else if(speed > threesholdOne && speed < threesholdTwo){
      this.gear = 2;
    }

    else if(speed > threesholdTwo && speed < threesholdThree){
        this.gear = 3;
    }

    else if(speed > threesholdThree && speed < threesholdFour){
        this.gear = 4;
    }

    else{
        this.gear = 5;
    }

  }


  public TransmissionBox speedUp(){
    TransmissionBox fasterCar = new TransmissionBox(speed+2, threesholdOne, threesholdTwo, threesholdThree, threesholdFour, threesholdFive);
    return fasterCar;
  }

  public TransmissionBox speedDown(){
    TransmissionBox slowerCar = new TransmissionBox(speed-2, threesholdOne, threesholdTwo, threesholdThree, threesholdFour, threesholdFive);
    return slowerCar;
  }

  public int currentSpeed(){
    return this.speed;
  }

  public int currentGear(){
    return this.gear;
  }


  public String toString(){
    return "speed: " + this.speed + "\n gear: " + this.gear;
  }
}

Answer 1

在错误的测试中调用speedUp()

Answer 2

注意您在这里所做的事情：

renault.speedUp();
assertEquals(25,renault.currentSpeed());

当你打电话给speedUp()时会发生什么？这确实是：

TransmissionBox fasterCar = new TransmissionBox(speed+2, threesholdOne, threesholdTwo, threesholdThree, threesholdFour, threesholdFive);
return fasterCar;

现有对象不变，并返回一个新对象。基本上，目前的汽车根本没有“加速”。你刚上了一辆更快的新车。

但是你怎么处理那个新返回的对象呢？什么也没有：

renault.speedUp();

renault对象不变，新返回的对象将被忽略/丢弃。renault仍在按设计运行25。

如果希望speedUp()影响当前汽车，请修改当前对象，而不是返回新对象：

public void speedUp(){
    this.speed += 2;
}

请注意，这不再返回一个对象。除非你真的需要它，否则这个流的语义是没有意义的。(并不是所有的东西都需要流畅的界面。)因此，这个测试需要改变：

@Test
public void speedIncrease(){
    assertEquals(27,renault.speedUp().currentSpeed());
}

像这样的事情：

@Test
public void speedIncrease(){
    renault.speedUp();
    assertEquals(27,renault.currentSpeed());
}