为什么 Java 将我的第二个实例的参数分配给一个类的两个实例？答案

【问题标题】：Why is Java assigning the parameters of my second instance to both instances of a class?为什么 Java 将我的第二个实例的参数分配给一个类的两个实例？
【发布时间】：2021-01-20 23:07:16
【问题描述】：

我正在上入门级大学编程课，这可能是一个新手问题，但我无法在其他地方找到答案，也无法弄清楚为什么 Java 会从我的第二个 Conveyor 实例中分配参数两个实例。

这是我的 Conveyor 课程：

    public class Conveyor
    {
       //fields
       private static String type;
       private static double speed; //Speed is measured in m/s (meters/second)
       
       //constructors
       public Conveyor(String type, double speed)
       {
          this.type = type;
          this.speed = 0;
          this.speed = setSpeed(speed);
       }
       
       //methods
       public String getType()
       {
          return this.type;
       }
       
       public double getSpeed()
       {
          return this.speed;
       }
       
       public double setSpeed(double speed)
       {  
          if(speed <= 0)
          {
             this.speed = this.speed;
          }
          else
          {
             this.speed = speed;
          }
          
          return this.speed;
       }
       
       public double timeToTransport(double distance)
       {
          double t = distance / getSpeed();
          return t;
       }
    }

以及用于测试它的 Conveyor 应用程序：

    public class ConveyorApp
    {
       public static void main(String[] args)
       {
          Conveyor c1 = new Conveyor("flat belt",0.9);
          Conveyor c2 = new Conveyor("roller", -0.5);
          
          System.out.printf("Conveyor 1: %s conveyor with a speed of %.1f\n", c1.getType(), c1.getSpeed());
          System.out.printf("Time to transport an item 50m: %.1f\n\n", c1.timeToTransport(50));
          
          System.out.printf("Conveyor 2: %s conveyor with a speed of %.1f\n", c2.getType(), c2.getSpeed());
          System.out.printf("Time to transport an item 50m: %.1f\n\n", c2.timeToTransport(50));
       }
    }

如果我将第二个实例移到第一个的打印语句下方，它会产生预期的行为并打印：

    Conveyor 1: flat belt conveyor with a speed of 0.9
    Time to transport an item 50m: 55.6
    
    Conveyor 2: roller conveyor with a speed of 0.0
    Time to transport an item 50m: Infinity

否则按照上面显示的顺序，这是我的输出：

    Conveyor 1: roller conveyor with a speed of 0.0
    Time to transport an item 50m: Infinity
    
    Conveyor 2: roller conveyor with a speed of 0.0
    Time to transport an item 50m: Infinity

我做错了什么？

【问题讨论】：

看Understanding Class Members ????????docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

标签： java class instance instance-variables

【解决方案1】：

您的 Conveyor 类的字段被标记为静态字段，但听起来您希望它们充当实例字段。

  public class Conveyor
    {
       //fields
       private String type;
       private double speed; //Speed is measured in m/s (meters/second)
       ...

只需像这样删除静态关键字就可以了！

【讨论】：

解决了！谢谢！