java - 计算人口密度: a beginners guide

Question

好吧，我对此还很陌生，我不知道我的代码发生了什么。它看起来是正确的，但是当我尝试运行它时，它给了我除以零的错误？

老师给我的一页上的代码 1:

 public class Country
{
   // fields
    private String name;
    private int population; 
    private int area; // in square miles

    // constructors

    // methods
    public String getName()
    {
        return name;
    }

    public int getPopulation()
    {
        return population;
    }   

    public int getArea()
    {
        return area;
    }   

    public void setName(String newName)
    {
        name = newName;
    }

    public void setPopulation(int newPopulation)
    {
        population = newPopulation;
    }

    public void setArea(int newArea)
    {
        area = newArea;
    }

    // population per square mile
    public int populationDensity()
    {
        int density;
        density = population / area; // integer division truncates
        return density;
    }
}

还有我做过的另一件事:

public class A03  
{
   public static void main(String[] args) 
   {
      Country country1 = new Country();

      String newName = "Macau";
      int newPopulation = 453000;
      int newArea = 6;
      int density = newPopulation / newArea;

      country1.setName(newName);
      country1.setPopulation(newPopulation);
      country1.setArea(newArea);

      System.out.printf("Name: " + country1.getName());
      System.out.printf("%nPopulation: %,d ", country1.getPopulation());
      System.out.printf("%nArea: "+ country1.getArea());      
      System.out.printf("%nPopulation Density: %,d", country1.populationDensity(), "%n");
      System.out.println();
      System.out.println();

      Country country2 = new Country();
      String name1 = "Libya";
      int population1 = 5900000;
      int area1 = 679358;
      int density1 = population1 / area1;

      System.out.printf("Name: %s%n", name1);
      System.out.printf("Population: %,d%n", population1);
      System.out.printf("Area: %,d%n", area1);
      System.out.printf("Population Density: %,d%", country2.populationDensity(),   "%n");

      System.out.println();

      Country country3 = new Country();
      String name2 = "USA";
      int population2 = 298500000;
      int area2 = 3539225;
      int density2 = population2 / area2;

      System.out.printf("Name: %s%n", name2);
      System.out.printf("Population: %,d%n", population2);
      System.out.printf("Area: %,d%n", area2);
      System.out.println();

   }
}

有人可以告诉我出了什么问题吗？或者我正在做的事情是错误的。我基本上复制、粘贴和更改我的变量以适应下一个国家的人口密度。

Answer 1

在 main 中，您没有调用 country2 或 country3 上的任何 setter 方法。您只声明了变量。当实例变量未分配值时，Java 会分配默认值，对于原始数字类型，该值是 0。这解释了为什么会出现被零除错误。

调用 country2 和 country3 的 setter(和 getter)。