如何从Java中的继承类生成对象？

Question

编辑：现在看起来更干净了，反映了我目前的状况和我正在努力实现的目标，并展示了我正在处理的新问题（下面有一个评论解释了我得到了什么）。

public class Main {
class Terrain
{
    private int length, width;
    Terrain(int l, int w)
    {
        length = l;
        width = w;
    }

    public String getTerrainSize()
    {
        return "Land has dimensions " + length + " X " + width;
    }
}
 public class Mountain extends Terrain{
    private int mounts;
    public Mountain(int num, int x, int y) {
//error above: Implicit super constructor Main.Terrain() is undefined. Must explicitly invoke another constructor
//What exactly does this error mean, and how should I fix it?
        mounts = num;
        length = x;
        width = y;
    }
    public String getMountainSize()
    {
        return "Mountains have the dimensions " + length + " X " + width + " with a total of " + mounts + " mountains";
    }
}
  public static void main(String[] args) {
    Terrain T1 = new Terrain(400, 200);
    T1.getTerrainSize();
    Mountain M1 = new Mountain(350, 150);
//error here: The constructor Main.Mountain(int, int) is undefined
//I have a feeling it'll be fixed if I fix the first error, but if not, let me know.
    M1.getMountainSize();
}
}

抱歉，如果帖子有点长，但我希望每个人都能看到整个画面。

Answer 1

如果您在方法中访问长度或宽度值。你必须把这个关键字放在长度或宽度之前。

代码：

public class Terrain
{
 private int length, width;
    public Terrain(int l, int w)
    {
        length = l;
        width = w;
    }
    public String getTerrainSize()
    {
        return "Land has dimensions " + this.length + " X " + this.width;
    }
}
public class Mountain extends Terrain{
    private int mounts,length,width;
    public Mountain(int num, int x, int y) {
        mounts = num;
        length = x;
        width = y;
    }
    public String getMountainSize()
    {
        return "Mountains have the dimensions " + this.length + " X " + this.width + " with a total of " + this.mounts + " mountains";
    }
  }```

Answer 2

最后：

public class Main {
static class Terrain
{
    public static int length, width;
    Terrain(int l, int w)
    {
        length = l;
        width = w;
    }

    public String getTerrainSize()
    {
        return "Land has dimensions " + length + " X " + width;
    }
}
 public static class Mountain extends Terrain{
    private int mounts;
    public Mountain(int num, int x, int y) {
      super(length, width);
        mounts = num;
        length = x;
        width = y;
    }
    public String getMountainSize()
    {
        return "Mountains have the dimensions " + length + " X " + width + " with a total of " + mounts + " mountains";
    }
 }
  public static void main(String[] args) {
    Terrain T1 = new Terrain(400, 200);
    System.out.println(T1.getTerrainSize());
    Mountain M1 = new Mountain(8, 350, 150);
    System.out.println(M1.getMountainSize());
}
}