已編輯：現在看起來更干凈了，反映了我目前所處的位置以及我正在努力完成的作業，并顯示了我正在處理的新問題（它下面有一條評論，解釋了我正在得到什么）。

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();
}
}

對不起，如果帖子有點長，但我希望每個人都能看到整個畫面。

uj5u.com熱心網友回復：

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());
}
}

uj5u.com熱心網友回復：

為了澄清，您需要知道位元組碼級別才能知道事情是如何作業的。

讓我們開始使用這段代碼：

public class ParentConstructor {
    public ParentConstructor() {
    }
}

然后運行編譯（javac-java compile）并提取資訊（javap-java print）命令：

$~ javac /path/to/ParentConstructor.java
$~ javap /path/to/ParentConstructor.class

你會得到：

{
  public ParentConstructor();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 2: 0
        line 3: 4
}

第一條指令壓this入運算元堆疊。第二條指令從堆疊中彈出這個值并呼叫類中<init>定義的方法Object。這對應于super()呼叫，即對超類的建構式的呼叫Object。

ParentConstructor可以改寫如下：

public class ParentConstructor {
    public ParentConstructor() {
       super();
    }
}

這意味著如果超類具有默認建構式，則可以在子類建構式中隱式呼叫超類的建構式，如果超類沒有默認建構式，則可以在子類建構式中顯式呼叫超類的建構式。

public static class Terrain
    {
        public 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) {
            //explicitly call superclass's constructor.
            super(x, y);

            //Error above, I'm not sure why this wont work when it works just fine in the parent class.
            mounts = num;
            length = x;
            width = y;
        }
        public String getMountainSize()
        {
            return "Mountains have the dimensions "   length   " X "   width   " with a total of "   mounts   " mountains";
        }
    }

或向超類添加默認建構式。

public static class Terrain
    {
        public int length, width;
        Terrain(int l, int w)
        {
            length = l;
            width = w;
        }
        
        public Terrain() {
        }
        
        public String getTerrainSize()
        {
            return "Land has dimensions "   length   " X "   width;
        }
    }

參考：

• 方法 java/lang/Object."<init>":()V 的定義在哪里？
• 建構式位元組碼

uj5u.com熱心網友回復：

嘗試從 w3School 學習 Java 建構式。

如果您在方法中訪問長度或寬度值。您必須 將此關鍵字放在長度或寬度之前。

代碼 ：

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";
    }
  }```

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