java，在超类中使用构造函数扩展类

Question

I have a class that defines field "coordinates" which is an array without length and a subclass where i need to set a length of array to work with it.So the question is how do I change the array of coordinates' length in subclass. Here is the code: Superclass:

abstract class Vector implements sample {       

    private int[] coordinates;  

    public Vector (int[] coordinates) {

    }
}

And subclass :

class Vector3D extends Vector {

    public Vector3D(int[] coordinates) {
        super(coordinates);

    }
}

Answer 1

abstract class Vector implements sample {       
    private int[] coordinates;  
     public Vector (int[] coordinates){
         this.coordinates=coordinates;
     }
}

If you want to initialize coordinates by calling new Vector3D(1,2,3):

class Vector3D extends Vector {

    public Vector3D(int n1,int n2,int n3) {
       super(new int[]{n1,n2,n3});
   }
}

Answer 2

Your parent class (Vector) needs to set the private field. Currently, the parameter and the instance variable are unrelated.

This would do the job:

public abstract class Vector implements Sample {
   private int[] coordinates;

   public Vector(int[] coordinates) {
      this.coordinates = coordinates;
   }
}



public class Vector3D {
   public Vector3D(int[] coordinates) {
      super(coordinates);
   }
}

// usage:
new Vector3D(new int[]{ 1, 2, 3 }); // Vectors coordinate size: 3. Content: 1, 2, 3

This would pass the Vector3Ds array to the parent and its size would be the same, as the one provided to the Vector3D class.