I have an ArrayClass and mergeSortArray extends it. And mergeSortArray contains a mergeSort() method. However, since I used super to call a constructor from the superclass, I do not know how to refer to the mergeSortArray (the subclass object / array) and pass it as a parameter in the mergeSort method. In fact, is this even feasible ? I know I can do this in a NON- OOP way. However, I am keen to know how to do this in an OOP way.

Please correct me if I have said incorrect, as I am new to Java and I want to learn more about it.

// ArrayClass Object
    import java.util.*;
    import java.io.*;
    import java.math.*;

public class ArrayClass{
    public int[] input_array;
    public int nElems;

    public ArrayClass(int max){
        input_array = new int [max];
        nElems = 0;
    }

    public void insert(int value){
        input_array[nElems++] = value;
    }

    public void display(){
        for(int j = 0; j < nElems; j++){
            System.out.print(input_array[j] + " ");
        }
        System.out.println("");
    }

}

import java.io.*;
import java.util.*;
import java.math.*;

class mergeSortArray extends ArrayClass{

    public mergeSortArray(int max){
        super(max);
    }

    public void methodOne(){
        int[] output_array = new int[super.nElems];
        mergeSort( // ************* // ,output_array,0, super.nElems -1);

     }
       ................
}

I am not sure what I should put to replace ****** such that I can pass mergeSortArray as a parameter into the mergeSort method.

There isn't a mergeSortArray. You inherit input_array like (and no need for super.nElems you inherit that too),

 mergeSort( input_array, output_array, 0, nElems - 1);

Your sub-class will inherit everything that is protected or greater visibility (not private), however your ArrayClass gives you both public fields

public int[] input_array;
public int nElems;

They should probably be protected and have accessor methods (getters).

protected int[] input_array;
protected int nElems;
public int size() {
  return nElems;
}
public int[] getInputArray() {
  return input_array;
}