Java OOP: referencing subclass object

Question

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.

Elliott Frisch · Accepted Answer · 2014-12-02 00:58:35Z

1

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;
}

answered Dec 2, 2014 at 0:58

Elliott Frisch

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3 Comments

mynameisJEFF Over a year ago

@Elliott_Frisch If the variables are protected in the ArrayClass, the sub-class can access them (as they are not private). In this case , why would I still want to specify public int size() and public int[] getInputArray() instance method ?

Elliott Frisch Over a year ago

Those are accessor methods. It's generally preferred over making the fields public (like they were in your question).

mynameisJEFF Over a year ago

but if the fields are protected, I can access them without accessor methods. Then why need accessor methods ?

Kleyguerth · Accepted Answer · 2014-12-02 01:02:52Z

1

First of all, I suggest you not to have public fields on OO code. You got two public fields (input_array and nElems), you should change them to private and create acessors if you need.

Then, to refer to those fields on the subclass, you can either use a protected acessor if you wish to hide the acessor from the rest of the API or a public one, if it's part of your public API. That way, on your ArrayClass:

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

    //this may be public
    protected int[] getInputArray() {
        return input_array;
    }

and when you get to call your mergeSort method, you can use getInputArray()

answered Dec 2, 2014 at 1:02

Kleyguerth

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