How do I access an array from a parent class in a child class?

Question

I have a class where arrays are globally declared and public these arrays are initialized though methods in this class. They are not inside of a constructor. I have another class where I have used extends to allow me access to these values. Of course, I am recieving a null pointer exception. How would I go about fixing this? I do not need to override these arrays. JUst need to use them inside of methods to fill other arrays.

I have been at this for awhile now. My experience with java is still pretty minimal. Any suggestions would be appreciated. Thank you everyone

An example of what I am talking about:

public class Parent{
  public double hey[ ];

  public double [] fillHey(){

  hey = new double[57]

  for(int k = 0; k<57; k++)
    {
         hey[k] = k+2;
    }
   }
 }

The child class:

public class Child extends Parent{

public double you[ ];

public double[ ] fillYou(){

you = new double[57];

 for(int k = 0; k<57; k++)
    {
         you[k] = (k+2) * hey[k];
    }
   }
 }

Ah looks like @Damien has me covered thanks for the heads up — handroski, Feb 18 '14 at 22:24

score 2 · Accepted Answer · answered Feb 18 '14 at 22:22

2

You did not instantiate hey array in the constructor of either class so it is still null when you use it in the child class.

You can either

instantiate it in a constructor
or instantiate it inside fillYou() method in the child class (before it's used).

answered Feb 18 '14 at 22:22

Szymon

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So if I bring it up with the same name from the Parent class, the compiler will recognize this? – handroski Feb 18 '14 at 22:27
1

Yes, it will recognise it. – Szymon Feb 18 '14 at 22:28

score 1 · Answer 2 · answered Feb 18 '14 at 22:23

1

If you want to access to the hey[] from the child class, you must call it like this:

double  a[] = super.hey;

Hope it works for you!

answered Feb 18 '14 at 22:23

Fabian Rivera

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I see. Didnt know about .super(). Would I have to change anything else to this code? – handroski Feb 18 '14 at 22:25
1

Take in count that super refers to the parent class, so, if parent class contains a public X attribute, you can access to it using super.X – Fabian Rivera Feb 18 '14 at 22:28
It is the same for calling parent methods and constructors. Nos, with this information, hope you can figure out how to do what you want. – Fabian Rivera Feb 18 '14 at 22:29
Thank you. So would I have to instantiate it in a constructor in either class like @Szymon mentioned? – handroski Feb 18 '14 at 22:30
1

Yeah, I think you must first initialize the array in the constructor you're calling in order to be able to use it. – Fabian Rivera Feb 18 '14 at 22:31

How do I access an array from a parent class in a child class?

2 Answers2