Instance variables with arrays

Question

I am trying to create an instance variable that is an array. I have many methods that will produce certain statistics about the array. I'm wondering if someone can explain to me if I am going about this the correct way. I'm rather new to Java, so any pointers is greatly appreciated. When I run the program I get errors, such like Null. I'm not looking to fix these errors now, I'm just wondering if I am going about this the correct way.

My data class:

   import java.util.Arrays;
   public class Stat {

private double data[];

public Stat()
{
    data = new double[1];
    data[0]= 0.0;
}

public Stat(double[] d)
{
    d = new double[d.length];

}

public double[] getData()
{
    return data;

}

public void setData(double[] d)
{

}

Main method:

  double[] data = {1,2,2,3,4,5};
    Stat stat1 = new Stat(data);

    System.out.println(stat1.getData());

    System.out.println("stat1 data = " + stat1.toString()); 
    System.out.println("stat1 min = " + stat1.min()); 
    System.out.println("stat1 max = " + stat1.max()); 
    System.out.println("stat1 average = " + stat1.average()); 
    System.out.println("stat1 mode = " + stat1.mode()); 
    System.out.println("stat1 data = " + stat1.toString());

Which line is giving you the NullPointerException? Including the stack trace is always a good idea when asking for help (and pointing out which line gets reported, because we don't have your exact file, so line numbers without some indication of where the problem actually is are meaningless. — Bob Gilmore
– Bob Gilmore, Commented Apr 6, 2014 at 23:50

Tyler · Accepted Answer · 2014-04-07 00:16:59Z

1

This constructor doesn't really do anything. You pass in an array in d, and then assign d to a different array when you say new, and additionally, d only lives on the stack until the method returns. Whenever this constructor is used data is never initialized and that's where your error is coming from.

Change:

public Stat(double[] d)
{
    d = new double[d.length];
}

to something like this:

public Stat(double[] d)
{
    data = d;
}

Here's what I ran on my computer:

public class Stat {

    private double data[];

    public Stat()
    {
        data = new double[1];
        data[0]= 0.0;
    }

    public Stat(double[] d)
    {
        data = d;

    }

    public double[] getData()
    {
        return data;

    }

}

public class JavaTest {

    public static void main(String[] args) {

        double[] data = {1,2,2,3,4,5};
        Stat stat1 = new Stat(data);

        System.out.println(stat1.getData()[0]);  //outputs 1.0

    }
}

edited Apr 7, 2014 at 0:16

answered Apr 6, 2014 at 23:49

Tyler

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2 Comments

Nadal Over a year ago

its saying that stat1.getData is null. Can you give me a hint on why so?

Tyler Over a year ago

Works for me: System.out.println(stat1.getData()[0]); // prints 1.0

Naili · Accepted Answer · 2014-04-07 00:40:29Z

0

Change the constructor to be

public Stat(double[] d)
{
    data = d.clone();
}

Because by using the new keyword, your are creating a new empty array. Second error, you can't print directly an array. You have to print its elements, one by one using a for loop for example. However, I suggest that you override the toString() method

@Override
public String toString(){
    return Arrays.toString(data);
}

Then printing your class will output the content of the array

System.out.println("stat1 data = " + stat1);

edited Apr 7, 2014 at 0:40

answered Apr 7, 2014 at 0:09

Naili

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1 Comment

Karol S Over a year ago

Arrays.toString(...) is your friend

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