Thread: NullPointException errors in code?

Hello!
I'm kind of super confused on this in general.

In this code, I'm testing it through the main method that I put at the end of the code and essentially, I'm supposed to be testing that the methods execute correctly.

I keep getting nullPointException errors which I essentially know what they are, but I'm having difficulty tracing it back to where exactly doesn't have any memory address path.

Also, I didn't create a mode method completely yet.

Here's my code:

public class Stat {
 
 
		private double [] data;
 
		int x = 0;
		public Stat()
		{
			//default constructor
			double [] dataArray = new double [x];
			dataArray[x] = 0.0;
		}
 
 
		public Stat(double[] d)
		{
 
			data = new double [d.length];
			for (int i = 0; i<d.length; i++)
			{
				data[i] = d[i];
			}
 
		}
 
		private double [] getDataArray;
		public double [] getData()
		{
			getDataArray = new double [data.length];
 
			for (int i = 0; i < data.length; i++)
			{
				data [i] = getDataArray[i];
			}
 
			return getDataArray;
		}
 
		private double [] setDataArray;
		public void setData (double [] d) 
		{
			setDataArray = new double [d.length];
 
			for (int i =0; i<d.length; i++)
			{
				d[i] = setDataArray[i];
			}
 
			data = setDataArray;
		}
 
		public boolean equals(Stat s)
		{
			if (data.length == s.getData().length)
			{
				for (int i = 0; i < s.getData().length; i++)
				{
					data[i] = s.getData()[i];
				}
				return true;
			}
			else
			{
				return false;
			}
		}
 
		public String toString()
		{
			return "[" + data + "]";
 
		}
 
		double min = data[0];
		public double min()
		{
			for (int i = 0; i < data.length; i++)
			{
				if (min > data[i])
				{
					min = data[i];
				}
			}
 
			return min;
		}
 
		double max = data[0];
		public double max()
		{
			for (int i = 0; i < data.length; i++)
			{
				if (max < data[i])
				{
					max = data[i];
				}
			}
 
			return max;
		}
 
		public double average()
		{
			double sum = 0;
			double numOfElements = 0;
			for (int i = 0; i<data.length; i++)
			{
				sum = sum + data[i];
				numOfElements = numOfElements + i;
			}
 
			return sum/numOfElements;
 
		}
 
		public double mode()
		{
			//go through each element in an array and compare it to the next element and see if it is the same, then add that one to count
 
			for (int i =0; i<data.length; i++)
			{
 
 
			}
 
			return 0.0;
 
		}
 
 
 
 
 
		public static void main(String[] args)
		{
			double[] data = {-5.3, 2.5, 88.9, 0, 0.0, 28, 16.5, 88.9, 109.5, -90, 88.9};
			double[] data2 = {100.34, 50.01, 50.01, -8};
			Stat stat1 = new Stat();
			System.out.println("stat1 data = " + stat1.toString());
			stat1 = new Stat(data);
			System.out.println("stat1 has been altered.");
			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() + "\n");
			Stat stat2 = new Stat();
			stat2.setData(data2);
			Stat stat3 = new Stat(stat1.getData());
			System.out.println("stat2 data = " + stat2.toString());
			System.out.println("stat3 data = " + stat3.toString());
			System.out.println();
			System.out.println("stat1 is equal to stat2 using \"equals()\"? " +
			stat1.equals(stat2));
			System.out.println("stat1 is equal to stat3 using \"equals()\"? " +
			stat1.equals(stat3));
			System.out.println("stat1 is equal to stat3 using \"==\"? " + (stat1== stat3));
		}
 
}

Thank you so much!!

Originally Posted by Norm

Please copy the full text of the error message and paste it here. It has important info about the error.

I actually kind of figured it out! I'm sorry! Can I ask you about another problem though?
Now when I print stuff out, it's also printing out memory addresses which I don't understand why.
Also I believe there's something wrong with the getData method.
Thank you so much for helping me sort this out!

Here's the code:

public class Stat {
 
 
	private double [] data;
 
 
	public Stat()
	{
		//default constructor
		data = new double[1];
		data[0] = 0.0;
	}
 
 
	public Stat(double[] d)
	{
 
		data = new double [d.length];
		for (int i = 0; i<d.length; i++)
		{
			data[i] = d[i];
		}
 
	}
 
 
	public double [] getData()
 
	{
		double[] temp = new double[data.length];
 
		for (int i = 0; i < data.length; i++) 
		{
			data[i] = temp[i];
		}
		return temp;
	}
 
	public void setData (double [] d) 
	{
 
		data = new double[d.length];
 
        for (int i = 0; i < d.length; i++) 
        {
                data[i] = d[i];
        }
	}
 
	public boolean equals(Stat s)
	{
 
		 double[] temp = s.getData();
 
         if (data.length != temp.length) 
         {
        	 return false;
         }
         for (int i = 0; i < data.length; i++)
         {
        	 if (data[i] != temp[i]) return false;
         }
         return true;
         //feel like the return true is in a wrong place?
 
 
	}
 
	public String toString()
	{
		System.out.println(data);
        String temp = "[";
 
        for (int i = 0; i < data.length; i++) {
                temp = temp + data[i];
                if (i < data.length - 1) 
                {
                	temp = temp + ", ";
                }
        }
        return temp + "]";
        //will be a pair of braces
 
	}
 
 
	public double min()
	{
 
		if (data.length == 0) 
		{
			return 0.0;
		}
 
        double min = data[0];
 
        for (int i = 1; i < data.length; i++) 
        {
        	if (min > data[i]) 
        	{
        		min = data[i];
        	}
        }
        return min;
 
	}
 
 
	public double max()
	{
		if (data.length == 0) 
		{
			return 0.0;
		}
        double max = data[0];
 
        for (int i = 1; i < data.length; i++) 
        {
        	if (max < data[i])
        	{
        		max = data[i];
        	}
        }
        return max;
	}
 
	public double average()
	{
 
		if (data.length == 0) 
		{
			return 0.0;
		}
 
        double sum = 0;
        for (int i = 0; i < data.length; i++)
        {
        	sum = sum + data[i];
        }
        return sum / data.length;
	}
 
	public double mode()
	{
		//go through each element in an array and compare it to the next element and see if it is the same, then add that one to count
 
		for (int i =0; i<data.length; i++)
		{
 
 
		}
 
		return 0.0;
 
	}
 
 
 
 
 
	public static void main(String[] args)
	{
		System.out.println("Example 1\n");
 
		double[] data = {-5.3, 2.5, 88.9, 0, 0.0, 28, 16.5, 88.9, 109.5, -90, 88.9};
		double[] data2 = {100.34, 50.01, 50.01, -8};
		Stat stat1 = new Stat();
		//is printing out the memory address when the above is executed
		System.out.println("stat1 data = " + stat1.toString());
		stat1 = new Stat(data);
		System.out.println("stat1 has been altered.");
		//is printing out the memory address 
		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() + "\n");
		Stat stat2 = new Stat();
		//printing out memory address
		stat2.setData(data2);
		Stat stat3 = new Stat(stat1.getData());
		//probably something wrong with getData
		System.out.println("stat2 data = " + stat2.toString());
		//printing out memory address
		System.out.println("stat3 data = " + stat3.toString());
		//not printing out correctly, is just printing out the default value of the stat3
		System.out.println();
		System.out.println("stat1 is equal to stat2 using \"equals()\"? " +
				stat1.equals(stat2));
		System.out.println("stat1 is equal to stat3 using \"equals()\"? " +
				stat1.equals(stat3));
		System.out.println("stat1 is equal to stat3 using \"==\"? " + (stat1== stat3));
 
               }
    }

lol! I'm so sorry, I keep figuring stuff out after you give me these helpful hints!
I was printing out data for some reason at the beginning of my toString method.

So, I was given this "example 1" from my teacher to test this code.
When I'm printing out "stat3" in the example, it just prints out 0.0 (b/c of the default constructor) the same number of times of the length of the array. But I'm trying to make it have the same elements in the same order as the data array. the example my teacher gave me uses the getData method to do this which makes me think the getData method isn't working.

What is being printed out is :
Example 1

stat1 data = [0.0]
stat1 has been altered.
stat1 data = [-5.3, 2.5, 88.9, 0.0, 0.0, 28.0, 16.5, 88.9, 109.5, -90.0, 88.9]
stat1 min = -90.0
stat1 max = 109.5
stat1 average = 29.80909090909091
stat1 mode = 0.0

stat2 data = [100.34, 50.01, 50.01, -8.0]
stat3 data = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

stat1 is equal to stat2 using "equals()"? false
stat1 is equal to stat3 using "equals()"? true
stat1 is equal to stat3 using "=="? false