How to write a program that reads a sequence of integers into an array and that computes the alternating sum of all elements in the array?

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Java

I am trying to write a program that reads a sequence of integers into an array and that computes the alternating sum of all elements in the array. For example, if the program is executed with the input data

1 4 9 16 9 7 4 9 11
then it computes

1 - 4 + 9 - 16 + 9 - 7 + 4 - 9 + 11 = - 2

I have below code so far:

import java.util.Arrays;

/**
   This class computes the alternating sum
   of a set of data values.
*/
public class DataSet
{
   private double[] data;
   private int dataSize;

   /**
      Constructs an empty data set.
   */
   public DataSet()
   {
      final int DATA_LENGTH = 100;
      data = new double[DATA_LENGTH];
      dataSize = 0;
   }

   /**
      Adds a data value to the data set.
      @param x a data value
   */
   public void add(double x)
   {
      if (dataSize == data.length)
         data = Arrays.copyOf(data, 2 * data.length);
      data[dataSize] = x;
      dataSize++;
   }

   /**
      Gets the alternating sum of the added data.
      @return sum the sum of the alternating data or 0 if no data has been added
   */
   public double alternatingSum()
   {
      . . .
   }
}

I have to use the following class as the tester class:

/**
   This program calculates an alternating sum.
*/
public class AlternatingSumTester
{
   public static void main(String[] args)
   {
      DataSet data = new DataSet();
      
      data.add(1);
      data.add(4);
      data.add(9);
      data.add(16);
      data.add(9);
      data.add(7);
      data.add(4);
      data.add(9);
      data.add(11);
      
      double sum = data.alternatingSum();
      System.out.println("Alternating Sum: " + sum);
      System.out.println("Expected: -2.0");
   }
}