/**
   This class computes the alternating sum
   of a set of data values.
*/
public class DataSet
{
   /**
      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)
      {
         // make a new array of twice the size
         double[] newData = new double[2 * data.length];
         // copy over all elements from data to newData
         System.arraycopy(data, 0, newData, 0, data.length);
         // abandon the old array and store in data
         // a reference to the new array
         data = newData;
   }
   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()
{
   . . .
}
   private double[] data;
   private int dataSize;
}

Use the following class as your 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");
   }
}