test programme not working - compilation error?

Cannot understanding why my test file does not compile?

/*This programme tests the DataLog class*/

public class DataLogTestProg
{
      public static void main(String [] args)
      {
            DataLog d1, d2;//declare two DataLog objects
            d1 = new DataLog();//create the first DataLog instance
            d2 = new DataLog();//create the second DataLog instance

            System.out.println("d1:" + d1.toString());
            System.out.println("d2:" + d2.toString());

            d1.setData(4);//send value 4 to ojbect 1
            d2.setData(6);//send value 6 to ojbect 2
      }
}

public class DataLog
{
      private int x;//the current data value
      private int lastx;//the previous data value

      //Constructors

      /*Default constructor, sets data to zero*/

      public DataLog()
      {
            x = 0;
            lastx = 0;
      }

      /*Sets data to initialValue.  This is also the previous value*/

      public DataLog(int initialValue)
      {
            x = initialValue;
            lastx = initialValue;
      }

      //Methods

      /*Sets data to dataValue*/

      public void setData(int dataValue)
      {
            lastx = x;
            x = dataValue;
      }

      /*Returns the current data value*/

      public int getData()
      {
            return x;
      }

      /*Restores the previous value of the data value*/

      public void undo()
      {
            x = lastx;
      }

      /*Returns a text representation of the current and last value of the data value*/

      public String toString()
      {
            return "new value" + x + "last value" + lastx;
      }
}