Putting all together

   import org.w3c.dom.*;
   import org.apache.xerces.parsers.DOMParser;

   class Grades
   { 
      static float grades[][] = new float[100][5];
      static int nstudent = 0;
      static int gi = -1;

      public static void computeGrades(String uri)
      {
         try {
         DOMParser parser = new DOMParser();
         parser.parse(uri);
         Document doc = parser.getDocument();
         traverse_tree(doc);
         compute_final_grades();
         } catch (Exception e) {
            e.printStackTrace(System.err);
         }
      }

      private static void compute_final_grades()
      {
         float Ave = 0;
         int i = 0, j = 0;
         System.out.println("Grades");
         for(i = 0; i < nstudent ; i++) 
         {
            float total = 0;
            for(j = 0; j < 4; j++) {
               total += grades[i][j];
            }		
            grades[i][4] = total/4;
            Ave += grades[i][4];
            System.out.println("Student " + i + "=" + grades[i][4]);
         }
      Ave /= nstudent;
      System.out.println("Class Average =" + Ave);  
      }

      private static void traverse_tree(Node node)
      {
         if(node == null) {
            return;
         }
         int type = node.getNodeType();
         switch (type) {
            case Node.DOCUMENT_NODE: {
               traverse_tree(((Document)node).getDocumentElement());
               break;
            }
            case Node.ELEMENT_NODE: {
               String elementName = node.getNodeName();
               gi = -1;
               if(elementName.equals("hw1"))
                  gi = 0;
               else if(elementName.equals("hw2"))
                  gi = 1;
               else if(elementName.equals("project"))
                  gi = 2;
               else if(elementName.equals("final"))
                  gi = 3;
               else if(elementName.equals("student"))
                  nstudent++;

               NodeList childNodes = node.getChildNodes();	
               if(childNodes != null) {
                  int length = childNodes.getLength();
                  for (int loopIndex = 0; loopIndex < length ; loopIndex++)
                  {
                     traverse_tree(childNodes.item(loopIndex));
                  }
               }
               break;
            }
            case Node.TEXT_NODE: {
               String chData = node.getNodeValue().trim();
               if(chData.indexOf("\n") < 0 && chData.length() > 0) {
                  if(gi >= 0)
                  grades[nstudent-1][gi] = Integer.parseInt(chData);
               }
            }
         }
      }
   }

   public class DOMGrades
   {
      public static void main(String[] args)
      {
         Grades.computeGrades(args[0]);
      }
   }