I'm using the program to send data from database to the Excel file . It works fine at the beginning and then becomes more and more slowly,finally it run out of the memory and the following error ocurrs: "java.lang.OutOfMemoryError: Java heap space...".

The problem can be resolved by adding the jvm heap sapce.But the question is that it spends too much time to run out the program.

After several minutes,it finished a loop with 4 seconds which can be finished with 0.5 seconds at the beginning . I can't found a solution to make it always run in a certain speed.

Is it my code problem?

Any clues on this?

Here is the code:

public void addAnswerRow(List<FinalUsers> finalUsersList,WritableWorkbook book){

   if (finalUsersList.size() >0 ) {
    try {
       WritableSheet  sheet = book.createSheet("Answer", 0);
       int colCount = 0;
       sheet.addCell(new Label(colCount++,0,"Number"));
       sheet.addCell(new Label(colCount++,0,"SchoolNumber"));
       sheet.addCell(new Label(colCount++,0,"District"));
       sheet.addCell(new Label(colCount++,0,"SchoolName"));
       sheet.setColumnView(1, 15);
       sheet.setColumnView(3, 25);

       List<Elements> elementsList = this.elementsManager.getObjectElementsByEduTypeAndQuestionnaireType(finalUsersList.get(0).getEducationType().getId(),     this.getQuestionnaireByFinalUsersType(finalUsersList.get(0).getFinalUsersType().getId()));

       Collections.sort(elementsList, new Comparator<Elements>(){

           public int compare(Elements o1, Elements o2) {

             for(int i=0; i< ( o1.getItemNO().length()>o2.getItemNO().length()?  o2.getItemNO().length(): o1.getItemNO().length());i++){
                  if (CommonFun.isNumberic(o1.getItemNO().substring(0, o1.getItemNO().length()>3? 4: o1.getItemNO().length()-1)) && !CommonFun.isNumberic(o2.getItemNO().substring(0, o2.getItemNO().length()>3? 4: o2.getItemNO().length()-1))){
                 return 1;
                  }
                 if (!CommonFun.isNumberic(o1.getItemNO().substring(0, o1.getItemNO().length()>3? 4: o1.getItemNO().length()-1)) && CommonFun.isNumberic(o2.getItemNO().substring(0,o2.getItemNO().length()>3? 4:o2.getItemNO().length()-1))){ 
                 return -1;
                 }  
                if ( o1.getItemNO().charAt(i)!=o2.getItemNO().charAt(i) ){

                    return   o1.getItemNO().charAt(i)-o2.getItemNO().charAt(i);
                 }
           }
           return  o1.getItemNO().length()> o2.getItemNO().length()? 1:-1;
       }});

       for (Elements elements : elementsList){
           sheet.addCell(new Label(colCount++,0,this.getTitlePre(finalUsersList.get(0).getFinalUsersType().getId(), finalUsersList.get(0).getEducationType().getId())+elements.getItemNO()+elements.getItem().getStem())); 
       }

       int sheetRowCount =1;
       int sheetColCount =0;

       for(FinalUsers finalUsers : finalUsersList){

          sheetColCount =0;

          sheet.addCell(new Label(sheetColCount++,sheetRowCount,String.valueOf(sheetRowCount)));
          sheet.addCell(new Label(sheetColCount++,sheetRowCount,finalUsers.getSchool().getSchoolNumber()));
          sheet.addCell(new Label(sheetColCount++,sheetRowCount,finalUsers.getSchool().getDistrict().getDistrictNumber().toString().trim()));
          sheet.addCell(new Label(sheetColCount++,sheetRowCount,finalUsers.getSchool().getName()));

          List<AnswerLog> answerLogList = this.answerLogManager.getAnswerLogByFinalUsers(finalUsers.getId());


          Map<String,String> answerMap = new HashMap<String,String>();

          for(AnswerLog answerLog :answerLogList ){
             if (answerLog.getOptionsId() != null)
             {
                answerMap.put(answerLog.getElement().getItemNO(), this.getOptionsAnswer(answerLog.getOptionsId()));
             }else if (answerLog.getBlanks()!= null){

                answerMap.put(answerLog.getElement().getItemNO(), answerLog.getBlanks());
             }else{

                answerMap.put(answerLog.getElement().getItemNO(), answerLog.getSubjectiveItemContent());  
             }   
          }
          for (Elements elements : elementsList){

             sheet.addCell(new Label(sheetColCount++,sheetRowCount,null==answerMap.get(elements.getItemNO())?"0":answerMap.get(elements.getItemNO())));

          }

         sheetRowCount++; 
       }

       book.write();
       book.close();

     } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
     } catch (RowsExceededException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
     } catch (WriteException e) {

      // TODO Auto-generated catch block
      e.printStackTrace();
   }

} }