清除列表還會清除Java中以前的列表記錄

Question

我有一個數據值列表，我將其插入到HashMap中供請求讀取，我將這些數據輸入到JSP中的表中。我在這裏遇到的問題是，我會成功將數據添加到相關列表中，然後將其添加到散列映射中，但是一旦我清除列表以檢索新年的新數據，它也會清除記錄的值哈希映射。如何阻止這個動態調整我的散列表？感謝您的時間。

這是我的代碼，雖然我不認爲這是問題，因爲我缺乏知識。

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { 

    try { 
     Class.forName("com.mysql.jdbc.Driver"); 
    } catch (ClassNotFoundException e1) { 
     e1.printStackTrace(); 
    } 

    List<MyDataType> dataList = null; 
    try { 
     dataList = ***; 
    } catch (SQLException e) { 
     // TODO Auto-generated catch block 
     e.printStackTrace(); 
    } 

    HashMap<String, List<BigDecimal>> recordYearlyData = new HashMap<>(); 
    String nwC = "New Customers "; 
    String alC = "All Customers "; 
    String lsC = "Lost Customers "; 
    String rtC = "Rate "; 

    List<BigDecimal> startCus = new ArrayList<>(); 

    List<BigDecimal> newCus = new ArrayList<>(); 

    List<BigDecimal> endCus = new ArrayList<>(); 

    List<BigDecimal> thisYearCus = new ArrayList<>(); 

    List<BigDecimal> lostCus = new ArrayList<>(); 

    List<BigDecimal> rateL = new ArrayList<>(); 

    for (MyDataType dataType : dataList) { 

     String createDate = dataType.getOrderDate().toString(); 
     String createDateN = createDate.substring(5, 7); 
     Integer getDateKey = Integer.parseInt(createDateN) - 1; 
     Integer getYear = Integer.parseInt(createDate.substring(0,4)); 
     BigDecimal tenant_id = dataType.getTenantID(); 

     createDateLabel = monthNames[getDateKey]; 

     if (getYear != yearChecker) { 
      String yearLabel = String.valueOf(getYear - 1); 
      for (BigDecimal tenant : thisYearCus) { 
       if (newCus.contains(tenant) || startCus.contains(tenant)) { 
        endCus.add(tenant); 
       } 
      } 
      for (BigDecimal tenant : startCus) { 
       if (!thisYearCus.contains(tenant)) { 
        lostCus.add(tenant); 
       } 
      } 

      double rate = 1.0; 
      if (startCus.size() == 0) { 
       rate=1.0; 
      } else if (startCus.size() > 0){ 
       rate = ((double) endCus.size() - (double) newCus.size())/(double) startCus.size(); 
      } 

      rateL.add(BigDecimal.valueOf(rate)); 

      recordYearlyData.put(alC+yearLabel, thisYearCus); 
      recordYearlyData.put(nwC+yearLabel, newCus); 
      recordYearlyData.put(lsC+yearLabel, lostCus); 
      recordYearlyData.put(rtC+yearLabel, rateL); 

      rateL.clear(); 
      startCus.clear(); 
      startCus.addAll(thisYearCus); 
      thisYearCus.clear(); 
      endCus.clear(); 
      newCus.clear(); 
      lostCus.clear(); 
      yearChecker = getYear; 
     } 

     if (!thisYearCus.contains(tenant_id)) { 
      thisYearCus.add(tenant_id); 
     } 
     if (!startCus.contains(tenant_id)) { 
      if(!newCus.contains(tenant_id)) { 
       newCus.add(tenant_id); 
      } 
     } 
    } 



    request.setAttribute("newCusTable", recordYearlyData.get("New Customers 2015")); 
    request.setAttribute("allCusTable", recordYearlyData.get("All Customers 2015")); 
    request.setAttribute("lostCusTable", recordYearlyData.get("Lost Customers 2015")); 
    request.setAttribute("retRate", recordYearlyData.get("Rate 2015")); 

    RequestDispatcher view = request.getRequestDispatcher("/Sales.jsp"); 
    view.forward(request, response); 
} 

Answer 1

會發生什麼事是，你繼續插入到你的地圖相同的列表引用，而不是每次創建一個新的列表。要修復，將表聲明爲for循環：

for (MyDataType dataType : dataList) { 
    List<BigDecimal> startCus = new ArrayList<>(); 
    List<BigDecimal> newCus = new ArrayList<>(); 
    List<BigDecimal> endCus = new ArrayList<>(); 
    List<BigDecimal> thisYearCus = new ArrayList<>(); 
    List<BigDecimal> lostCus = new ArrayList<>(); 
    List<BigDecimal> rateL = new ArrayList<>(); 
    ...