map集合优化

通过map集合减少查询数据库次数

说明：将数据优先提取，减少访问数据库次数
示例：

if(paymentData!=null&&paymentData.size()>0) {
                 Map<String, List<PaymentBudget>> paymentBudgetMap = dao.getPaymentBudgetByYearAndNo(applyYear, budget.getCostCenterNo(), budget.getItemNo());
                 for(ContractPayment payment:paymentData) {
                     List<PaymentBudget>budgetData= paymentBudgetMap.get(payment.getPaymentId());
                     if(budgetData!=null&&budgetData.size()>0) {
                         for(PaymentBudget bud:budgetData) {
                             if(StringUtils.isBlank(bud.getBudgetNo())) {
                                 double untaxAmount=bud.getUnTaxAmount();
                                 useSubject=Arith.add(useSubject, untaxAmount);
                             }
                         }
                     } 
                 }
             }

public Map<String, List<PaymentBudget>> getPaymentBudgetByYearAndNo(int applyYear,String costCenterNo,String itemNo) {
        String hql = " from " + PaymentBudget.class.getName() + " where  year =  ? and costCenterNo =? and itemNo=  ? ";
        List<PaymentBudget> list =  find(hql,applyYear,costCenterNo,itemNo);
        Map<String, List<PaymentBudget>> mapData = new HashMap<String, List<PaymentBudget>>();
        if (list == null || list.size() == 0) {
            return mapData;
        }
        for (PaymentBudget data : list) {
            String paymentId = data.getPaymentId();
            if (mapData.containsKey(paymentId)) {
                List<PaymentBudget> listData = mapData.get(paymentId);
                listData.add(data);
                mapData.put(paymentId, listData);
            }
            else {
                List<PaymentBudget> listData = new ArrayList<PaymentBudget>();
                listData.add(data);
                mapData.put(paymentId, listData);
            }
        }
        return mapData;
    }

反例：

if(paymentData!=null&&paymentData.size()>0) {
                 for(ContractPayment payment:paymentData) {
                     List<PaymentBudget>budgetData=dao.find(" from "+PaymentBudget.class.getName()+" where  year =  ? and costCenterNo =? and itemNo=  ? and  paymentId=? ",applyYear,budget.getCostCenterNo(),budget.getItemNo(),payment.getPaymentId());
                     if(budgetData!=null&&budgetData.size()>0) {
                         for(PaymentBudget bud:budgetData) {
                             if(StringUtils.isBlank(bud.getBudgetNo())) {
                                 double untaxAmount=bud.getUnTaxAmount();
                                 useSubject=Arith.add(useSubject, untaxAmount);
                             }
                         }
                     } 
                 }
             }

将嵌套for循环简化成单层for循环，降低时间复杂度

if(paymentData!=null&&paymentData.size()>0) {
                 Map<String, List<PaymentBudget>> paymentBudgetMap = dao.getPaymentBudgetByYearAndNo(applyYear, budget.getCostCenterNo(), budget.getItemNo());
                 List<PaymentBudget> list = new ArrayList<PaymentBudget>(); 
                 for(String paymentId : paymentBudgetMap.keySet()) {
                     List<PaymentBudget>budgetData= paymentBudgetMap.get(paymentId);
                     list.addAll(budgetData);
                 }
                 if(list!=null&&list.size()>0) {
                     for(PaymentBudget bud:list) {
                         if(StringUtils.isBlank(bud.getBudgetNo())) {
                             double untaxAmount=bud.getUnTaxAmount();
                             useSubject=Arith.add(useSubject, untaxAmount);
                         }
                     }
                 } 
             }

通过map集合减少某些循环嵌套次数

说明：可将外层for循环的内容存放在map集合中，降低时间复杂度

            public void test1() {
                    List<Map<String, Object>> result = new ArrayList<>();
                    List<Map<String, Object>> result1 = new ArrayList<>();
                    List<Map<String, Object>> result2 = new ArrayList<>();
                    for (int i = 0; i < result2.size(); i++) {
                        Map<String, Object> timeNow = new HashMap<>();
                        timeNow.put("index_order", result2.get(i).get("index_order"));
                        for (int m = 0; m < result1.size(); m++) {
                            if (result2.get(i).get("index_id").equals(result1.get(m).get("index_id"))) {
                                timeNow.put("compare_value", Math.round(Double.parseDouble(result2.get(i).get("index_value") + "")/ Double.parseDouble(result1.get(m).get("index_value") + "") * 100)/ 100.00);
                            }
                        }
                        result.add(timeNow);
                    }
                }
            /**
             * map集合优化嵌套循环  
             */
            public void test2() {
                List<Map<String, Object>> result = new ArrayList<>();
                List<Map<String, Object>> result1 = new ArrayList<>();
                List<Map<String, Object>> result2 = new ArrayList<>();
                Map<Object, Map> map = new HashMap<>();
                for (int j = 0; j < result1.size(); j++) {
                    map.put(result1.get(j).get("index_id"), result1.get(j));
                }
                for (int i = 0; i < result2.size(); i++) {
                    Map<String, Object> timeNow = new HashMap<>();
                    timeNow.put("index_order", result2.get(i).get("index_order"));
                    if (map.get(result2.get(i).get("index_id")) != null) {
                        timeNow.put("compare_value",Math.round(Double.parseDouble(result2.get(i).get("index_value") + "")/ Double.parseDouble(map.get(result2.get(i).get("index_id")).get("index_value") + "")* 100) / 100.00);
                    }
                    result.add(timeNow);
                }
            }