Java 8 （lambda）表達式 Collectors中的統計、分組、排序、過濾等

#使用filter()過濾List

//添加測試數據
List<User> userList = new ArrayList<>();
studentList.add(new StudentInfo("張三",14)); studentList.add(new StudentInfo("李四",18)); studentList.add(new StudentInfo("王五",21)); studentList.add(new StudentInfo("趙六",17));

//查找年齡在18歲以上的
List<User> userFilterList = userList.stream().filter(s->s.getAge() >= 18).collect(Collectors.toList());
//輸出查找結果
StudentInfo.printStudents(userFilterList);

#使用groupingBy()進行分組后 使用averagingInt()進行求平均值

//添加測試數據　　
List<ResultModel> selectResultList = new ArrayList<>();
selectResultList.add("傳輸網IP",12,10,22);
selectResultList.add("傳輸網IP",14,10,30);
selectResultList.add("傳輸網IP",12,10,33);
selectResultList.add("傳輸網IP",17,11,41);
selectResultList.add("傳輸網IP",12,10,15);
//根據數據源id，策略id進行分組
Map<String, List<ResultModel>> countMap = sevenDaySqlList.stream().collect(Collectors.groupingBy(item -> item.getDatasourceId() + "_" + item.getPolicyId()));

for(String keys : countMap.keySet()){
    String[] temp = keys.split("_");
    Integer datasourceId = Integer.valueOf(temp[0]);
    Integer policyId = Integer.valueOf(temp[1]);
    List<ResultModel> list = countMap.get(keys);
　　 //進行求平均值計算  
    Double resultAvg = list.stream().collect(Collectors.averagingInt(ResultModel::getResultNum));
}

 

//過濾 采集結果為0的數據 且不在范圍內的數據后  並求平均值
Double resultAvg = list.stream().filter(s->s.getResultNum() > 0  && Math.max(reduc, s.getResultNum()) == Math.min(s.getResultNum(), plus) ).collect(Collectors.averagingInt(ResultModel::getResultNum));

//獲取平均值，總數，最大數，最小數，求和
IntSummaryStatistics summaryStatistics = getNowList.stream().collect(Collectors.summarizingInt(ResultModel::getResultNum));
double avgResult = summaryStatistics.getAverage();int countResult = Math.toIntExact(summaryStatistics.getCount());
int maxResult = Math.toIntExact(summaryStatistics.getMax());
int minResult = Math.toIntExact(summaryStatistics.getMin());
int sumResult = Math.toIntExact(summaryStatistics.getSum());