Java SE 8 学习笔记

Java SE 8 学习笔记 一 Stream流的使用 1.Stream流获取总和

  double sum = retIndustryPercentages.stream().mapToInt(RetIndustryPercentage::getCount).sum();

2.Stream流转换为map

  Map map = retIndustryPercentages.stream().collect(Collectors.toMap(RetIndustryPercentage::getIndustry, RetIndustryPercentage::getCount));

转换为对象：
  Map map = industryList.getData().stream().collect(Collectors.toMap(RetIndustry::getDisplayName, Function.identity()));

3. Stream流的filter使用

获得任意一个匹配的值
Optional result = words.parallel().filter(s->s.startWith("Q")).findAny();
或者
Optional result = words.parallel().anyMatch(s->s.startWith("Q"));
获得第一个
Optional result = words.filter(s->s.startWith("Q")).findFirst();

4. Stream流的map使用

 Set ids = data.stream().map(UserEntity::getId).collect(Collectors.toSet());

5. Stream收集结果

（1）获得指定返回类型的数组

String[] result = words.toArray(String[]::new);

（2）获得指定返回类型的HashSet

HashSet result = stream.collect(HashSet::new,HashSet::add,HashSet::addAll);

（3）获得指定返回类型的set

 Set ids = data.stream().map(UserEntity::getId).collect(Collectors.toSet());

（4）获得指定返回类型的map

 Map map = retIndustryPercentages.stream().collect(Collectors.toMap(RetIndustryPercentage::getIndustry, RetIndustryPercentage::getCount));

（5）获得指定返回拼接的字符串

String result = stream.map(Object::toString).collect(Collections.joining(","));

(6）获得总和，平均值，最值

（7）根据实体的字段属性去重

 List list =  result.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(()->new TreeSet<>(Comparator.comparing(RetElementTypeRole::getElementType))),ArrayList::new));

（8）将结果用逗号拼接字符串

 auditTargetNodeEntity.setUsers(userIdList.stream().map(e -> e.toString()).collect(Collectors.joining(",")));

（9）将结果保存为集合

List userIds = new ArrayList();
reqStartFlow.getTargetNodeUsers().forEach(reqTargetNodeUser -> userIds.addAll(reqTargetNodeUser.getUserIds()));

6. Stream的分组分片

（1）goupingBy分组

收集每个州所有城市的人口总数
Map result = cities.collect(groupingBy(City::getState,summingInt(City::getPopulation)));
收集每个州中人口最多的城市
Map result = cities.collect(groupingBy(City::getState,maxBy(Comparator.comparing(City::getPopulation))));
收集每个州的城市的名称并按照其最大长度进行聚合
Map> result = cities.collect(groupingBy(City::getState,mapping(City::getName,maxBy(Comparator.comparing(String::length)))));

（2）reducing分片

收集每个州中的所有城市名称的字符串，名字之间用逗号分隔
Map result = cities.collect(groupingBy(City::getState,reducing("",City::getName,(s,t)->s.length==0?t:s+", "+t)));
或者
Map result = cities.collect(groupingBy(City::getState,mapping(City::getName,joining(", ")))));

二 NumberFormat的使用 1 百分数精度值格式化

private static String formattedDecimalToPercentage(double decimal) {
        //获取格式化对象
        NumberFormat nt = NumberFormat.getPercentInstance();
        //设置百分数精确度2即保留两位小数
        nt.setMinimumFractionDigits(2);
        return nt.format(decimal);
    }

三 Collections集合的使用 1. 排序

   Collections.sort(retMonthDocCounts, Comparator.comparing(RetMonthDocCount::getMonthStr));
参数说明：集合，集合里的对象的属性进行排序

四 Sql语句 1. 日期格式化

     SELECt DATE_FORMAT((CURDATE() - INTERVAL 12 MONTH), '%Y-%m')                         monthStr,
               count(DISTINCT (IF(industry IN (#{kecdl}, #{kacdl}), doc_no, NULL)))          businessCount,
               count(DISTINCT (IF(industry IN (#{gcjx}, #{gydl}, #{nyzbdl}), doc_no, NULL))) nonRoadCount,
               count(DISTINCT doc_no)                                                        sumCount
        FROM doc_file
        WHERe deleted = FALSE
          AND cancel_flag = false
          AND `status` = 10
          AND sub_category = '6'
          AND industry IN (#{kecdl}, #{kacdl}, #{gcjx}, #{gydl}, #{nyzbdl})
          AND created_at < CONCAt(DATE_FORMAT((CURDATE() - INTERVAL 11 MONTH), '%Y-%m'), '-01')