一:多字段分组统计
方法一
public static void main(String[] args) {
List<Test> list = new ArrayList<>();
for(int i = 0; i< 5; i++){
Test test = new Test();
//主键id
test.setId(Long.valueOf(i));
//金额
test.setMoney(new BigDecimal(i));
//数量
test.setCount(new BigDecimal(i));
list.add(test);
}
for(int i = 0; i< 2; i++){
Test test = new Test();
//主键id
test.setId(Long.valueOf(i));
//金额
test.setMoney(new BigDecimal(i));
//数量
test.setCount(new BigDecimal(i));
list.add(test);
}
List<Test> resultList = new ArrayList<>();
//先根据id分组
Map<Long,List<Test>> map = list.stream().collect(Collectors.groupingBy(m->m.getId()));
//分组后求 总金额,总数量
map.keySet().stream().forEach(key->{
Test info = new Test();
List<Test> listInfo = map.get(key);
//总金额
BigDecimal sumMoney = listInfo.stream().map(Test::getMoney).reduce(BigDecimal.ZERO,BigDecimal::add);
//总数量
BigDecimal sumCount = listInfo.stream().map(Test::getCount).reduce(BigDecimal.ZERO,BigDecimal::add);
info.setId(key);
info.setMoney(sumMoney);
info.setCount(sumCount);
resultList.add(info);
});
System.out.println(resultList);
}
方法二
List<Student> collect = stus.stream().collect(Collectors.toMap(k ->k.getId()+k.getBj(), e -> e, (o1, o2) -> {
o1.setK0qmkcje((null==o1.getK0qmkcje()?BigDecimal.ZERO:o1.getK0qmkcje()).add((null==o2.getK0qmkcje()?BigDecimal.ZERO:o2.getK0qmkcje())).setScale(0,BigDecimal.ROUND_HALF_UP));
return o1;
})).values().stream().collect(Collectors.toList());
获取此类格式数据 Map<String, BigDecimal>
public static void main(String[] args) {
Map<String, BigDecimal> ageGroup = list.stream().collect(Collectors.groupingBy(Student::getName
, Collectors.collectingAndThen(Collectors.toList()
, x -> x.stream().map(Student::getStature).reduce(BigDecimal.ZERO, BigDecimal::add))));
}
public static void main(String[] args) {
Map<String, BigDecimal> ageGroup = list.stream().collect(Collectors.groupingBy(Student::getName
, MyCollector.summingDecimal(Student::getStature)));
System.out.println(ageGroup);
}