Lambda使用

  // 查询所有栏目集合
List<ColumnEntity> columns = columnApiService.queryAllColumns();

 // 过滤type为4的数据
List<ColumnEntity> list = columns.stream().filter(p -> p.getType() == 4).collect(Collectors.toList());

// 根据Sort字段排序（正序）
List<ColumnEntity> sortList = parentList.stream().sorted(Comparator.comparing(ColumnEntity::getSort)).collect(Collectors.toList());
// 根据Sort字段排序（倒序）
List<ColumnEntity> sortList = parentList.stream().sorted(Comparator.comparing(ColumnEntity::getSort).reversed()).collect(Collectors.toList());
//ParentId>0且ParentId=Id
 List<ColumnEntity> collect2 = list.stream().filter(item -> item.getParentId() > 0 && item.getId().equals(parentList.get(0))).collect(Collectors.toList());

//转换
List<Long> userIds = strList.stream().map(s -> Long.parseLong(s == null ? "-1000" : s.trim())).collect(Collectors.toList());

//转成set
Set<Integer> ageSet = list.stream().map(Student::getAge).collect(Collectors.toSet()); // [20, 10]
 

//转成map key不能相同 否则报错
Map<String, Integer> studentMap = list.stream().collect(Collectors.toMap(Student::getName, Student::getAge)); // {cc=10, bb=20, aa=10}

//字符串分隔符连接
String joinName = list.stream().map(Student::getName).collect(Collectors.joining(",", "(", ")")); // (aa,bb,cc)

//查询总数
Long count = list.stream().collect(Collectors.counting()); 

//最大年龄 (最小的minBy同理)
Integer maxAge = list.stream().map(Student::getAge).collect(Collectors.maxBy(Integer::compare)).get(); /

//所有人的年龄
Integer sumAge = list.stream().collect(Collectors.summingInt(Student::getAge)); 

//平均年龄
Double averageAge = list.stream().collect(Collectors.averagingDouble(Student::getAge)); 

//带上以上所有方法
DoubleSummaryStatistics statistics = list.stream().collect(Collectors.summarizingDouble(Student::getAge));
System.out.println("count:" + statistics.getCount() + ",max:" + statistics.getMax() + ",sum:" + statistics.getSum() + ",average:" + statistics.getAverage());

//分组
Map<Integer, List<Student>> ageMap = list.stream().collect(Collectors.groupingBy(Student::getAge));

//多重分组,先根据类型分再根据年龄分
Map<Integer, Map<Integer, List<Student>>> typeAgeMap = list.stream().collect(Collectors.groupingBy(Student::getType, Collectors.groupingBy(Student::getAge)));

//分区
//分成两部分，一部分大于10岁，一部分小于等于10岁
Map<Boolean, List<Student>> partMap = list.stream().collect(Collectors.partitioningBy(v -> v.getAge() > 10));


       

标签：getAge,Collectors,stream,list,collect,Student,使用,Lambda
来源： https://blog.csdn.net/qq_42839232/article/details/122308141