JDK1.8 Stream Collectors常用方法 分组 平均值 去重 BigDecimal求和等

 首先初始化一个集合，用于我们下方的操作

Student stu1 = new Student(1, "dugt", 20);
Student stu2 = new Student(1, "lym", 10);
Student stu3 = new Student(3, "zs", 18);
Student stu4 = new Student(4, "ls", 18);
Student stu5 = new Student(4, "ww", null);
List<Student> list = Arrays.asList(stu1, stu2, stu3, stu4, stu5);

averagingInt ... averagingDouble 平均值

// 求平均年龄
Double averageAge = list.stream().collect(Collectors.averagingInt(item -> item.getAge() == null ? 0 : item.getAge()));

 count 求个数

// 求年龄为18的人数
long ageIs18 = list.stream().filter(item -> item.getAge() != null).filter(item -> item.getAge().compareTo(18) == 0).count();

 groupingBy 分组

// 根据id分组
Map<Integer, List<Student>> map = list.stream().collect(Collectors.groupingBy(Student::getId));

 groupingByConcurrent 保证线程安全分组

// 线程安全分组
ConcurrentMap<Integer, List<Student>> concurrentMap = list.stream().collect(Collectors.groupingByConcurrent(Student::getId));

 map 根据某字段分组

//根据学生姓名集合
List<String> nameList = list.stream().map(Student::getName).collect(Collectors.toList());

 根据id去重

// 根据id去重

List<Student> collect = new ArrayList<>(list.stream().collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Student::getId)))));

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

max min 最大值最小值 

// 求出年龄最大的同学
Student maxAgeStu = list.stream().filter(item -> item.getAge() != null).max(Comparator.comparing(Student::getAge)).get();

数求和 

// BigDecimal 
BigDecimal lastMonthMoneySum = lastMonthBtBillList.stream().map(OrderBtDto::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);

//double int long
double lastMonthMoneySum = list.stream().mapToDouble(OrderBtDto::getMoney).sum();

 jdk1.8新特性stream