标签：operator group 元素 system ide hashset 实现 均值 list()
1、reduce（T identity,Binaryoperator)/reduce(Binaryoperator)一可以将流中元素反复结合起来，得到一个值。
List<Employee> employees = Arrays.asList(// new Employee(20, "张三", 5000.35, Status.FREE), // new Employee(40, "李四", 6500.63, Status.BUSY), // new Employee(30, "王五", 4000.93, Status.FREE), // new Employee(50, "赵六", 9005.36, Status.BUSY), // new Employee(10, "马七", 1050.93, Status.VOCATION), // new Employee(20, "朱八", 3000.73, Status.BUSY)// ); @Test public void test1() { List<Integer> asList = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); Integer reduce = asList.stream().reduce(0, (x, y) -> x + y); System.out.println(reduce); System.out.println("===================="); Optional<Double> reduce2 = employees.stream().map(Employee::getSalary).reduce(Double::sum); System.out.println(reduce2.get()); }
2、收集co1lect—将流转换为其他形式。接收一个Co1lector接口的实现，用于给Stream中元素做汇总的方法
@Test public void test2() { employees.stream().map(Employee::getName).collect(Collectors.toList()).forEach(System.out::println); System.out.println("===================="); employees.stream().map(Employee::getName).collect(Collectors.toSet()).forEach(System.out::println); System.out.println("===================="); employees.stream().map(Employee::getName).collect(Collectors.toCollection(HashSet::new)) .forEach(System.out::println); }
3、统计
@Test public void test3() { // 数量 Long collect = employees.stream().collect(Collectors.counting()); System.out.println(collect); // 平均值 System.out.println("===================="); Double collect2 = employees.stream().collect(Collectors.averagingDouble(Employee::getSalary)); System.out.println(collect2); // 总和 System.out.println("===================="); Double collect3 = employees.stream().collect(Collectors.summingDouble(Employee::getSalary)); System.out.println(collect3); // 最大值 System.out.println("===================="); Optional<Employee> collect4 = employees.stream() .collect(Collectors.maxBy((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()))); System.out.println(collect4.get()); // 最小值 System.out.println("===================="); Optional<Double> collect5 = employees.stream().map(Employee::getSalary) .collect(Collectors.minBy(Double::compare)); System.out.println(collect5.get()); }
4、分组
@Test public void test4() { Map<Status, List<Employee>> collect = employees.stream().collect(Collectors.groupingBy(Employee::getStatus)); System.out.println(collect); System.out.println("===================="); // 多级分组 Map<Status, Map<String, List<Employee>>> collect2 = employees.stream() .collect(Collectors.groupingBy(Employee::getStatus, Collectors.groupingBy((e) -> { if (((Employee) e).getAge() <= 35) { return "青年"; } else if (((Employee) e).getAge() <= 50) { return "中年"; } else { return "老年"; } }))); System.out.println(collect2); }
5、分区partitioningBy
@Test public void test5() { Map<Boolean, List<Employee>> collect = employees.stream() .collect(Collectors.partitioningBy((e) -> e.getSalary() > 5000)); System.out.println(collect); }
6、summarizingDouble
@Test public void test6() { DoubleSummaryStatistics collect = employees.stream().collect(Collectors.summarizingDouble(Employee::getSalary)); System.out.println(collect.getCount()); System.out.println(collect.getAverage()); System.out.println(collect.getMax()); System.out.println(collect.getMin()); }
7、joining
@Test public void test7() { String collect = employees.stream().map(Employee::getName).collect(Collectors.joining(",", "====", "===")); System.out.println(collect); }
标签：operator group 元素 system ide hashset 实现 均值 list()
原文地址：https://www.cnblogs.com/zhanh247/p/11854451.html
