使用Java Stream API将List按自定义分组规则转换成Map的一个例子
本文完整测试代码见文末。 测试数据是List里的4个员工对象实例： 根据员工所在的城市进行分组： 结果分成了三组： 第一组的员工在上海： 第二组的员工在成都： 统计每组员工个数： 把员工进行分组，得分大于101分的在一组，小于等于101的在另一组： 分组结果： package java8; import java.util.ArrayList; import java.util.List; import java.util.Map; import java.util.Map.Entry; import java.util.function.Consumer; import java.util.stream.Collectors; class Employee { private String city; private String name; private int score; public Employee(String name, String city, int score){ this.city = city; this.name = name; this.score = score; } public String getCity(){ System.out.println("city: " + this.city); return this.city; } public String getName() { return this.name; } public int getScore() { return this.score; } @Override public String toString() { return String.format("Employee: " + this.name + " city: " + this.city); } } class Person { private String name; private int age; Person(String name, int age) { this.name = name; this.age = age; } @Override public String toString() { return String.format("Person{name='%s', age=%d}", name, age); } } // Jerry 2016-01-15 20:51PM ? 多用于extends generic的type，接受所有Object的sub class public class StreamTest { private static void printMap(Map<? extends Object, ? extends Object> map) { for(Entry<? extends Object, ? extends Object> entry:map.entrySet()) { System.out.println("key = " + entry.getKey() + " , Value = " + entry.getValue()); } } public static void main(String[] args) { ArrayList<Employee> employees = new ArrayList<Employee>(); employees.add(new Employee("A", "Shanghai",100)); employees.add(new Employee("B", "Chengdu",101)); employees.add(new Employee("C", "Shenzhen",102)); employees.add(new Employee("D", "Chengdu",104)); // group by City Map<String, List<Employee>> employeesByCity = employees.stream().collect( Collectors.groupingBy(Employee::getCity)); // default void forEach(Consumer<? super T> action) { for(Map.Entry<String, List<Employee>> entry:employeesByCity.entrySet()) { System.out.println("key= " + entry.getKey() + " , Value = " + entry.getValue()); entry.getValue().forEach(System.out::println); } // 2016-01-15 20:37PM Consumer<Employee> aa = a -> { System.out.println("Employee: " + a.getName() + " : " + a.getScore()); }; List<Employee> chengduEmployee = employeesByCity.get("Chengdu"); chengduEmployee.forEach(aa); // test for counting Map<String, Long> employeesByCity2 = employees.stream().collect( Collectors.groupingBy(Employee::getCity, Collectors.counting())); printMap(employeesByCity2); // calculate average score Map<String, Double> employeesByCity3 = employees.stream().collect( Collectors.groupingBy(Employee::getCity, Collectors.averagingInt(Employee::getScore))); printMap(employeesByCity3); /*Stream<Person> people = Stream.of(new Person("Paul", 24), new Person("Mark", 30), new Person("Will", 28)); Map<Integer, List<String>> peopleByAge = people.collect(groupingBy(p -> p.age, mapping((Person p) -> p.name, toList()))); System.out.println(peopleByAge);*/ /* * 分区是一种特殊的分组，结果 map 至少包含两个不同的分组——一个true，一个false。 * 例如，如果想找出最优秀的员工，你可以将所有雇员分为两组，一组销售量大于 N， * 另一组小于 N，使用 partitioningBy 收集器： */ System.out.println("partition result"); Map<Boolean, List<Employee>> partitioned = employees.stream().collect(Collectors.partitioningBy(e -> e.getScore() > 101)); printMap(partitioned); /* * 你也可以将 groupingBy 收集器传递给 partitioningBy 收集器来将联合使用分区和分组。例如，你可以统计每个分区中的每个城市的雇员人数： Map<Boolean, Map<String, Long>> result = employees.stream().collect(partitioningBy(e -> e.getNumSales() > 150, groupingBy(Employee::getCity, counting()))); 这样会生成一个二级 Map: {false={London=1}, true={New York=1, Hong Kong=1, London=1}} */ } } 本文来自云栖社区合作伙伴“汪子熙”，了解相关信息可以关注微信公众号"汪子熙"。
