Java 8 - Stream Collectors类 · BeginnersBook 中文系列教程

# Java 8 - `Stream Collectors`类 > 原文： [https://beginnersbook.com/2017/10/java-8-stream-collectors-class-with-examples/](https://beginnersbook.com/2017/10/java-8-stream-collectors-class-with-examples/) 收集器是[最终类](https://beginnersbook.com/2014/07/final-keyword-java-final-variable-method-class/)，它扩展了`Object`类。在本教程中，我们将看到使用 [lambda 表达式](https://beginnersbook.com/2017/10/java-lambda-expressions-tutorial-with-examples/)，[Java 流](https://beginnersbook.com/2017/10/java-8-stream-tutorial/) 和 Java 8 的其他新功能的 Java 流收集器类的示例。 ```java java.lang.Object | |___java.util.stream.Collectors ``` ## Java - `Stream Collectors groupingBy`和计数示例在此示例中，我们使用`Collectors`类的`groupingBy()`方法对列表的元素进行分组，并打印列表中每个元素的出现次数。 ```java import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.function.Function; import java.util.stream.Collectors; public class Example { public static void main(String[] args) { List<String> names = Arrays.asList("Jon", "Ajeet", "Steve", "Ajeet", "Jon", "Ajeet"); Map<String, Long> map = names.stream().collect( Collectors.groupingBy( Function.identity(), Collectors.counting() ) ); System.out.println(map); } } ``` 输出： ```java {Steve=1, Jon=2, Ajeet=3} ``` ## Java - `Stream Collector`将数据提取为`List`的示例 ```java import java.util.stream.Collectors; import java.util.List; import java.util.ArrayList; class Student{ int id; String name; int age; public Student(int id, String name, int age) { this.id = id; this.name = name; this.age = age; } } public class Example { public static void main(String[] args) { List<Student> studentlist = new ArrayList<Student>(); //Adding Students studentlist.add(new Student(11,"Jon",22)); studentlist.add(new Student(22,"Steve",18)); studentlist.add(new Student(33,"Lucy",22)); studentlist.add(new Student(44,"Sansa",23)); studentlist.add(new Student(55,"Maggie",18)); //Fetching student names as List List<String> names = studentlist.stream() .map(n->n.name) .collect(Collectors.toList()); System.out.println(names); } } ``` 输出： ```java [Jon, Steve, Lucy, Sansa, Maggie] ``` ## Java 收集器示例 - 将数据收集为集合在这个例子中，我们将学生列表转换为[流](https://beginnersbook.com/2017/10/java-8-stream-tutorial/)，然后我们应用[ Java `Stream`过滤器](https://beginnersbook.com/2017/10/java-8-stream-filter/)从流中获取所选记录，之后我们将使用`Collectors.toSet()`方法，将该流转换为集合。 ```java import java.util.stream.Collectors; import java.util.List; import java.util.Set; import java.util.ArrayList; class Student{ int id; String name; int age; public Student(int id, String name, int age) { this.id = id; this.name = name; this.age = age; } } public class Example { public static void main(String[] args) { List<Student> studentlist = new ArrayList<Student>(); //Adding Students studentlist.add(new Student(11,"Jon",22)); studentlist.add(new Student(22,"Steve",18)); studentlist.add(new Student(33,"Lucy",22)); studentlist.add(new Student(44,"Sansa",23)); studentlist.add(new Student(55,"Maggie",18)); //Fetching student data as a Set Set<Student> students = studentlist.stream() .filter(n-> n.id>22) .collect(Collectors.toSet()); //Iterating Set for(Student stu : students) { System.out.println(stu.id+" "+stu.name+" "+stu.age); } } } ``` 输出： ```java 44 Sansa 23 33 Lucy 22 55 Maggie 18 ``` ## Java 收集器示例 - 使用`averageInt()`方法获取学生的平均年龄 ```java import java.util.stream.Collectors; import java.util.List; import java.util.ArrayList; class Student{ int id; String name; int age; public Student(int id, String name, int age) { this.id = id; this.name = name; this.age = age; } } public class Example { public static void main(String[] args) { List<Student> studentlist = new ArrayList<Student>(); //Adding Students studentlist.add(new Student(11,"Jon",22)); studentlist.add(new Student(22,"Steve",18)); studentlist.add(new Student(33,"Lucy",22)); studentlist.add(new Student(44,"Sansa",23)); studentlist.add(new Student(55,"Maggie",18)); //Getting the average Age Double avgAge = studentlist.stream() .collect(Collectors.averagingInt(s->s.age)); System.out.println("Average Age of Students is: "+avgAge); } } ``` 输出： ```java Average Age of Students is: 20.6 ``` 我已经展示了 Java `Collectors`类的极少数方法的例子。要获得完整的方法列表，请参阅 javadoc：[Java 8 `Stream Collectors` - JavaDoc](https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html)