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本文主要介绍一下lambda表达式中的Collectors.groupingBy的使用。

//groupingBy使用        System.out.println("=======groupingBy==========");        Stream
   
     stream = Stream.of(new Person("1", "aa", "12"), new Person("1", "bb", "13"), new Person("3", "cc", "14"));        System.out.println(stream.collect(Collectors.groupingBy(x -> x.id)));         //groupingBy        Map
    
     > tempMap = Stream.of(new Person("1", "aa", "12"), new Person("1", "bb", "13"), new Person("3", "cc", "14"))                .collect(Collectors.groupingBy(x -> x.id));        for (String s : tempMap.keySet()) {            tempMap.get(s).stream().forEach(x -> System.out.println(x));        }         Map
     
      > collectGroup = Stream.of(1, 2, 3, 4)                .collect(Collectors.groupingBy(it -> it > 3));        System.out.println("collectGroup : " + collectGroup);
     
    
   

这里用到的Person类的代码：

class Person {    String id;    String name;    String age;     public String getId() {        return id;    }     public void setId(String id) {        this.id = id;    }     public String getName() {        return name;    }     public void setName(String name) {        this.name = name;    }     public String getAge() {        return age;    }     public void setAge(String age) {        this.age = age;    }     public Person() {    }     public Person(String id, String name, String age) {        this.id = id;        this.name = name;        this.age = age;    }     @Override    public String toString() {        return "Person{" +                "id='" + id + '\'' +                ", name='" + name + '\'' +                ", age='" + age + '\'' +                '}';    }}

看一下运行结果：