Java8 Stream及collect收集器的使用

Posted 2021-05-20 忘忧草~

• 测试数据的准备

@Data
@NoArgsConstructor
@AllArgsConstructor
class Student{
    private String id;
    private String name;
    private String officeNo;
    private Double score;
}

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public static List<Student> getData(){
    List<Student> students = new ArrayList<>();
    Student student1 = new Student(UUID.randomUUID().toString(),"张三","N001",50.0);
    Student student2 = new Student(UUID.randomUUID().toString(),"李四","N002",66.0);
    Student student3 = new Student(UUID.randomUUID().toString(),"王五","N003",79.0);
    Student student4 = new Student(UUID.randomUUID().toString(),"赵六","N004",100.0);
    Student student5 = new Student(UUID.randomUUID().toString(),"钱七","N005",89.0);
    Student student6 = new Student(UUID.randomUUID().toString(),"王六","N005",90.0);
    Student student7 = new Student(UUID.randomUUID().toString(),"李五","N005",98.0);
    Student student8 = new Student(UUID.randomUUID().toString(),"张七","N002",99.0);
    Student student9 = new Student(UUID.randomUUID().toString(),"李五","N005",92.0);
    Student student10 = new Student(UUID.randomUUID().toString(),"张七","N002",94.0);
    Student student11 = new Student(UUID.randomUUID().toString(),"李四","N002",69.1);
    Student student12 = new Student(UUID.randomUUID().toString(),"张三","N001",90.1);
    students.add(student1);students.add(student2);students.add(student3);students.add(student4);
    students.add(student5);students.add(student6);students.add(student7);students.add(student8);
    students.add(student9);students.add(student10);students.add(student11);students.add(student12);
    return students;
}

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• 通过Map收集list集合中对象属性集合

/**
 * @description:  获取 List<Student> 中Student 对象的某个属性集合
 *  stream() 将集合转换成流对象
 *  map(Student::getOfficeNo) 获取对象集合的某个属性集合
 *  collect(Collectors.toList()) 收集 ->  Collectors.toList() 收集到一个list集合;
 *  forEach(System.out::println) 遍历结合打印到控制台
 **/
@Test
public void testMap(){
    List<Student> students = getData();
    students.stream()
            .map(Student::getOfficeNo)
            .collect(Collectors.toList())
            .forEach(System.out::println);
}

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• 通过filter过滤删选出list集合中特定属性的集合

/**
 * @description: 删选出 List<Student> 中 officeNo 为"N002"的对象集合
 **/
@Test
public void  testFilter(){
    List<Student> students = getData();
    students.stream()
            .filter(student -> student.getOfficeNo().equals("N002"))
            .collect(Collectors.toList())
            .forEach(System.out::println);
}

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• 通过Collectors.groupingBy，Collectors.summarizingDouble先分组再统计

/**
 * @description: 按照name分组统计 List<Student> 统计分数
 * Collectors.groupingBy(Function<? super T, ? extends K> classifier,Collector<? super T, A, D> downstream)
 * @author Mr.Zhang
 * @date 2019/11/29 11:30
 **/
 @Test
public void testCount(){
    List<Student> students = getData();
    Map<String, DoubleSummaryStatistics> collect =
            students.stream()
                    .collect(
                            Collectors.groupingBy(
                                    Student::getName,
                                    Collectors.summarizingDouble(Student::getScore)
                            )
                    );

    for (Map.Entry<String,DoubleSummaryStatistics> entry : collect.entrySet()){
        System.out.println(entry.getKey().toString()+ "->" +
                        "  count:" + entry.getValue().getCount()+
                         "  avg:" + entry.getValue().getAverage()+
                         "  max:" +entry.getValue().getMax()+
                         "  min:" +entry.getValue().getMin()+
                          "  sum:" + entry.getValue().getSum());
    }

    Long counting = students.stream().count();
    System.out.println("counting:"+counting);
}

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• 通过max()获取list集合中属性最大值的对象

/**
　* @description: 获取list中最大的值
　* @param
　* @return
　* @author Mr.Zhang
　* @date 2019/11/29 17:00
　**/
@Test
public void testGetMax(){
    List<Student> students = getData();
    Optional<Student> collection = students.stream().max(Comparator.comparing(Student::getScore));
    System.out.println(collection.get());
}

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• 先分组再单独获取每组中的最大值

/**
 * @description: 分组统计后获取每个分组中最大的值
 * @param
 * @return
 * @author Mr.Zhang
 * @date 2019/11/29 16:18
 **/
@Test
public void testGroupMax(){
    List<Student> students = getData();
    Map<String, Optional<Student>> collect =
            students.stream()
                    .collect(
                            Collectors.groupingBy(
                                    Student::getName,
                                    Collectors.maxBy(Comparator.comparing(Student::getScore))
                            )
                    );

    for (Map.Entry<String,  Optional<Student>> entry : collect.entrySet()){
        System.out.println(entry.getKey()+":"+entry.getValue().get());
    }
}

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• 将list对象转换成map对象

/**
　* @description: 将list对象转换成map对象
　* @param 
　* @return  
　* @author Mr.Zhang
　* @date 2019/12/1 2:12 
　**/
public void testToMap(){
    List<Student> students = getData();
    Map<String, Student> collect = students.stream().collect(Collectors.toMap(Student::getId,student -> student));
    collect.entrySet().stream().forEach(t->{
        System.out.println(t.getKey()+":"+t.getValue().getName());
    });
}

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