java-list-分组方法

2019-08-31

1.简介

项目开发经常会碰到数据分组，大多数通过sql group by处理，但有时也需要在java做分组。

2.方法

定义一个Student类


public class Student {
	  private Integer id;
	  private Integer age;
	  private String name;
	  public Student(Integer couponId, Integer price, String name) {
	    this.id = couponId;
	    this.age = price;
	    this.name = name;
	  }
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public Integer getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
}

定义一个测试类


public class ListGroupBy {
	public static void main(String[] args) {
		List<Student> studentList = new ArrayList<Student>();
		Student student1 = new Student(1, 16, "学生a");
		Student student2 = new Student(2, 16, "学生b");
		Student student3 = new Student(3, 17, "学生c");
		Student student4 = new Student(3, 18, "学生d");
		studentList.add(student1);
		studentList.add(student2);
		studentList.add(student3);
		studentList.add(student4);
		
}

2.1 分组方法1

// 分组
	Map<String, List<Student>> groupMap2 = new HashMap<String, List<Student>>();
	for (Student Student : studentList) {
		String key = Student.getAge() + "";
		if (groupMap2.containsKey(key)) {
			groupMap2.get(key).add(Student);
		} else {
			List<Student> grouplist = new ArrayList<Student>();
			grouplist.add(Student);
			groupMap2.put(key, grouplist);
		}
	}
	System.out.println(groupMap2);
}

2.2分组方法2

使用JDK1.8 stream 方法，一行代码搞定，Collectors还有多种方法，有兴趣自行尝试。

1 2	Map<Integer, List<Student>> groupMap1 = studentList.stream().collect(Collectors.groupingBy(Student::getAge)); System.out.println(groupMap1);

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