将list中的数据通过某一个字段来分类存储的实例

现有学生表

public class Student {
	/**
	 * 班级id
	 */
	private String classId;
	/**
	 * 学生name
	 */
	private String name;
	public Student() {
	}
	
	public Student(String classId, String name) {
		super();
		this.classId = classId;
		this.name = name;
	}
	public String getClassId() {
		return classId;
	}
	public void setClassId(String classId) {
		this.classId = classId;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
}

往student里存数据，并通过classId将student的list进行分类

public static void main(String[] args) {
		List<Student>allData=new ArrayList<Student>();
		allData.add(new Student("class1","张三"));
		allData.add(new Student("class1","李四"));
		allData.add(new Student("class1","王五"));
		allData.add(new Student("class2","赵六"));
		allData.add(new Student("class2","scy"));
		
		Map<String, List<Student>>map=new HashMap<String, List<Student>>();
		for(Student student:allData){
			if(map.get(student.getClassId())==null){
				List<Student>list=new ArrayList<Student>();
				list.add(student);
				map.put(student.getClassId(), list);
			}else{
				List<Student>list=map.get(student.getClassId());
				list.add(student);			}
		}
		for (Student stu:map.get("class1")) {
			System.out.println("班级id为："+stu.getClassId()+"的"+stu.getName());
		}
		System.out.println("------------------------------");
		for (Student stu:map.get("class2")) {
			System.out.println("班级id为："+stu.getClassId()+"的"+stu.getName());
		}
		
	}

输出结果：

班级id为：class1的张三
班级id为：class1的李四
班级id为：class1的王五
------------------------------
班级id为：class2的赵六
班级id为：class2的scy

这样之后，map中key为class1的对象有三个，key为class2的对象有两个，可以通过这样的分类之后再将数据进行插入或修改就很方便了

顺便说一下遍历map，执行插入或修改操作的相关代码:

List<Student> students = new ArrayList<Student>();
Student stu1=new Student();
for (String key : map.keySet()) {
	for (Student stu2 : map.get(key)) {
		stu1.setClassId(stu2.getClassId());
		stu1.setName(stu2.getName());
		students.add(stu1);
	}
	addStudent(students);//执行插入方法
	students.clear();//插入完后清空，第二次循环在往里面存值
}