我们知道TreeMap可以实现Map的键元素的排序功能，但是要求键元素的类型是可比较的(Comparable)，也就是键元素类型必须实现Comparable接口。

例如：

class Students {


    private String sid; //学号
    private String name; //姓名
    private int score; //分数

    public Students() {
    }

    public Students(String sid, String name, int score) {
        this.sid = sid;
        this.name = name;
        this.score = score;
    }

    public String getSid() {
        return sid;
    }

    public void setSid(String sid) {
        this.sid = sid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }

    @Override
    public String toString() {
        return "Students{" +
                "sid='" + sid + '\'' +
                ", name='" + name + '\'' +
                ", score=" + score +
                '}';
    }
}

public class Test {

    public static void main(String[] args) {


        Map map = new TreeMap();

        map.put(new Students("S001", "张三", 89), "北京市");
        map.put(new Students("S002", "李四", 98), "北京市");
        map.put(new Students("S003", "王五", 78), "北京市");
        map.put(new Students("S004", "赵六", 90), "北京市");

        Set<Map.Entry> entries = map.entrySet();
        for(Map.Entry entry:entries){
            System.out.println(entry.getKey() + "=" + entry.getValue());
        }
    }
}

运行时抛出以下异常：
Exception in thread "main" java.lang.ClassCastException: setdemo.Students cannot be cast to java.lang.Comparable

改写Students类，实现Comparable接口，按照学生的考试成绩降序排序。

class Students implements Comparable<Students> {


    private String sid; //学号
    private String name; //姓名
    private int score; //分数

    public Students() {
    }

    public Students(String sid, String name, int score) {
        this.sid = sid;
        this.name = name;
        this.score = score;
    }

    public String getSid() {
        return sid;
    }

    public void setSid(String sid) {
        this.sid = sid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }

    @Override
    public int compareTo(Students s) {
        return s.getScore()-this.getScore();
    }

    @Override
    public String toString() {
        return "Students{" +
                "sid='" + sid + '\'' +
                ", name='" + name + '\'' +
                ", score=" + score +
                '}';
    }
}

public class Test {

    public static void main(String[] args) {


        Map map = new TreeMap();

        map.put(new Students("S001", "张三", 89), "北京市");
        map.put(new Students("S002", "李四", 98), "北京市");
        map.put(new Students("S003", "王五", 78), "北京市");
        map.put(new Students("S004", "赵六", 90), "北京市");

        Set<Map.Entry> entries = map.entrySet();
        for(Map.Entry entry:entries){
            System.out.println(entry.getKey() + "=" + entry.getValue());
        }
    }
}

运行结果如下：
Students{sid='S002', name='李四', score=98}=北京市
Students{sid='S004', name='赵六', score=90}=北京市
Students{sid='S001', name='张三', score=89}=北京市
Students{sid='S003', name='王五', score=78}=北京市

==注意：不建议使用复杂的自定义对象类型作为键(例如：Students类型)，通常情况下强烈建议使用String类型作为键，Object类型作为值。==