java Map接口TreeMap涉及類的比較

*類的比較需要實現接口Comparable和重寫接口的抽象方法compareTo（）：
*class em implements Comparable<em> //需要增加類對象的泛型
public int compareTo(em o) //負數小于，0等于，正數大于

    Map<em,String> treemap2=new TreeMap<>();   //按照類中的方法遞增
    treemap2.put(new em(2,"af",30),"yes1");
    treemap2.put(new em(1,"af",30),"yes2");
    treemap2.put(new em(6,"af",30),"yes3");
    treemap2.put(new em(5,"af",30),"yes4");

    for(em f:treemap2.keySet())
    {
        System.out.println(f+"+"+treemap2.get(f) );
    }
class em implements Comparable<em>{     //比較類時需要實現Comparable接口,泛型是對象
    int id;
    String name;
    double salary;
    public em(int id, String name, double salary) {
        super();
        this.id = id;
        this.name = name;
        this.salary = salary;
    }
    @Override
    public int compareTo(em o) {    //負數小于，0等于，正數大于
        if(this.salary>o.salary)
        {
            return 1;
        }
        else if(this.salary<o.salary)
        {
            return -1;
        }
        else
        {
            if(this.id>o.id)
            {
                return 1;
            }
            else if(this.id<o.id)
            {
                return -1;
            }
            else
            {
                return 0;
            }
        }
    }