原理

class Grade{
    private String level;
    //私有化
    private Grade(){}
    private Grade(String level){
        this.level = level;
    }
    public static Grade A = new Grade("90-100");
    public static Grade B = new Grade("80-90");
    public static Grade C = new Grade("70-80");
    public static Grade D = new Grade("60-70");
    public static Grade E = new Grade("<60");
}

枚举

// jdk5.0以后
enum Grade{
    A("90-100"),B("80-90"),C("70-80"),D("60-70"),E("<60");
    private String level;
    private Grade(){}
    private Grade(String level){
        this.level = level;
    }
}