求教关于enum的一个基本问题

public class JAVA_enum {
    enum Constants2{
        Constants_A,
        Constants_B,
        Constants_D
    }
    public static void doit(int c)
    {
        switch (c)
        {
            case 1:
                System.out.println("doit() Constants_A");
                break;
            case 3:
                System.out.println("doit() Constants_B");
                break;
        }
    }
    public static void doit2(Constants2 c)
    {
        switch (c)
        {
            case Constants_A:
                System.out.println("doit2() Constants_A");
                break;
            case Constants_B:
                System.out.println("doit2() Constants_B");
                break;
        }
    }
    public JAVA_enum() {
        // TODO Auto-generated constructor stub
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        JAVA_enum.doit(3);
        JAVA_enum.doit2(Constants2.Constants_A);
        JAVA_enum.doit2(Constants2.Constants_B);
        JAVA_enum.doit(1);
        //JAVA_enum.doit2(3);
    }

}