java中如何定义一个表示性别的多例类？

class Sex {

public static final int MALE_CMD=1;

public static final int FEMALE_CMD=2;

private static final Sex MALE =new Sex("男");

private static final Sex FEMALE= new Sex("女");

private String title ;

private Sex (String title) {//构造方法必须私有化

    this . title = title;

}

public static Sex getInstance (int ch) {

    switch (ch) {

        case MALE_CMD:

            return MALE :

        case FEMALE_CMD:

            return FEMALE;

        default:

            return null;

    }

}

    public String toString() {

        return this.title ;

    }

} public class TestSingleton {

public static void main(String args[]) {

    Sex a = Sex.getInstance(Sex.MALE_CMD);

    System.out.println(a);

}

}