有关Java类变量与类方法

为什么不能用类方法来代替类变量

import java.util.*;

public class Main_S {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);

    Student student = new Student();
    student.setSex(Student.SEX_FEMALE);
    student.name = sc.next();
    student.age = sc.nextInt();
    System.out.println(student.toString());

    student.setSex(Student.SEX_MALE);
    System.out.println(student.toString());

    sc.close();
}

}

public class Student {
public String name;
public int age;
private String sex;
/public static String SEX_FEMALE(){
return "female";
}
public static String SEX_MALE(){
return "male";
}/
public static String SEX_FEMALE = "female";
public static String SEX_MALE = "male";
public void setSex(String sex){
this.sex = sex;
}
public String toString(){
return "name = "+name+", age = "+age+", sex = "+sex;

}

}

见类中的注释为什么不能替代类变量SEX_FEMALE和SEX_MALE