我可以/如何在非静态环境中使用静态方法

Question

public class Transcript {
    private List<Grade> grades = new ArrayList<Grade>();
    private int id;

    public void assignVariables(){
        for(int i = 0; i < grades.size(); i++){
            if(grades.get(i).getLetter() == null){
                grades.get(i).setLetter(GPA2Letter(grades.get(i).gradePoint()));

            }
        }
    }

    public class Registrar {

    static double gpa;

    public Registrar(){

    }

    public static double letter2Point(String gradeIn){
        if(gradeIn == "A" ){
            gpa = 4.0;
        }
        else if(gradeIn == "B"){
            gpa = 3.0;
        }
        else if(gradeIn == "C"){
            gpa = 2.0;
        }
        else if(gradeIn == "D"){
            gpa = 1.0;
        }
        else if(gradeIn == "F"){
            gpa = 0;
        }
        return gpa;
    }

    public static String GPA2Letter(double gradePointIn){
        String letter = "";
        if(gradePointIn >= 4){
            letter = "A";
        }
        else if((gradePointIn >= 3) && (gradePointIn < 4)){
            letter = "B";
        }
        else if((gradePointIn >= 2) && (gradePointIn < 3)){
            letter = "C";
        }
        else if((gradePointIn >= 1) && (gradePointIn < 2)){
            letter = "D";
        }
        else if((gradePointIn >= 0) && (gradePointIn < 1)){
            letter = "F";
        }
    }
}

我正在尝试使用GPA2Letter（）方法来处理另一个类中的信息我想知道如何访问此方法，或者我是否可以注意到我正在执行此类代码的类不是静态环境