前提条件方法应该是公共的还是私有的

Question

方法markStudent(...(和getUnitMark(...(中的前提条件代码使用了类的其他方法：isEnrolled(...(和hasCompleteAssessments(...(。

import java.util.ArrayList; 
import java.util.HashMap;
public class Unit {
    private String code;
    private String name;
    private HashMap<Integer, Student> enrolledStudents = new HashMap<Integer, Student>(); 
    private AssessmentScheme assessmentScheme = null;
    private HashMap<Assessment, HashMap<Student, Mark> > Marks
    public Unit(String newCode, String newName) { code = newCode;
             name = newName;
       }
    public void enrolStudent(Student newStudent) {
             enrolledStudents.put(newStudent.getPersonID(), newStudent);
       }
    public void unenrolStudent(Student student) {
             enrolledStudents.remove(student.getPersonID());
       }
    public boolean isEnrolled(Student student) {
            return enrolledStudents.containsKey(student.getPersonID()); }
    public ArrayList<Student> getEnrolledStudents() {
            ArrayList<Student> students = new ArrayList<Student (enrolledStudents.values());
            return students; }
    public boolean hasCompletedAssessments(Student student) { boolean hasCompleted = true;
            for (Assessment a : assessmentScheme.getAssessments()) {       hasCompleted &= Marks.get(a).containsKey(student);
} 
    return hasCompleted; }
    public void markStudent(Assessment assessment, Student student, int score, String comment) throws Exception {
        /* Start Preconditions */
        // Precondition: studentEnrolledInUnit
        if (! isEnrolled(student)) {
             } throw new Exception("Precondition violated: studentEnrolledInUnit");
        // Precondition: scoreInValidRange
        if ((score < 0 || (score > assessment.getWeight()) } 
            throw new Exception("Precondition violated: scoreInValidRange"); /* End Preconditions */
        Mark mark = new Mark(assessment, student, score, comment);
               Marks.get(assessment).put(student, mark);
}

问题是，前提条件代码中使用的方法，例如isEnrolled(...并且已完成评估(...(，是公开的还是私有的？

根据我对合同设计的理解，客户应该检查其论点是否符合前提条件。这意味着前提条件代码中使用的方法应该是公共的，以便客户端类执行检查。但是，在 markStudent 方法中，可以清楚地看到该方法正在检查自己的前提条件。有人可以帮忙吗？

Answer 1

公开这些方法没有害处。这使客户能够确保他们的呼叫是合法的。如果客户知道他们的电话是合法的，他们就不必这样做。

方法实现也可以通过调用公共方法来执行自己的检查。