如何让我的程序只输入特定的单词，如果输入的不是所说的单词，请重试输入

Question

I want to make a registration program. So there is 5 enrollments possbiel, lets say music, English, history, sports, photography. I want to make it so it will ask the user to enter to enroll into only 3 units out of the 5 options listed above. only those 5 are the options, if anything else is entered say like "math" or "jdwhagdwa" then it will ask the user to re enter it. Once the student enters the unit it should tell them the classes they have enrolled into and the fee for each class

import java.util.*;
        
    public class CheckMarkEnroll
       {    
         public void Enroll()
           {    
             Scanner in = new Scanner(System.in);
             Scanner sc = new Scanner(System.in);
         
             int i = 0;
             String studentlogin = "u123";
             //Unit name
             String english="Math";
             String history="History";
             String sports="Sports";
             String photography="Photography";
             String music="Music";             
           
             String musicfee ="$1320";
             String englishfee="$1890";
             String historyfee="$1890";
             String sportsfee="$1600";
             String photographyfee="$1500";
            
             String studentId="";
             String course1="";
             String course2="";
             String course3="";

        
             //============================================START OF 
             PROGRAM==================================================================//
             System.out.println("Welcome to student enrollment");
             System.out.println("Please enter your student ID");
             
             studentId = in.nextLine();
             
             System.out.println("Please enter name of units you would like to enroll into. You may enroll 
             into only 4 units. Upon entering please enter if this is your first time or you are 
             repeating this unit. At the end your fees required will be shown"); 
             System.out.println("List of units available to enroll:");
             System.out.println("music");
             System.out.println("english");
             System.out.println("history");
             System.out.println("sports");
             System.out.println("photography");
             
                               
             System.out.println("Please enter name of unit #1 : ");
             course1= sc.nextLine();
             
             System.out.println("Please enter name of unit #2 : ");
             course2= sc.nextLine();
             
             System.out.println("Please enter name of unit #3 : ");
             course3= sc.nextLine();
             
             
             
             
             
            }
        }
             
             
             

Answer 1

You could work with Sets (or any Collection). One containing all your possible courses and one that saves your entered courses.

Set<String> courses = new HashSet<>();
Set<String> possibleCourses = new HashSet<>();
possibleCourses.add(english);
possibleCourses.add(history);
possibleCourses.add(sports);
possibleCourses.add(photography);
possibleCourses.add(music);

System.out.println("List of units available to enroll: " + possibleCourses);

while (courses.size() < 3) {
    System.out.println("Please enter name of unit #" + (courses.size() + 1) + " : ");
    String course = sc.nextLine();
    if (!possibleCourses.contains(course)) {
        System.out.println("Please enter a valid course name! Your options are: " + possibleCourses);
        continue;
    }
    if (!courses.contains(course)) {
        courses.add(course);
    } else {
        System.out.println("You have already taken course: " + course);
    }
}

System.out.println("You have taken the courses: " + courses);