Lesson 3
Managing Student Enrollments Using Java HashMap and ArrayList
Managing Student Enrollments Using Java HashMap and ArrayList

In this lesson, we'll learn how to manage student enrollments using Java's HashMap and ArrayList collections. These data structures allow efficient data organization and access, which are essential for building an enrollment system. We'll implement methods to handle enrolling and unenrolling students, checking enrollment status, and listing students in a course. This practical approach will enhance your understanding of these structures. Now, let's dive into the methods we need to implement.

Introducing Methods to Implement

Here are the methods we need to implement in our enrollment system:

  • enroll(String student, String course): This method adds a student to a course. If the student is already enrolled, it does nothing.
  • unenroll(String student, String course): This method removes a student from a course. It returns true if the student was enrolled and has now been removed. Otherwise, it returns false. If, after unenrolling the student, the course becomes empty (no one is enrolled there), remove the course as well.
  • isEnrolled(String student, String course): This method checks if a student is enrolled in a course. It returns true if the student is enrolled and false otherwise.
  • listStudents(String course): This method returns an ArrayList<String> of all students enrolled in a given course. If no students are enrolled, it returns an empty ArrayList<String>.

Let's look at how to implement each of these methods step by step.

Step 1: Define the Class

We'll start by defining our class and then add each method one by one.

First, we define our EnrollmentSystem class:

Java
1import java.util.ArrayList;
2import java.util.HashMap;
3
4class EnrollmentSystem {
5    private HashMap<String, ArrayList<String>> enrollments;
6
7    public EnrollmentSystem() {
8        enrollments = new HashMap<>();
9    }
10}
11
12public class Solution {
13    public static void main(String[] args) {
14        EnrollmentSystem es = new EnrollmentSystem();
15        System.out.println("EnrollmentSystem initialized.");
16    }
17}

This code initializes an EnrollmentSystem class with a HashMap named enrollments that maps courses to lists of students.

Step 2: Implement Enroll Method

Next, we implement the enroll method:

Java
1import java.util.ArrayList;
2import java.util.HashMap;
3
4class EnrollmentSystem {
5    private HashMap<String, ArrayList<String>> enrollments;
6
7    public EnrollmentSystem() {
8        enrollments = new HashMap<>();
9    }
10
11    public void enroll(String student, String course) {
12        enrollments.putIfAbsent(course, new ArrayList<>());
13        ArrayList<String> students = enrollments.get(course);
14        if (!students.contains(student)) {
15            students.add(student);
16        }
17    }
18
19    public ArrayList<String> getStudents(String course) {
20        return enrollments.getOrDefault(course, new ArrayList<>());
21    }
22}
23
24public class Solution {
25    public static void main(String[] args) {
26        EnrollmentSystem es = new EnrollmentSystem();
27        es.enroll("Alice", "Math101");
28        System.out.println(String.join(", ", es.getStudents("Math101")));  // Output: Alice
29    }
30}

In the enroll method:

  1. We check if the course exists in the HashMap. If it doesn't, we initialize a new ArrayList for that course.
  2. We then add the student to the list if they are not already enrolled.
Step 3: Implement Unenroll Method

Let's move on to the unenroll method:

Java
1import java.util.ArrayList;
2import java.util.HashMap;
3
4class EnrollmentSystem {
5    private HashMap<String, ArrayList<String>> enrollments;
6
7    public EnrollmentSystem() {
8        enrollments = new HashMap<>();
9    }
10
11    public void enroll(String student, String course) {
12        enrollments.putIfAbsent(course, new ArrayList<>());
13        ArrayList<String> students = enrollments.get(course);
14        if (!students.contains(student)) {
15            students.add(student);
16        }
17    }
18
19    public boolean unenroll(String student, String course) {
20        if (enrollments.containsKey(course)) {
21            ArrayList<String> students = enrollments.get(course);
22            if (students.remove(student)) {
23                if (students.isEmpty()) {
24                    enrollments.remove(course);
25                }
26                return true;
27            }
28        }
29        return false;
30    }
31
32    public ArrayList<String> getStudents(String course) {
33        return enrollments.getOrDefault(course, new ArrayList<>());
34    }
35
36    public boolean containsCourse(String course) {
37        return enrollments.containsKey(course);
38    }
39}
40
41public class Solution {
42    public static void main(String[] args) {
43        EnrollmentSystem es = new EnrollmentSystem();
44        es.enroll("Alice", "Math101");
45        es.enroll("Bob", "Math101");
46        System.out.println(String.join(", ", es.getStudents("Math101"))); // Output: Alice, Bob
47        es.unenroll("Alice", "Math101");
48        System.out.println(String.join(", ", es.getStudents("Math101"))); // Output: Bob
49        es.unenroll("Bob", "Math101");
50        System.out.println(es.containsCourse("Math101"));  // Output: false
51    }
52}

This method first checks whether the course exists in the enrollments and whether the student is enrolled in that course. If they are, it removes the student from the course. If the course list becomes empty after removal, it deletes the course from the HashMap. The method returns true if the student was successfully unenrolled and false otherwise.

Step 4: Implement IsEnrolled Method

Next, let's implement the isEnrolled method:

Java
1import java.util.ArrayList;
2import java.util.HashMap;
3
4class EnrollmentSystem {
5    private HashMap<String, ArrayList<String>> enrollments;
6
7    public EnrollmentSystem() {
8        enrollments = new HashMap<>();
9    }
10
11    public void enroll(String student, String course) {
12        enrollments.putIfAbsent(course, new ArrayList<>());
13        ArrayList<String> students = enrollments.get(course);
14        if (!students.contains(student)) {
15            students.add(student);
16        }
17    }
18
19    public boolean isEnrolled(String student, String course) {
20        if (enrollments.containsKey(course)) {
21            ArrayList<String> students = enrollments.get(course);
22            return students.contains(student);
23        }
24        return false;
25    }
26}
27
28public class Solution {
29    public static void main(String[] args) {
30        EnrollmentSystem es = new EnrollmentSystem();
31        es.enroll("Alice", "Math101");
32        System.out.println(es.isEnrolled("Alice", "Math101"));   // Output: true
33        System.out.println(es.isEnrolled("Bob", "Math101"));     // Output: false
34    }
35}

This method checks whether the specified course exists in the enrollments and whether the student is enrolled in that course. If both conditions are met, it returns true; otherwise, it returns false.

Step 5: Implement ListStudents Method

Finally, let's implement the listStudents method:

Java
1import java.util.ArrayList;
2import java.util.HashMap;
3
4class EnrollmentSystem {
5    private HashMap<String, ArrayList<String>> enrollments;
6
7    public EnrollmentSystem() {
8        enrollments = new HashMap<>();
9    }
10
11    public void enroll(String student, String course) {
12        enrollments.putIfAbsent(course, new ArrayList<>());
13        ArrayList<String> students = enrollments.get(course);
14        if (!students.contains(student)) {
15            students.add(student);
16        }
17    }
18
19    public ArrayList<String> listStudents(String course) {
20        return enrollments.getOrDefault(course, new ArrayList<>());
21    }
22}
23
24public class Solution {
25    public static void main(String[] args) {
26        EnrollmentSystem es = new EnrollmentSystem();
27        es.enroll("Alice", "Math101");
28        es.enroll("Bob", "Math101");
29        System.out.println(String.join(", ", es.listStudents("Math101")));  // Output: Alice, Bob
30        System.out.println(String.join(", ", es.listStudents("Physics101")));  // Output:
31    }
32}

The listStudents method returns an ArrayList<String> of students enrolled in the given course. If the course is not in the HashMap, it returns an empty ArrayList<String>.

Lesson Summary

In today's lesson, we learned how to manage student enrollments using Java HashMap and ArrayList objects. We implemented methods to enroll and unenroll students, check enrollments, and list students in a course. This task provided a practical way to reinforce your understanding of hash maps and array lists in Java.

I encourage you to move on to the practice to undertake similar challenges to deepen your understanding. Keep experimenting and honing your skills. Happy coding!

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