CODEWITHSUNDEEP
javalisthashmap
RaoulDuke316
RaoulDuke316

Reputation: 1

How can I populate a List from a HashMap in Java?

As part of my assignment, we are given an interface which we can't change, and some pre-defined tests to use to develop our methods.

The interface is as follows:

public interface ContactTracer {
    public void addCourse(String courseID, String courseName);
    public void addStudent(String id, String name, String email);
    public void loadStudentCourseList(Map<String, String> enrolments);
    
    public String getStudentName(String studentID);
    public String getStudentEmail(String studentID);
    public String getCourseForStudent(String studentID);
    public String getCourseName(String courseID);
    public List<String> findStudentsForCourse(String courseID);
    public List<String> findContacts(String studentID);
}

The particular test I'm running:

@Test
void testLoadStudentCourseList()    {
    ContactTracer ct = new PartOneContactTracer();
    Map<String, String> enrolments = new HashMap<>();
    enrolments.put("S101", "SOFT01");
    enrolments.put("S102", "NET02");
    enrolments.put("S103", "SOFT01");
    ct.loadStudentCourseList(enrolments);
    List<String> students = ct.findStudentsForCourse("SOFT01");
    assertEquals(2, students.size());
    assertTrue(students.contains("S101"));
    assertTrue(students.contains("S103"));
}

And the method I need to create from the test:

@Override
public List<String> findStudentsForCourse(String courseID) {
    return null;
}

I can tell from the test that within the findStudentsForCourse method I need to create List<String> students and fill it with matching students from the enrolments Map, and then return the the List.

I have created the following fields:-

public class PartOneContactTracer implements ContactTracer {
    
    private Map<String, String> courseDetails = new HashMap<>();
    private Map<String, String> studentName = new HashMap<>();
    private Map<String, String> studentEmail = new HashMap<>();
    private Map<String, String> enrolments = new HashMap<>();

    ...
}

What I can't get my head around is how to pass the data in the enrolments Map into the students List in order to make the Assertion in the test true.

Upvotes: 0

Views: 114

Answers (1)

Kaneda
Kaneda

Reputation: 721

You don't need to create a list and fill it just search through the map all the keys (students) that has the value (course id):

public List<String> findStudentsForCourse(String courseId) {
    return enrolments.entrySet().stream()
        .filter(e -> e.getValue().equals(courseId))
        .map(e -> e.getKey())
        .collect(Collectors.toList());
}

Upvotes: 1

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