CODEWITHSUNDEEP
javahashset
user2259968
user2259968

Reputation: 35

How do I add a value to a hash set?

I'm working on a practice problem that requires me to add a value into a hashmap. But I can't figure out why I keep getting an error message on the line courseName.add(student);

Here is my code:

public class StudentDatabase {

    // add instance variables
    private Map<String, HashSet<Integer>> dataContent = new LinkedHashMap<String, HashSet<Integer>>();

    // Prints a report on the standard output, listing all the courses and all the
    // students in each.  If the map is completely empty (no courses), prints a message
    // saying that instead of printing nothing.
    public void report() {
        if (dataContent.isEmpty()) {
            System.out.println("Student database is empty.");
        } else {
            for (String key : dataContent.keySet()) {
                System.out.println(key + ":" + "\n" + dataContent.get(key));
            }
        }
    }

    // Adds a student to a course.  If the student is already in the course, no change
    // If the course doesn't already exist, adds it to the database.
    public void add(String courseName, Integer student) {
        if (dataContent.containsKey(courseName)) {
            courseName.add(student);
        } else {
            Set<Integer> ids = new HashSet<Integer>();
            ids.add(student);
            dataContent.put(courseName, ids);
        }
    }
}

Upvotes: 3

Views: 20619

Answers (2)

prashantsunkari
prashantsunkari

Reputation: 959

courseName.add is not possible.. courseName is a String, which being a immutable object doesn't allow any add method...

check out: http://docs.oracle.com/javase/7/docs/api/java/lang/String.html

I think this is what you are looking for: 

    public void add(String courseName, Integer student) {
        if (dataContent.containsKey(courseName)) {
            HashSet<Integer> newhashSet=dataContent.get(courseName);
            if(newhashSet!=null)
            {
                newhashSet.add(student);
            }
            dataContent.put(courseName, newhashSet);
            //courseName.add(student);
        }

        else {
            Set<Integer> ids = new HashSet<Integer>();
            ids.add(student);
            dataContent.put(courseName, (HashSet<Integer>) ids);
        }
       // System.out.println("Data:"+dataContent.toString());
    } // end add
 }

Hope this helps!

Upvotes: 0

Rob
Rob

Reputation: 11733

Ok, this construct:

if (dataContent.containsKey(courseName)) {
    courseName.add(student);
}

is completely whacky. What you want is:

if (dataContent.containsKey(courseName)){
    Set<Integer> studentsInCourse = dataContent.get(courseName);
    studentsInCourse.add(student);
}

Should fix it.

Upvotes: 2

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