java changing value of hashmap

Question

i have quick question, what I am trying to do here is to, get the Student from HashMap and adds a double mark to Student's mark.

lets say i have class called Student and Student class has method called addToMark() and Hashmap called theStudent = new HashMap<String, Student>.

public void addExtraMark(String studentNumber, double mark) {
    if(stuentNumber != null && mark >= 0) {
        Student extraMark = theStudent.get(studentNumber);
        extraMark.addToMark(mark)};
    }
}

my question is, does mark adds to hashmap? automatically? or do i have to use

theStudent.put(studentNumber, extraMark);

on the bottome of my code?

Michael Markidis · Accepted Answer · 2016-05-17 04:58:13Z

2

Since Student extraMark is a reference to that Student, anything you do to the reference will be reflected in the HashMap.

No, you do not have to make the call:

theStudent.put(studentNumber, extraMark);

answered May 17, 2016 at 4:58

Michael Markidis

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Comments

ajb · Accepted Answer · 2016-05-17 04:59:36Z

In this code:

Student extraMark = theStudent.get(studentNumber);
extraMark.addToMark(mark)}};

If the studentNumber doesn't exist in the hash map, get returns null. Then extraMark.addToMark will throw a NullPointerException.

So you have to check yourself, e.g.:

Student extraMark;
if (theStudent.containsKey(studentNumber)) {
    extraMark = theStudent.get(studentNumber);
} else {
    extraMark = new Student(.......);
    theStudent.put(studentNumber, extraMark);
}
extraMark.addToMark(mark);

or you do the get first and check the result for null.

Note: I've assumed that your question "does mark adds to hashmap" meant "would it add a new student to the hashmap if it weren't already there", but after rereading your question, I'm not clear on what you meant.

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