contains() method in ArrayList, Java

Question

I am using an ArrayList to save Questions and their answers. The ArrayList is made up of Answer Class which has Question No and an ArrayList of answers_marked as follows:

private class Answer {
    private long question_no;
    private ArrayList<long> answer;

    public boolean equals(Object o) {
        if (o instanceof Answer) 
            if (((Answer)o).question_no == this.question_no)
                 return true;
        return false;
    }
}

public ArrayList<Answer> answers = new ArrayList<Answer>();

Now, when the user changes his answer I want to look into the answers arraylist and check if the question_no already exists in the answers. If it does then update the answer value for which answer was changed. I am trying to use contains method to check if the question_no already exists, but it always return false. What am I doing wrong here? Which other data-structure would be best suited for to do this?

I am using answers.contains(new Answer(10,20)) to see if the question_no 10 was already answered.

I don't see where you used contains(). but still I would say: Why not use a set. And you have to use brackets with if-clauses. and you should define getters/setters. — PKlumpp
– PKlumpp, Commented Jun 30, 2014 at 12:57
when the user changes his answer I want to look into the answers arraylist and check if the question_no already exists in the answers - I've serious concerns about your design.. — TheLostMind
– TheLostMind, Commented Jun 30, 2014 at 12:58
contains() works just fine with your equals() implementation, as you can see here. Your problem must be elsewhere. — Keppil
– Keppil, Commented Jun 30, 2014 at 13:03

rob · Accepted Answer · 2014-06-30 13:12:37Z

0

if you have a ArrayList you shouldn't do answers.contains(10) - this won't work.. 10 is an int and, in your equals(), if o is not an Answer then you are returning false.

Rather, try:

Answer answer1 = new Answer(1);
Answer answer2 = new Answer(2);

answers.add(answer1);

answers.contains(answer1) ;//= true
answers.contains(answer2) ;//= false
answers.contains(10) ;//= false

answered Jun 30, 2014 at 13:12

rob

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2 Comments

Keppil Over a year ago

The answers.contains(10) was apparently some kind of typo, so this answer isn't really valid anymore.

TheLostMind Over a year ago

I am using answers.contains(new Answer(10,20)) to see if the question_no 10 was already answered

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