Not allowing duplicates of an object in an Array List

Question

I am currently working on a family feud game in Java. I have an array list of an object question called questions, which stores random questions to use throughout the game. I am trying to populate the array list in a way to make sure there are no duplicates, however it is not working and I am getting duplicates.

Initialized as a field is my array list of questions:

private ArrayList<Question> questions = new ArrayList<>();

Here is what I am trying currently:

//Populates questions list
        for(int i=0; i<15; i++)
        {
           Question q = new Question();
           //makes sure there are no duplicates
            if(!questions.contains(q))
                questions.add(q); //not working for some reason cry

           System.out.println(q.getQuestion());
        }

When I print out the questions I still get duplicates:

Name something you would see on the Jerry Springer show:
Name something that would get you thrown out of most bars:
What accent might an American pretend to have in order to sound more attractive:
Name a breed of dog that might be used as a guard dog:
Name a place where a child might get seperated from its parents:
Name something you would see on the Jerry Springer show:
Name something that would get you thrown out of most bars:
Name a place where a child might get seperated from its parents:
Name something for which you need a warranty:
Name a game that would be inappropriate at a company party:
Name something for which you need a warranty:
Name a place where a child might get seperated from its parents:
Name an activity that could be rained out:
Name something for which you need a warranty:
Name something that would get you thrown out of most bars:

I do override the equals method in my question class, however it isn't doing the trick. Here is my question class:

/**
 * Question proposed to players to guess one of the answers on the board
 *
 * Each question has 5 possible answers
 *
 * The top answer to each question is stored at Answers(0) and is worth the most
 * The last answer is stored at Answers(4) and is worth the least
 *
 *
 * @author Stefan Gligorevic
 */
import java.util.ArrayList;
import java.util.Random;
import java.lang.String;
import java.lang.Object;

public class Question {

    private ArrayList<Answer> answers;
    private String question;

    /**** MIGHT HAVE TO INITIALIZE ANSWERS TO AN ARRAY LIST WITH NO SIZE SO ADD CAN WORK ****/
    //constructors
    public Question(){
        answers = new ArrayList<>();
        question=getRandomQuestion();
    }

    //makes a random question with a set of answers
    public String getRandomQuestion() {
        String ask = "";

        Random rand = new Random();
        int n = 1 + rand.nextInt(10);

        switch (n)
        {
            case 1:
                ask="Name a place where a child might get seperated from its parents:";
                answers.add(new Answer("Mall", 38));
                answers.add(new Answer("Park", 23));
                answers.add(new Answer("Zoo", 16));
                answers.add(new Answer("Theme Park", 16));
                answers.add(new Answer("Airport", 5));
                break;
            case 2:
                ask="Name something for which you need a warranty:";
                answers.add(new Answer("Car", 54));
                answers.add(new Answer("TV", 23));
                answers.add(new Answer("Watch", 8));
                answers.add(new Answer("Computers", 4));
                answers.add(new Answer("Appliance", 3));
                break;
            case 3:
                ask="Name a fruit you can buy dried:";
                answers.add(new Answer("Grape", 22));
                answers.add(new Answer("Banana", 21));
                answers.add(new Answer("Apricot", 21));
                answers.add(new Answer("Prune", 17));
                answers.add(new Answer("Apple", 15));
                break;
            case 4:
                ask="Name an activity that could be rained out:";
                answers.add(new Answer("Sports Event", 45));
                answers.add(new Answer("Picnic", 34));
                answers.add(new Answer("Wedding", 10));
                answers.add(new Answer("Concert", 7));
                answers.add(new Answer("Barbecue", 3));
                break;
            case 5:
                ask="What accent might an American pretend to have in order to sound more attractive:";
                answers.add(new Answer("French", 61));
                answers.add(new Answer("British", 18));
                answers.add(new Answer("Italian", 8));
                answers.add(new Answer("Spanish", 8));
                answers.add(new Answer("Australian", 3));
                break;
            case 6:
                ask="Name a sport that might be played at a family reunion:";
                answers.add(new Answer("Football", 54));
                answers.add(new Answer("Baseball", 21));
                answers.add(new Answer("Horseshoe", 8));
                answers.add(new Answer("Frisbee", 7));
                answers.add(new Answer("Basketball", 6));
                break;
            case 7:
                ask="Name a game that would be inappropriate at a company party:";
                answers.add(new Answer("Spin the Bottle", 41));
                answers.add(new Answer("Strip Poker", 32));
                answers.add(new Answer("Twister", 11));
                answers.add(new Answer("Truth or Dare", 11));
                answers.add(new Answer("Beer Pong", 3));
                break;
            case 8:
                ask="Name something that would get you thrown out of most bars:";
                answers.add(new Answer("Getting in a fight", 45));
                answers.add(new Answer("Drinking too much", 29));
                answers.add(new Answer("Not Paying", 6));
                answers.add(new Answer("Stripping", 5));
                answers.add(new Answer("Being underage", 3));
                break;
            case 9:
                ask="Name something you would see on the Jerry Springer show:";
                answers.add(new Answer("Fighting", 56));
                answers.add(new Answer("Nudity", 22));
                answers.add(new Answer("Security", 6));
                answers.add(new Answer("Jerry Springer", 4));
                answers.add(new Answer("Chairs Thrown", 3));
                break;
            case 10:
                ask="Name a breed of dog that might be used as a guard dog:";
                answers.add(new Answer("German Shepard", 36));
                answers.add(new Answer("Pit Bull", 23));
                answers.add(new Answer("Doberman Pinscher", 20));
                answers.add(new Answer("Rottweiler", 8));
                answers.add(new Answer("Bulldog", 5));
                break;
            default:
                //won't reach this hehe
        } //end switch
        return ask;
    }

    @Override
    public boolean equals(Object obj)
    {
        if(obj == null)
            return false;

        if(!Question.class.isAssignableFrom(obj.getClass()))
            return false;

        final Question q = (Question) obj;

        if(q.getQuestion() == null || this.getQuestion() == null)
            return false;

        if(!this.getQuestion().equalsIgnoreCase(q.getQuestion()))
            return false;

        return true;
    }

    //getters and setters
    public String getQuestion() { return question; }
    public ArrayList<Answer> getAnswers() { return answers; }

    //returns answer at given index
    public Answer answerAt(int index) { return answers.get(index); }
    //returns String of answer at specified index
    public String getAnswerAt(int index) { return answers.get(index).getAnswer(); }
    //returns point value of answer at specified index
    public int getAnswerPoints(int index) { return answers.get(index).getValue(); }

    //returns the points earned for this question
    //points are the point values of all the answers that have been found
    public int pointsEarned() {
        int points=0;
        for (int i=0; i<answers.size(); i++)
        {
            if(answers.get(i).isFound())
                points += answers.get(i).getValue();
        }
        return points;
    }

    //Sets a particular answer's value
    public void setAnswerVal(int index, int val) { answers.get(index).setValue(val); }

    //Add a multiplier to each answer's value for the question
    public void addMultiplier(int multiplier)
    {
        for(int i=0; i<answers.size(); i++)
        {
            int val = answers.get(i).getValue();
            answers.get(i).setValue(multiplier * val);
        }
    }

    //returns string of each answer
    public String topAnswer() { return answers.get(0).getAnswer(); }
    public String Answer2() { return answers.get(1).getAnswer(); }
    public String Answer3() { return answers.get(2).getAnswer(); }
    public String Answer4() { return answers.get(3).getAnswer(); }
    public String lastAnswer() { return answers.get(4).getAnswer(); }

    //returns values of each answer
    public int topAnswerVal() { return answers.get(0).getValue(); }
    public int Answer2Val() { return answers.get(1).getValue(); }
    public int Answer3Val() { return answers.get(2).getValue(); }
    public int Answer4Val() { return answers.get(3).getValue(); }
    public int lastAnswerVal() { return answers.get(4).getValue(); }

}

I don't get why it's not working :( Help would be much appreciated!

Just try useing: if(!this.getQuestion().compareTo(q.getQuestion()==0){return false;}, or try implementing Comparable and override compareTo() method — Kumar Ashutosh, Jan 05 '20 at 04:30
Is the output you have shown coming from the `//Populates questions list` bit of code ? If so, that is not what is in your `questions` list. Could you put another `for` loop afterwards to display what IS in the list ? — racraman, Jan 05 '20 at 05:22
You are printing out each and every Question, regardless of whether it gets added to the ArrayList or not. — VGR, Jan 05 '20 at 05:34
Does this answer your question? [Remove duplicates from a Java List](https://stackoverflow.com/questions/59594433/remove-duplicates-from-a-java-list) — xenteros, Jan 08 '20 at 23:11

Ashishkumar Singh · Accepted Answer · 2020-01-05T07:17:16.463

Your overridden equals method logic is fine. It just that you are doing the sysout in the wrong place making you believe that the logic is not working. Instead of printing the question inside the for-loop which is adding the question, print it outside that loop. Modify your logic like below and you would get the expected result

    for(int i=0; i<15; i++)
    {
        Question q = new Question();
        //makes sure there are no duplicates
        if(!questions.contains(q))
            questions.add(q); //not working for some reason cry
    }
    for (Question question : questions) {
        System.out.println(question.getQuestion());
    }

Note, whenever you override equals method, you should override hashCode as well. Because if you don't, then the default implementation in Object class will be used. So, even if they are equal according to the equals() method, they might have different hashcode. You are using Arraylist, so you will not face an issue here but some collections like HashSet, HashMap which uses both equals and hashcode for adding and retrieving object. This might give you result that you don't expect. So always override them consistently. Link

public int hashCode(){
    return question.hashCode();
    }

Also, if your logic for equality is based on String question only, I advise you to either make your Question class implement Comparable or create a Comparator for your Question class and instead of using ArrayList to hold list of question, use Set(TreeSet) so that 1) you don't have to worry about implementing equals and hashcode method and 2) you don't have to always check on adding a new Question. Below is a code snippet

Implementing Comparable<Question>

@Override
public int compareTo(Question question) {
    return this.getQuestion().compareTo(question.getQuestion());
}

Adding a question to Set

Set<Question> questions = new TreeSet<>();
    for(int i=0; i<15; i++)
    {
        questions.add(new Question());
    }
    for (Question question : questions) {
        System.out.println(question.getQuestion());
    }

Oof, derpy mistake on my part. Thank you for the suggestions! Makes much more sense now. — DerpDude21, Jan 13 '20 at 20:21

score 0 · Answer 2 · edited Jan 05 '20 at 05:11

0

You must override hashCode()as well when you override equals(). Read the reason here.

edited Jan 05 '20 at 05:11

Naresh Kumar

794
8
25

answered Jan 05 '20 at 04:39

Prakash Joshi

11
1

While this statement is true, it has no bearing on adding objects to an ArrayList. ArrayList does not make use of hashCodes. – VGR Jan 05 '20 at 05:32
I think it does when it checks for contains – orirab Jan 05 '20 at 05:59

score 0 · Answer 3 · answered Jan 05 '20 at 06:28

Its better to use HashSet(if order of objects is not important)or LinkedHashSet(if 
order of objects is important) using an ArrayList will decrease the performance of the 
game on which you are currently working.
If you want to use LinkedHashSet you have to override hashCode and equals method.

eg program...

import java.util.LinkedHashSet;
public class Question {

    private ArrayList<Answer> answers;
    private String question;

    public Question(){
        answers = new ArrayList<>();
        question=getRandomQuestion();
    }
    @Override
    public int hashCode(){
        return question.hashCode();
    }
    @Override
    public boolean equals(Object o){
            if(o==null)return false;
            if(o.getClass()!=this.getClass())return false;
            Question q=(Question)o;
            return q.question.equals(this.question);
    }
    ...
    </>
    ...
    public static void main(String $[]){
        LinkedHashSet<Question>hash=new LinkedHashSet<>;
    }
}

Not allowing duplicates of an object in an Array List

3 Answers3