Check if two parameters from an object already exists in an arrayList, if so do not add to arrayList?

Question

When trying to identify if duplicates exist inside of the arrayList it doesn't seem to check, I am trying to override equals but the statement will just always pass as false and add the object to the list. In the action listener what is wrong with my statement?

The issue is in the action listener statement.

public class addTrackLayout extends JFrame{

private JTextField artist; 
private JTextField trackTitle;
private JTextField genre;
private JTextField duration;
private JTextField year;

private JLabel artistLabel;
private JLabel titleLabel;
private JLabel genreLabel;
private JLabel durationLabel;
private JLabel yearLabel;
private JLabel addLabel;

private String addArtist;
private String addTitle;
private String addGenre;
private double addDuration;
private int addYear;

private static final long serialVersionUID = 1L;

public addTrackLayout(String title){

    super (title);

    JPanel contentPane = new JPanel();
    contentPane.setLayout(new GridBagLayout());

    addLabel = new JLabel("Add Record");
    artistLabel = new JLabel("Artist Name:");
    titleLabel = new JLabel("Track Title:");
    genreLabel = new JLabel("Music Genre:");
    durationLabel = new JLabel("Duration:");
    yearLabel = new JLabel("Release Year:");

    artist = new JTextField(15);
    trackTitle = new JTextField(15);
    genre = new JTextField(15);
    duration = new JTextField(15);
    year = new JTextField(15);
    ArrayList<Music> music = TimelineLayout.music;

    JButton addButton = new JButton("Add");
    addButton.setActionCommand("add");


        //Actions for buttons.
        addButton.addActionListener(new ActionListener(){

                public void actionPerformed(ActionEvent e){

                    addArtist = artist.getText();
                    addTitle = trackTitle.getText();
                    addGenre = genre.getText();
                    addDuration = Double.parseDouble(duration.getText());
                    addYear = Integer.parseInt(year.getText());

                    Music m = new Music(addArtist, addTitle, addGenre, addDuration, addYear);


                    if(music.contains(m)){

                        JOptionPane.showMessageDialog(null, "already exists");
                    }
                    else{

                        music.add(m);       
                        JOptionPane.showMessageDialog(null,"added to db");
                    }

                }   

            });

Music Class:

public class Music {

private String artist;
private String title;
private String genre;
private double duration;
private int year;


public Music(String initArtist, String initTitle, String initGenre, double initDuration, int initYear)
{   
    artist =  initArtist;
    title= initTitle;
    genre = initGenre;
    duration = initDuration;
    year = initYear;
}

public String getArtist() {
    return artist;
}

public void setArtist(String artist) {
    this.artist = artist;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public double getDuration() {
    return duration;
}

public void setDuration(double duration) {
    this.duration = duration;
}

public int getYear() {
    return year;
}

public void setYear(int year) {
    this.year = year;
}

public String getGenre() {
    return genre;
}

public void setGenre(String genre) {
    this.genre = genre;
}

Override and hash.

public boolean equals(Object obj)   {

 if (this == obj) 
 {
     return true;
 }

 else if (obj == null)
 {
     return false;              
 }

 else if (obj instanceof Music) 
 {
     Music music = (Music) obj;

     if (music.getArtist().equals(this.getArtist()) && (music.getTitle().equals(this.getTitle())))       
        {
            return true;
        }
     }
        return false;
    }

@Override
public int hashCode()
{
    int hash = 7;
    hash = 31 * hash + Objects.hashCode(this.getArtist());
    hash = 31 * hash + Objects.hashCode(this.getTitle());
    return hash;
}

Answer 1

The problem is that you need to understand object equality in Java, as well as how it is implemented in ArrayList.

The method list.contains(obj):

at least one element e such that (o==null ? e==null : o.equals(e)).

(from https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#contains(java.lang.Object))

This means that the array list is running Music.equals(...)

I can only assume that you did not override .equals in your Music object to ensure that objects could be compared.

Quick refresher:

obj == obj

checks if the objects point to the same thing. So...

Music m;
Music m2;
m == m2; //would be false, they're seperate.
m.equals(m2) //you should implement this so it is true.

You said you wanted to see if an element already exists in the array.

This is done by implementing equals(Object o) in the Music class.

@override
public boolean equals(Object o){
     boolean equals = true;
     if(o instanceof Music){
        //here, we need to compare all the fields in music
        //repeat for title, genre, duration, year, etc.

        if(!(Music) o.artist.equals( this.artist){
             equals = false;
        }
    }
    return equals;
}

Further reading:

1. Compare two objects with .equals() and == operator - Take special attention to the answer marked as accepted, if you're gonna compare instances of a custom class you will need to override the equals() method and, consequently, the hashCode() method.

2. https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html - all the methods for ArrayList - to really learn the datastructure, you should essentially memorize this.

Answer 2

You have to override the hashcode and equals method in your Music class and implement the comparable interface. And when you're adding new music to your music list check to see if your music list contains it, if not add it.