Help constructing a class with ArrayList as a parameter

Question

Ive been trying to work this out for a few hours now, but am stuck, hence I am coming here for some help. N.B. I am using BlueJ, to construct these classes, as im still learning.

What I am trying to do is create a PlayList which has a two parameters: name and a ArrayList of tracks. It then creates a playlist and copies the tracks from the list (in order) onto the playlist; but I dont want any help with this part, as of yet.

My issue is I am unsure how to call the ArrayList when constructing the PlayList.. Because the specified type is of .

public class PlayList
{    
private String myName;
private ArrayList<Track> myTracks;
private int myDuration;

public PlayList(String name, ArrayList<Track> tracks) {
    name = myName;
    myTracks = new ArrayList<Track>();
    for (Track t : tracks) {
        myTracks.add(t);
    }
}

}

What happens, in BlueJ, is when I construct a new PlayList class, it provides an empty field box for String name, and for ArrayList tracks. String name is fine, as I can simply put "anything" but am stuck as to the ArrayList tracks?

I know this is probably isnt a very specific question, but I am still learning.

Are you asking us how to design the GUI so that the user can enter the data for some number of Track objects? — Jeff Grigg
– Jeff Grigg, Commented Sep 4, 2011 at 12:27

Arnout Engelen · Accepted Answer · 2011-09-04 12:21:00Z

2

sunadorer's response should answer your direct question. I have some general remarks about naming conventions for the fields in your class and the parameters of your constructor.

I would not prefix your fields with 'my'. It is a bit of a matter of style, but I would not have much against using the same names for the constructor parameters. You can distinguish between the parameter and the field by using this.name when referring to the field.

Also, using ArrayList here seems unnecessarily restrictive: you could use the more general List interface (which ArrayList is just one implementation of, so you don't have to touch PlayList if you want another kind of List later o):

public class PlayList {    

  private String name;
  private List<Track> tracks;
  private int duration;

  public PlayList(String name, List<Track> tracks) {
    this.name = name;
    this.tracks = new ArrayList<Track>(tracks);
  }
}

edited Sep 4, 2011 at 12:21

answered Sep 4, 2011 at 11:55

Arnout Engelen

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7 Comments

Philipp Reichart Over a year ago

You should add the type parameters to the List tracks again, without them it's worst than having ArrayList in your constructor signature IMHO.

yjw Over a year ago

+1 for suggesting List instead of ArrayList. Just to go a bit further, will it be better to have List<Track>?

amit Over a year ago

He Should NOT use List if he knows what type of List it is, maybe List<Track>. this is also more appropriate as a comment then an answer.

Philipp Reichart Over a year ago

Might also want to use the copy-constructor of ArrayList to save the whole loop-and-add thing: new ArrayList<Track>(tracks);

Philipp Reichart Over a year ago

Just looked at the source of @Arnout's answer and the type paramters are already in there, he just used <code><pre> tags instead of the 4-spaces-indentation so SO filtered out the <Track> as HTML. I edited his answer and it's pending review.

|

sunadorer · Accepted Answer · 2011-09-04 11:52:20Z

The constructor of your PlayList wants an object of type ArrayList filled with elements of type Track. So to create a PlayList you must provide an already created one.

It depends on your surrounding of the call, maybe you already build a list there and can just put it into your PlayList constructor, but you need something like this anywhere:

// Create an empty list
ArrayList<Track> tracks = new ArrayList<Track>();
// Add a track. e.g. when receiving a gui event
tracks.add(track); // track was created with new Track()

Or maybe you don't need/want lists outside of PlayList objects. You could use PlayList objects to manage and encapsulate those, by changing the constructor to only create an empty list on its own and allow others to add tracks to it via an addTrack method.

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