Basic java arraylist

Question

I'm not sure why my code isn't working. I'm trying to create a graph using an arraylist but this code doesn't appear to work. Whenever I try and get a node ID from the arraylist it returns 0. I'm sure I've just done something clumsy. Can ayone point out my mistake?

private ArrayList<Node> NodeList = new ArrayList<Node>();

public void addNode(int id, String Label, List connections) {   
    NodeList.add(new Station(id, Label, connections));
}

public ArrayList<Node> getNodes() {
    return NodeList;
}

Then in my main method (these are just for testing purposes)

ArrayList<Integer> connections = new ArrayList<Integer>();
     connections.add(2);
     connections.add(5);
     g.addNode(6, "first",connections );

    System.out.println(""+g.getNodes().get(0).getID());

Thanks for the interest guys! Here is the station class:

    private int id;
    private String stopName;
    private ArrayList connections;

    public Station(int id, String stopName, List connection) {
        id = this.id;
        stopName = this.stopName;
        setConnections(connection);
    }


    public List getConnections() {
        return connections;
    }



    public int getID() {

        return id;
    }


    public String getLabel() {

        return stopName;
    }

Can you provide an example we can run? Otherwise we can only guess you are using an id of 0 which is why that is what you find. — Peter Lawrey
– Peter Lawrey, Commented Oct 12, 2013 at 22:39
Please show the implementation of Station(int, String, List) (and possibly its super constructor), Node#getID() and/or Station#getID(). — Koraktor
– Koraktor, Commented Oct 12, 2013 at 22:45

kunal18 · Accepted Answer · 2013-10-12 23:00:26Z

6

These are two mistakes:

id = this.id;
stopName = this.stopName;

It should be:

this.id = id;
this.stopName = stopName;

See, 'this' is used to refer to calling object. So when you write like above, you say that "This object's id = id (argument one)".

And when you write as you have written in your question,

id = this.id;

you are changing the value of 'passed argument id' and assigning it the value of object's id, whose default value is 0! That's why you were getting the result as 0.

edited Oct 12, 2013 at 23:00

answered Oct 12, 2013 at 22:54

kunal18

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3 Comments

Alex Over a year ago

Thanks, this is the solution. I'm an idiot :( sorry for wasting your time.

kunal18 Over a year ago

no worries! It happens.. Hope your concept is clear now! Enjoy :)

Sentry Over a year ago

@John Don't forget to mark the right answer so stalin get's his well deserved points ;)

Dennis Meng · Accepted Answer · 2013-10-12 22:54:38Z

2

In your constructor, the assignments are the wrong way around. You want

public Station(int id, String stopName, List connection) {
    this.id = id;
    this.stopName = stopName;
    setConnections(connection);
}

instead of

public Station(int id, String stopName, List connection) {
    id = this.id;
    stopName = this.stopName;
    setConnections(connection);
}

answered Oct 12, 2013 at 22:54

Dennis Meng

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5 Comments

Peter Lawrey Over a year ago

+1 id = this.id; is the wrong way around and copies the value the wrong way as well.

Eric Jablow Over a year ago

You can enforce this by writing your constructor's signature as public Station(final int id, final String stopName, final List connection). Then, id = this.id will be a compile-time error, which you want. By the way, don't use raw Lists. And, watch out in your unshown setConnections method; you really want to create a new List, not share the input list.

Dennis Meng Over a year ago

@EricJablow that sounds like a comment to the question asker, not to me.

Eric Jablow Over a year ago

I'm not sure where to make that comment. The poster's question has no hook to make that comment.

Dennis Meng Over a year ago

It certainly sounds like it's directed at the poster, and not at me.

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