Arraylists within a class Java

Question

I am trying to add a CD object into the Band Object's ArrayList member field of an ArrayList of CDs. The band_index is the index of the Band ArrayList when it is selected from a combobox, and i have checked that band_index does assign the correct index of the selected band. I am getting a Null Pointer Exception on this line of code band.get(band_index).addCD(cd); when i go to call the current Band's addCD method.

Main class:

public void addCD() {
    CD cd = new CD(t, y);

    band.get(band_index).addCD(cd); //NULL pointer Exception on this line
            updateCDs();
}

//Method to print out all the CDs of a band
public void updateCDs() {
    String list = "";
    for(int i = 0; i < band.size(); i++)
    {
          //band_index is the index of the selected band in the combobox    
          if(i == band_index) {
            for(int j = 0; j < band.get(i).getCDs().size(); j++) {
                list += "Title: " + band.get(i).getCDs().get(j).getTitle();
                list += "Year: " + band.get(i).getCDs().get(j).getYear();
            }
        }
    }
    System.out.println(list);
}

Band class:

private ArrayList<CD> cds;

public void addCD(CD cd) {
    cds.add(cd);
}

CD class:

private String title;
private int year;

public CD(String t, int y) {
    title = t;
    year = y;
}

public getTitle() { return title; }
public getYear() { return year; }

What does band.get(band_index) do ? Initialize private ArrayList<CD> cds = new ArrayList<>(); — AllTooSir
– AllTooSir, Commented May 28, 2013 at 11:25
Use a StringBuilder in updateCDs() and looking at its code I don't understand why you don't call it printCDs() or something? — Ravi K Thapliyal
– Ravi K Thapliyal, Commented May 28, 2013 at 11:29
The general approach is: fire up a debugger, and figure out which subexpression is null first. — millimoose
– millimoose, Commented May 28, 2013 at 11:30

nano_nano · Accepted Answer · 2013-05-28 11:25:33Z

6

your cds is null.

try this:

private List<CD> cds = new ArrayList<CD>();

public void addCD(CD cd) {
    cds.add(cd);
}

btw. maybe band is also null. There is not enough source code to determine this.

answered May 28, 2013 at 11:25

nano_nano

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1 Comment

user1352609 Over a year ago

doh that was a simple mistake. i forgot to initialize the cds... thanks

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