sorting custom object array on two fields

Question

I'm sorting an array of custom objects (ListData[]) on two fields. I want it to be sorted by theme, and them by name. I thought i made a nice comparator in the custom object class and that i could use Arrays.sort(ld) to make my code working and sorting my array. But apparently im doing something wrong...

my custom object:

   public class ListData implements Comparable<ListData>{
public int venueID;
public String name;
public String photoUrl;
public String tip;
public String theme;
@Override
public int compareTo(ListData ld0) {
    return this.venueID- ld0.venueID;
}

public static Comparator<ListData> ListDataThemeAndNameComparator = new Comparator<ListData>() {
    @Override
    public int compare(ListData ld1, ListData ld2) {

        String compareTheme1 = ld1.theme.toUpperCase();
        String compareTheme2= ld2.theme.toUpperCase();
        String compareName1 = ld1.name.toUpperCase();
        String compareName2= ld2.name.toUpperCase();

        //ascending
        int comp = compareTheme1.compareTo(compareTheme2); // comp themes
        if(comp==0){ // same theme
            comp= compareName1.compareTo(compareName2); // compare names 
        }
        return comp;
        }
};


}

And in my main activity i have:

 ListData ld[]= new ListData[jsonResponse.size()];
(some code filling my ListData array)
 Arrays.sort(ld, ListData.ListDataThemeAndNameComparator); // compare by theme and then by name

Does anyone know what i'm doing wrong?

I edited my code But still it fails, now on a nullpointerexception on the compareTheme1 = ld1.theme.toUpperCase();. But i am sure my array is not empty, i logged it the line before sorting it and its filled with about 500 items.

biegleux · Accepted Answer · 2012-07-23T10:31:11.880

1

Your ListData object should implements Comparable not Comparator interface.

EDIT:

To make things clear, you can sort an array by Array.sort(). To make custom sort, you can specify your comparator in Array.sort(), if you don't do that, array will be sorted in natural order which you can define by implementing Comparable interface. So you have two options how to custom sort:

by using custom comparator and specifying it in Array.sort()
by implementing Comparable interface to your items

I would suggest you to go with implementing Comparable. You save memory by not creating new comparator objects and Comparator is useful if you are comparing objects of different types which is not your case.

edited Jul 23 '12 at 10:31

answered Jul 23 '12 at 08:54

biegleux

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I edited my code, and now i have an other error(explained in my question). Thank you for your contribution, but only changing comparator to comparable (and ofcourse imports etc) isn't solving my problem. – Jasper Jul 23 '12 at 10:04
Your array may not be empty, but it seems that some of your elements have not theme field initialized. Check theme and name for null. – biegleux Jul 23 '12 at 10:08
You are right. I had some empty fields at the end of the array, because i declared a to big array. Im changing it to arraylist, but when i used the correct number it worked. – Jasper Jul 23 '12 at 10:22

sorting custom object array on two fields

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