在两个字段上对自定义对象数组进行排序

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.

Answer 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.