Collections.sort

Question

Collections.sort(apps, new ResolveInfo.DisplayNameComparator(manager)); This is a sort function.The first parameter is an array to be sorted. Why we need the second parameter?What can the second parameter do?

Answer 1

Sort the second argument isComparable so you can provide custom function for comparing the object.

Collections.sort(apps, new ResolveInfo.DisplayNameComparator(manager));

It's seems this uses a comparator that compares objects by display name.

Answer 2

apps is a list of ResolveInfo objects. ResolveInfo isn't Comparable , meaning that because it doesn't implement Comparable interface, there is no "rule" on how to compare two ResolveInfo objects. sort(List<T> list) method requires objects in the list to be comparable:

public static <T extends Comparable<? super T>> void sort(List<T> list)

DisplayNameComparator defines logic on how to compare ResolveInfo objects by the display names of apps they represent.

Source code:

  public static class DisplayNameComparator
        implements Comparator<ResolveInfo> {
    public DisplayNameComparator(PackageManager pm) {
        mPM = pm;
        mCollator.setStrength(Collator.PRIMARY);
    }

    public final int compare(ResolveInfo a, ResolveInfo b) {
        CharSequence  sa = a.loadLabel(mPM);
        if (sa == null) sa = a.activityInfo.name;
        CharSequence  sb = b.loadLabel(mPM);
        if (sb == null) sb = b.activityInfo.name;

        return mCollator.compare(sa.toString(), sb.toString());
    }

    private final Collator   mCollator = Collator.getInstance();
    private PackageManager   mPM;
}