Difference between getPositionForSection and getSectionForPosition

Question

I am implementing ListView with section in which I show custom section headers apart from the conventional alphabets as the header. In order for me to implement the custom SectionIndexer correctly, I wish to understand the difference between the two methods getSectionForPosition and getPositionForSection.

I understand (not sure if that's correct) that getSectionForPosition returns the alphabet we want to show in the section header.

I don't understand the other method. Also, are they similar in any sense (if at all) and in what way they differ (if they do, which I think they do :) )

Anybody having understanding about this may kindly post an answer to this. Appreciate your time going through the question.

Update:

I have gone through the documentation at this official page; I'm looking for some elaborated insight with respect to custom SectionIndex implementation

Answer 1

getPositionForSection(section) returns the first position at which the cursor data at the indexed column starts with the section.

For example if the index of section B is 1 and the indexed column of the cursor has the following data

Position        Data               getSectionForPosition(position)

_________       __________         ______________________________

0               Abhfdf              0

1               Achahtkh            0

2               Ahtjlarej           0

3               Bchatkd             1

4               Bjklhdsfoi          1

5               Bzhafdlsfk          1

6               Cj fadsfkj          2

then getPositionForSection(1) returns 3

also getPositionForSection(2) returns 6

Hope this helps you

Answer 2

Here's a sample implementation I created. Perhaps it clarifies the SectionIndexer a bit.

For holding your data, you could use private inner classes:

private class Item {
    public int sectionNumber;

    public Long id;
    public String name;
}

private class Section {
    public int startPosition;

    public String header;

    @Override
    public String toString(){
        return header;
    }
}

Use a list for your items and one for your sections:

private List<Item> items = new ArrayList<Item>();
private List<Section> sections = new ArrayList<Section>();

The implementation in your list adapter for SectionIndexer could be like the following now:

@Override
public int getPositionForSection(int sectionNumber) {
    return sections.get(sectionNumber).startPosition;
}

@Override
public int getSectionForPosition(int itemPosition) {
    return items.get(itemPosition).sectionNumber;
}

@Override
public Object[] getSections() {
    return sections.toArray();
}

Ofcourse you should implement a method now to fill your adapter's data lists.

Answer 3

The method getPositionForSection(int section) returns you the starting index in list for section

while the method getSectionForPosition(int position) returns you the index of section which contains item at position