how does BaseAdapter works in when we extend it?

Question

How these methods works when we extend Baseadapter .

public int getCount() 
public Object getItem(int position) 
public long getItemId(int position) 
public View getView(int position, View convertView, ViewGroup parent) 

because if we have some_return_type_from_getCount() , than what getView() will get from it and when we return getView()_return_type who else there to get_the_return_value_of getView() .

I am totally confused with these methods .

Answer 1

The post below is according to what I understood. So feel free If You want improve it instead of criticizing for some particular point.

private ArrayList<HashMap<String, String>> mProjectsList = new ArrayList<HashMap<String, String>>(); (You can use any cursor or array that actually consists data and you want to bind it using adapter)

public int getCount() -> gives you the Total Elements present in your adapter (like the size of the array)

@Override
    public int getCount() {
        // TODO Auto-generated method stub
        return mProjectsList.size();
    }

public Object getItem(int position) -> tells which item was clicked simply return here the way I did specifying the position . It actually return the whole biwe that you clicked with all its properties and thats why we just return here the view of the position we clicked in order to tell the BASEADAPTER Class that this view is cliked

@Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return mProjectsList.get(position);
    }

public long getItemId(int position) will give the primary Id that you want to return when you will taap on some list item. when you actually click some item of list view it returns two things primarykey of long format and position of int format..

from this getItemId() method actually that returns the primary key.

we usually specify primary key as "_id" in our database so when we use simple adapters instead of extending the baseadapter class it automatically returns the _id field as primary id in long format. But we have to manually specify here in BaseAdapter what we want to return

@Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return Long.parseLong(mProjectsList.get(position).get("ID")) ;
// retuning my Primary id from the arraylist by
    }

public View getView(int position, View convertView, ViewGroup parent) actaully creates the view where you bind your custom layout

@Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub


        //**position**  index of the item whose view we want.
        //**convertView**   the old view to reuse, if possible. Note: You should 
//check that this view is non-null and of an appropriate type before using. If it is 
//not possible to convert this view to display the correct data, this method can create a 
//new view.
  //                        (Lets see if you have total 100 elements in listview , but 
//currently only 10 are visible in screen. So it will create only 10 items at a time as 
//only those are visible.
                //and when you will scroll that listView it will use these same 10 
//Views(elemnts in rows to display new data, instead of creating 10 more new views, which 
//obviously will be efficeient)
                //While alternatively if we dont extend base adapter (but rather 
//use simple binding using simpleadapter) it will then generates whole list of 100 Views 
//in one short whic will be time consuimng/ memory consuming depending uopn the amount of 
//data to be bind   



        //**parent**    the parent that this view will eventually be attached to


        View rowView = convertView; 
        if (rowView == null) { //always required to be checked as mentioned in google docs 
                               // this line checks for if initially row View is null then we have to create the Row View. Once it will be created then it will always Surpass this check and we will keep on reusing this rowView (thats what actually we are looking for) 
          LayoutInflater inflater = mActivitycontext.getLayoutInflater();  // creating instance of layout inflater to inflate our custom layout
          rowView = inflater.inflate(R.layout.projectslist_row, null); //assigend our custom row layout to convertview whic is to be reused
          ViewHolder viewHolder = new ViewHolder();     // ViewHolder is a custom class in which we are storing are UI elaments of Row  
          viewHolder.mprojectslistRowView = (TextView) rowView.findViewById(R.id.projectslist_row); //assigned the id of actual textView thats on our custom layout to the instance of TextView in our static class       
          rowView.setTag(viewHolder);
        }

        ViewHolder holder = (ViewHolder) rowView.getTag();
        String projectName = mProjectsList.get(position).get("ProjectName"); // here i am fetching data from my HashMap ArrayList
        holder.mprojectslistRowView.setText(projectName);       // here i am just assigning what to show in my text view 

        return rowView;
    }

    I created this as inner class 
    static class ViewHolder 
    {
        // create instances for all UI elemnts of your Row layout (here i am showing only text data as row of my list view so only instance of TextView has been created)
        // It is a statci class hence will keep this instance alive all the time and thats Why we will be able to  reuse it again and again.
        TextView mprojectslistRowView;
    }

You just have to bind this adapter to your Control, as we are overriding the methods here everything will be handled automatically on its own.