How does the BaseAdapter work when we extend it?

Question

How these methods work when we extend the 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() , then what getView() will get from it and return getView()_return_type , who else is there, in get_the_return_value_of getView() .

I am completely confused by these methods.

Answer 1

The message below corresponds to what I understood. So do not be shy if you want to improve it, instead of criticizing a particular point.

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

public int getCount () -> provides you with Total Elements present in your adapter (e.g. array size)

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

public Object getItem (int position) -> indicates which element was clicked, just return here the way I specified the position. It actually returns the entire biwa that you clicked with all its properties, and so we just return here the view of the position we clicked to tell the BASEADAPTER class that this view is clicked

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

public long getItemId (int position) will provide the main identifier that you want to return when you melt in any element of the list. when you actually click on any element of a list, it returns two things of primarykey of a long format and an int format position.

from this method getItemId (), which returns the primary key.

we usually specify the 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 the main identifier in a long format. But we must manually indicate here in the BaseAdapter that 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, parent group of ViewGroup) actaully creates a view in which you link your own 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 need to bind this adapter to your control, since we redefine the methods here everything will be processed automatically on its own.