Making a ListView holder for MainActivity

问题

I have my listview working to display it on the MainActivity and the layout is set to have 3 TextView properties of a new Person object.

My previous code was working but the scrolling was slow, so i looked into using the Holder pattern for optimization. However, when trying to implement this i am getting NullPointerException on lines TextView personID=(TextView) view.findViewById(R.id.person_field1)

listView is set to a ListView on the MainActivity and i need the person objects to display on this view within the MainActivity.

MainActivity class:

public void onButtonClick() 
{
   //put the persons objects on the listView within MainActivity
   listView.setAdapter(new personAdapter(MainActivity.this, R.layout.list_layout, 
                        personList));
}

class personAdapter extends ArrayAdapter<Person>
{
    private LayoutInflater layoutInflator;

    public personAdapter(Context context, int resource, List<Person> p) {
        super(context, resource, p);
        layoutInflator = LayoutInflater.from(context);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View view = convertView;
        Holder holder = null;
        Person p = getItem(position);

        if (view == null) {
            layoutInflator.inflate(R.layout.list_layout, null);  

            //getting null pointer exceptions here
            TextView personID = (TextView) view.findViewById(R.id.person_field1);
            TextView personFName = (TextView) view.findViewById(R.id.person_field2);
            TextView personLName = (TextView) view.findViewById(R.id.person_field3);

            holder = new Holder(personID, personFName, personLName);

            view.setTag(holder);
        }
        else {
            holder = (Holder) view.getTag();
        }

        holder.stop.setText(person.getID());
        holder.location.setText(person.getFName());
        holder.time.setText(person.getLName()); 
        return view;
    }
}

static class Holder 
{
    public TextView id;
    public TextView FName;
    public TextView LName;

    public Holder(TextView id, TextView FName, TextView LName) {
        this.id = id;
        this.FName = FName;
        this.LName = LName;
    }
}

回答1:

because there really is null pay attention to check view == null, and then view never initialized try this

view = layoutInflator.inflate(R.layout.list_layout, null);

回答2:

Just add reference to your view:

view = layoutInflator.inflate(R.layout.list_layout, null);

回答3:

view is null, you're even checking for it:

if (view == null) { ...
}

What you're missing is assigning the inflated layout to view

view = layoutInflator.inflate(R.layout.list_layout, null);

来源：https://stackoverflow.com/questions/28730417/making-a-listview-holder-for-mainactivity

标签

java

android

android-viewholder