Android: listview: custom items: nullpointerexception, findviewbyid returns null

问题

I have been googling and searching to resolve this error for some time and I can`t seem to find out why and how to solve it.

I`m using a customAdapter to fill in my listview. I inflate the xml describing my listItem. I make viewHolder object and load in my textview using findViewById. After that i want to set the text of that textview but it findViewbyid returns a null value. So automaticcally resolving into a nullpointerexception.

Adapter:

/** CLASS : Custom Adapter for the listview */
private class CustomAdapter extends ArrayAdapter<Track>{
    private List<Track> items;
    private Context mContext;
    private int layoutResourceId;       
    private ViewHolder mHolder;

    public CustomAdapter(Context context, int layoutId, List<Track> objects) {
        super(context, layoutId, objects);
        layoutResourceId=layoutId;
        items = objects;
        mContext=context;       
    }       

    public View getView(int position, View convertView, ViewGroup parent) {
         if(convertView == null){
             LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
             convertView = inflater.inflate(layoutResourceId, parent, false);
             mHolder = new ViewHolder();
             ****** RETURNS NULL DURING DEBUGGING ******
             mHolder.textViewCenter = (TextView)findViewById(R.id.textview_list_item_central);
             convertView.setTag(mHolder);
         }else{
             mHolder = (ViewHolder) convertView.getTag();
         }
         Track track = items.get(position);
         ****** ERROR HAPPENS HERE ******
         mHolder.textViewCenter.setText(track.getTrack());
         return convertView;
    }

    class ViewHolder{
        ImageView imageView;
        TextView textViewCenter;
        TextView textViewRight;
    }
}

XML:

?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="horizontal" >

<ImageView
    android:id="@+id/imageview_list_albumart_icon"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="left"
    android:src="@drawable/ic_music" />

<TextView
    android:id="@+id/textview_list_item_central"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:layout_weight="0.5"
    android:text="Center"
    android:textSize="20sp" >
</TextView>

<TextView
    android:id="@+id/textview_list_item_right"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="right"
    android:paddingRight="10dip"
    android:text="Right" />

Any help is appreciated!

回答1:

Search for the views in the convertView that you inflate(and not in the current Activity layout like you currently do):

 mHolder.textViewCenter = (TextView) 
                convertView.findViewById(R.id.textview_list_item_central);

回答2:

You are inflating the layout, but not using it while fetching it's views

Do this:

 mHolder.textViewCenter = (TextView)convertView.findViewById(R.id.textview_list_item_central)

回答3:

TextView initialization is wrong

if(convertView == null){
             LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
             convertView = inflater.inflate(layoutResourceId, parent, false);
             mHolder = new ViewHolder();
             ****** RETURNS NULL DURING DEBUGGING ******
             mHolder.textViewCenter = (TextView) convertView .findViewById(R.id.textview_list_item_central); //Change done
             convertView.setTag(mHolder);
         }else{
             mHolder = (ViewHolder) convertView.getTag();
         }

回答4:

change the line

mHolder.textViewCenter = (TextView)findViewById(R.id.textview_list_item_central);

which is below * RETURNS NULL DURING DEBUGGING * by

mHolder.textViewCenter = (TextView) convertView.findViewById(R.id.textview_list_item_central);

回答5:

try replacing this:

convertView = inflater.inflate(R.layout.listitemview, null);

where listitemview is your xml in which you defined your ImageView and TextViews..
hope this helps you

来源：https://stackoverflow.com/questions/10699422/android-listview-custom-items-nullpointerexception-findviewbyid-returns-null

标签

android

listview

android-listview

nullpointerexception

adapter