自定义适配器上的nullpointer异常

Question

I have created a project to display list with custom row layout. Upon clicking on the list item, calling an activity which retrieves data from sqlite and display in listview using custom row. But, I am getting null pointer exception when doing setadapter in

DisplayCards.java

import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.database.Cursor;
import android.graphics.Color;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;

public class DisplayCards extends Activity{


        private Divyadesamdb dbHelper;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            dbHelper = new Divyadesamdb(this);
            dbHelper.open();

            // Clean all data
            dbHelper.deleteAllPasurams();
            // Add some data
            dbHelper.insertPasurams();

            // Generate ListView from SQLite Database
            displayListView();

        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.main, menu);
            return true;
        }

        private void displayListView() {
            String category = null;
            String starting = "25";
            String ending = "30";
            int rowsFound = 0;
            Cursor cursor = dbHelper.fetchAllPasurams(category,starting,ending);
            /*
             * How many rows returned from the query
             */
            rowsFound = cursor.getCount();
            Log.d("VC", "Query returned "+rowsFound+" rows.");
            // the desired columns to be bound
            String[] columns = new String[] { Divyadesamdb.PASURAMS_COLUMN_PAASURAMNUMBER,
                    Divyadesamdb.PASURAMS_COLUMN_AAYIRAM, Divyadesamdb.PASURAMS_COLUMN_AZHWAAR,
                    Divyadesamdb.PASURAMS_COLUMN_CATEGORY, Divyadesamdb.PASURAMS_COLUMN_MANGALASASANAMON,
                    Divyadesamdb.PASURAMS_COLUMN_PAASURAM_EN_STR, Divyadesamdb.PASURAMS_COLUMN_SUBCATEGORY,
                    Divyadesamdb.PASURAMS_COLUMN_TITLE, Divyadesamdb.PASURAMS_COLUMN_TITLENUMBER };



            // the XML defined views which the data will be bound to
            int[] to = new int[] { R.id.tvPaasuramNumber, R.id.tvAayiram, R.id.tvAzhwaar,
                    R.id.tvCategory, R.id.tvMangalasasanamOn, R.id.tvPaasuram,
                    R.id.tvSubCategory, R.id.tvTitle, R.id.tvtvTitleNumber};
Log.d("VC","Just before creating dataAdapter");
            // create the adapter using the cursor pointing to the desired data
            // as well as the layout information
            MyCursorAdapter dataAdapter = new MyCursorAdapter(this,
                    R.layout.paasuram_single_item, cursor, columns, to, 0);
            Log.d("VC","Just after creating dataAdapter");
            ListView listView = (ListView) findViewById(R.id.listView1);
            // Assign adapter to ListView
            Log.d("VC","created listView");
            int myCount = dataAdapter.getCount();
            Log.d("VC","got data from dataAdapter "+myCount);
            //listView.getAdapter();
            listView.setAdapter(dataAdapter);
            Log.d("VC","completed setadapter dataAdapter");
        }

        // extend the SimpleCursorAdapter to create a custom class where we
        // can override the getView to change the row colors
        private class MyCursorAdapter extends SimpleCursorAdapter {

            public MyCursorAdapter(Context context, int layout, Cursor c,
                    String[] from, int[] to, int flags) {
                super(context, layout, c, from, to, flags);
            }

            @Override
            public View getView(int position, View convertView, ViewGroup parent) {

                // get reference to the row
                View view = super.getView(position, convertView, parent);
                // check for odd or even to set alternate colors to the row
                // background
                if (position % 2 == 0) {
                    view.setBackgroundColor(Color.rgb(238, 233, 233));
                } else {
                    view.setBackgroundColor(Color.rgb(255, 255, 255));
                }
                return view;
            }

        }

    }

paasuram_single_item.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@drawable/mybackground"
    android:orientation="vertical" >

    <ImageView
        android:id="@+id/ivAzhwaar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="false"
        android:layout_margin="10dp"
        android:contentDescription="@string/Azhwaar"
        android:padding="5dp"
        android:src="@drawable/nammalwar" />

    <TextView
        android:id="@+id/tvtvTitleNumber"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignTop="@+id/ivAzhwaar"
        android:layout_toRightOf="@+id/ivAzhwaar"
        android:text="TitleNumber"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/tvHyphan1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/tvtvTitleNumber"
        android:layout_alignBottom="@+id/tvtvTitleNumber"
        android:layout_toRightOf="@+id/tvtvTitleNumber"
        android:text=" "
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/tvTitle"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_alignBaseline="@+id/tvHyphan1"
        android:layout_alignBottom="@+id/tvHyphan1"
        android:layout_alignParentRight="true"
        android:layout_toRightOf="@+id/tvHyphan1"
        android:text="Title"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/tvAayiram"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/tvtvTitleNumber"
        android:layout_below="@+id/tvtvTitleNumber"
        android:text="Aayiram"
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvHyphan2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/tvtvTitleNumber"
        android:layout_toRightOf="@+id/tvAayiram"
        android:text=" - "
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvCategory"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/tvHyphan2"
        android:layout_alignBottom="@+id/tvHyphan2"
        android:layout_toRightOf="@+id/tvHyphan2"
        android:text="Category"
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvHyphan3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/tvCategory"
        android:layout_alignBottom="@+id/tvCategory"
        android:layout_toRightOf="@+id/tvCategory"
        android:text=" - "
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvSubCategory"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/tvHyphan3"
        android:layout_alignBottom="@+id/tvHyphan3"
        android:layout_toRightOf="@+id/tvHyphan3"
        android:text="SubCategory"
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvPaasuram"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/tvAayiram"
        android:layout_below="@+id/tvAayiram"
        android:padding="5dp"
        android:text="paasuram"
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvPaasuramNumber"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/tvPaasuram"
        android:layout_alignBottom="@+id/tvPaasuram"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:text="PaasuramNumber"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/tvAzhwaar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/tvMangalasasanamOn"
        android:layout_alignBottom="@+id/tvMangalasasanamOn"
        android:layout_alignLeft="@+id/tvPaasuram"
        android:text="Azhwaar"
        android:textAppearance="?android:attr/textAppearanceSmall" />

    <TextView
        android:id="@+id/tvMangalasasanamOn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/tvCategory"
        android:layout_below="@+id/tvPaasuram"
        android:layout_centerHorizontal="true"
        android:text="MangalasasanamOn"
        android:textAppearance="?android:attr/textAppearanceSmall" />

</RelativeLayout>

paasuram_list_fragment.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >



    <ListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >
    </ListView>



</RelativeLayout>

NPE is happening on DisplayCards.java

listView.setAdapter(dataAdapter);

Able to get the correct count in dataAdapter

any help is appreciated.

Answer 1

在activity_main.xml中没有id为listView1元素，这就是listView为null的原因。

Answer 2

Change

ListView listView = (ListView) findViewById(R.id.listView1);

into

ListView listView = (ListView) findViewById(android.R.id.list);