ListView中的NullPointException

Question

Please have a look at the following code

activity_form.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"
    tools:context=".Form" >

    <ImageView
        android:id="@+id/logo"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:src="@drawable/logo" />

    <TextView
        android:id="@+id/title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/logo"
        android:layout_marginTop="10dp"
        android:text="@string/title" />

    <ListView
        android:id="@+id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/title"
        android:layout_marginTop="10dp"
        />

</RelativeLayout>

list_layout.xml

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

    <ImageView
        android:id="@+id/listImage"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:src="@drawable/star" />

    <TextView
        android:id="@+id/listText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingLeft="5dp"
        android:paddingTop="10dp"
        android:text="Large Text"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</LinearLayout>

Form.java

package com.example.callmanager;

import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;

public class Form extends Activity {

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

        ListView lv = (ListView)findViewById(android.R.id.list);
        String arr[] = getResources().getStringArray(R.array.branch_list);
        lv.setAdapter(new MyAdapter(this,android.R.layout.simple_list_item_1,R.id.listText,arr));
    }

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

    private class MyAdapter extends ArrayAdapter
    {

        public MyAdapter(Context context, int resource, int textViewResourceId,
                Object[] objects) {
            super(context, resource, textViewResourceId, objects);
            // TODO Auto-generated constructor stub
        }

        @Override
        public View getView(int position, View contentView, ViewGroup parent)
        {
            LayoutInflater inflater = (LayoutInflater)getSystemService(LAYOUT_INFLATER_SERVICE);
            View view = inflater.inflate(R.layout.list_layout, parent,false);

            TextView tv = (TextView)findViewById(R.id.listText);
            ImageView iv = (ImageView)findViewById(R.id.listImage);

            String arr[] = getResources().getStringArray(R.array.branch_list);
            tv.setText(arr[position]);

            if(arr[position].equals("Anuradhapura"))
            iv.setImageResource(R.drawable.star);

            return view;

        }

    }

}

strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Esoft Call Manager</string>
    <string name="hello_world">Hello world!</string>
    <string name="menu_settings">Settings</string>
    <string name="title"><b>Select a Branch To Call</b></string>

    <string-array name="branch_list">
        <item>Anuradhapura</item>
        <item>Avissawella</item>

    </string-array>

</resources>

When I run this code, I get the ' NullPointException '. I am adding the complete log file here

02-08 21:04:01.463: D/dalvikvm(490): GC_EXTERNAL_ALLOC freed 43K, 53% free 2551K/5379K, external 1949K/2137K, paused 82ms
02-08 21:04:01.713: D/AndroidRuntime(490): Shutting down VM
02-08 21:04:01.713: W/dalvikvm(490): threadid=1: thread exiting with uncaught exception (group=0x40015560)
02-08 21:04:01.733: E/AndroidRuntime(490): FATAL EXCEPTION: main
02-08 21:04:01.733: E/AndroidRuntime(490): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.callmanager/com.example.callmanager.Form}: java.lang.NullPointerException
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.os.Looper.loop(Looper.java:123)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.ActivityThread.main(ActivityThread.java:3683)
02-08 21:04:01.733: E/AndroidRuntime(490):  at java.lang.reflect.Method.invokeNative(Native Method)
02-08 21:04:01.733: E/AndroidRuntime(490):  at java.lang.reflect.Method.invoke(Method.java:507)
02-08 21:04:01.733: E/AndroidRuntime(490):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
02-08 21:04:01.733: E/AndroidRuntime(490):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
02-08 21:04:01.733: E/AndroidRuntime(490):  at dalvik.system.NativeStart.main(Native Method)
02-08 21:04:01.733: E/AndroidRuntime(490): Caused by: java.lang.NullPointerException
02-08 21:04:01.733: E/AndroidRuntime(490):  at com.example.esoftcallmanager.Form.onCreate(Form.java:24)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
02-08 21:04:01.733: E/AndroidRuntime(490):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
02-08 21:04:01.733: E/AndroidRuntime(490):  ... 11 more

I am adding the folder structure below if it helps

Why I am getting this error? How to solve this? Please help!

Answer 1

try this:

change this

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

for:

ListView lv = (ListView)findViewById(R.id.list);

Normally they use android.R.id.list when class extends a ListActivity

Answer 2

Change:

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

to

    ListView lv = (ListView)findViewById(R.id.list);

Unless you have an id in your XML in the format of android:id="@android:id/myID" , you don't need to use android.R . Whenever you use one of your own IDs, you can simply use R.id.Whatever

Answer 3

The issue is right here. ListView lv = (ListView)findViewById(android.R.id.list); you reference the Id android.R.id.list however in your layout you reference android:id="@+id/list" which creates an Id in the Resources class called list. so you should be referencing R.id.list if you debug you will find that the lv variable is null because it wasn't found because you referenced the wrong id.

Answer 4

Try this.

I hope it will solve your NPE.

Change it

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

to

ListView lv = (ListView)findViewById(R.id.list);

Actually You have defined the list view in the layout activity_form.xml have name list so you need to write the R.id.list .

Answer 5

如果您使用自己的布局，则无需再编写android.R只需删除android并重新组织您的类

Answer 6

I managed to solve this by my self. Following edits were required.

activity_form.xml

The "id" of the ListView should be

android:id="@android:id/list"

Form.java

It should call the ListView in this way

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

Inside getView() , the initialization of the TextView and ImageView should be like this

TextView tv = (TextView) view.findViewById(R.id.listText);
ImageView iv = (ImageView) view.findViewById(R.id.listImage);

Note the view.findViewById() part.