Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object ref [duplicate]

问题

I'm trying to display data in a listView. I have the current code attempting to take an array that is passed in from the myDatabase class, and then display this within my listView.

However I got an error:

"Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference".

My code is working in main activity when I remove the code for the listView element, in it's place I've tested to ensure the passed in array outputs correctly (which it does). I'm not sure what I'm doing wrong to have this error thrown up.

MainScreenActivity.java

public class MainScreenActivity extends AppCompatActivity {

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

    // Find View-elements
    Button trackButton = (Button) findViewById(R.id.trackSelectButton);

    // Create click listener for trackSelectionButton
    trackButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {                
            trackInput();

            // Start Track List Activity
            Intent trackScreen = new Intent(v.getContext(), TrackListActivity.class);
            startActivity(trackScreen);
        }
    });        
}

public void trackInput(){

    MyDatabase mDb = new MyDatabase(this);

    String[] tracks = mDb.fetchDatabaseTracks();
    ArrayAdapter<String> trackAdapter =
            new ArrayAdapter<String>(this,
                    android.R.layout.simple_list_item_1,
                    android.R.id.text1, tracks);

    ListView lv = (ListView) findViewById(R.id.listView);
    lv.setAdapter(trackAdapter);
}

TrackListActivity.java

public class TrackListActivity extends AppCompatActivity {

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

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}

}

activity_track_list.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
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="com.example.autoplaylist.catalog.TrackListActivity">

<ListView
    android:id="@+id/listView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

activity_main_screen.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
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=".MainScreenActivity">

<TextView
    android:id="@+id/welcomeMessage"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/opening_message"
    android:textSize="18sp"
    android:fontFamily="sans-serif-light"
    android:layout_marginBottom="10dp"/>

<RelativeLayout
    android:id="@+id/relLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">

    <Button
        android:id="@+id/trackSelectButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:text="Track selection" />

</RelativeLayout>

</LinearLayout>

Error log

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.autoplaylist.catalog.MainScreenActivity.trackInput(MainScreenActivity.java:61)
at com.example.autoplaylist.catalog.MainScreenActivity$1.onClick(MainScreenActivity.java:29)
at android.view.View.performClick(View.java:5204)
at android.view.View$PerformClick.run(View.java:21153)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

回答1:

You are trying to find the ListView element in your activity_main_screen xml where there is no such element. So, either add the ListView to your activity_main_screen or use the proper xml file.

Use

setContentView(R.layout.activity_track_list);

instead of

setContentView(R.layout.activity_main_screen);

and move

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

to onCreate()

回答2:

You dont have a ListView in mainScreen.xml. yOu are actually wiring up your activity to mainscreen.xml. and trying to display your List in tracklist.xml which is not possible. So if you want to display your list then do this.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
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=".MainScreenActivity">

<TextView
    android:id="@+id/welcomeMessage"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/opening_message"
    android:textSize="18sp"
    android:fontFamily="sans-serif-light"
    android:layout_marginBottom="10dp"/>

<RelativeLayout
    android:id="@+id/relLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">

    <Button
        android:id="@+id/trackSelectButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:text="Track selection" />

<ListView
    android:id="@+id/listView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
</RelativeLayout>

</LinearLayout>

来源：https://stackoverflow.com/questions/35882414/attempt-to-invoke-virtual-method-void-android-widget-listview-setadapterandroi

标签

android

listview

android-adapter