<分区>
我正在尝试在 listView 中显示数据。我有当前代码尝试获取从 myDatabase 类传入的数组，然后在我的 listView 中显示它。
但是我得到一个错误:
"Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference".
当我删除 listView 元素的代码时，我的代码在主要 Activity 中工作，我已经测试过它以确保传入的数组输出正确(确实如此)。我不确定抛出此错误我做错了什么。
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>
错误日志
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)