我正在尝试构建一个简单的列表视图应用程序，但我不断收到这个恼人的错误。我对Android很新，所以我真的不知道如何修复它。任何帮助表示赞赏。
错误行位于BirthdayReminderActivity.java中：
listView.setOnItemClickListener（new OnItemClickListener（）{
Logcat错误输出为：
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: premprakash.birthdayreminder, PID: 2266
java.lang.RuntimeException: Unable to start activity ComponentInfo{premprakash.birthdayreminder/premprakash.birthdayreminder.BirthdayReminderActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setOnItemClickListener(android.widget.AdapterView$OnItemClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setOnItemClickListener(android.widget.AdapterView$OnItemClickListener)' on a null object reference
at premprakash.birthdayreminder.BirthdayReminderActivity.onCreate(BirthdayReminderActivity.java:51)
at android.app.Activity.performCreate(Activity.java:6662)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Application terminated.
BirthdayReminderActivity.java
package premprakash.birthdayreminder;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Gravity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import android.widget.AdapterView;
import android.os.Handler;
import android.util.Log;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
public class BirthdayReminderActivity extends AppCompatActivity {
private CustomCursorAdapterBirthday customAdapter;
private BirthdayDatabaseHelper databaseHelper;
private static final int ENTER_BIRTHDAY_REQUEST_CODE = 1;
private ListView listView;
private static final String TAG = BirthdayReminderActivity.class.getSimpleName();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_birthday_reminder);
Button add = (Button) findViewById(R.id.add);
databaseHelper = new BirthdayDatabaseHelper(this);
listView = (ListView) findViewById(R.id.list_birthday);
listView.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Log.d(TAG, "clicked on item: " + position);
}
});
// Database query can be a time consuming task ..
// so its safe to call database query in another thread
// Handler, will handle this stuff for you <img src="http://s0.wp.com/wp-includes/images/smilies/icon_smile.gif?m=1129645325g" alt=":)" class="wp-smiley">
new Handler().post(new Runnable() {
@Override
public void run() {
customAdapter = new CustomCursorAdapterBirthday(BirthdayReminderActivity.this, databaseHelper.getAllBirthday());
System.out.println("dsafjkdsjflkdsjfkdsjf");
listView.setAdapter(customAdapter);
}
});
}
public void onClickAdd(View add) {
add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast toast = Toast.makeText(getApplicationContext(), "Successfully Added", Toast.LENGTH_SHORT);
toast.setGravity(Gravity.BOTTOM, 0, 0);
toast.show();
startActivity(new Intent(BirthdayReminderActivity.this, ReminderTypesActivity.class));
}
});
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent birthday) {
super.onActivityResult(requestCode, resultCode, birthday);
if (requestCode == ENTER_BIRTHDAY_REQUEST_CODE && resultCode == RESULT_OK) {
databaseHelper.insertBirthday(birthday.getExtras().getString("tag_name"), birthday.getExtras().getString("tag_date"), birthday.getExtras().getString("tag_setalarm"), birthday.getExtras().getString("tag_date1"), birthday.getExtras().getString("tag_time"));
customAdapter.changeCursor(databaseHelper.getAllBirthday());
}
}
}
birthdayreminderactivity.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_birthday_reminder"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="premprakash.birthdayreminder.BirthdayReminderActivity">
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:id="@+id/add"
android:visibility="visible"
android:contextClickable="true"
tools:text="@string/add"
android:onClick="onClickAdd"
tools:ignore="UnusedAttribute" />
<SearchView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignParentBottom="true"
android:layout_toRightOf="@+id/add"
android:layout_alignTop="@+id/add"
android:id="@+id/search"
android:orientation="vertical"
android:filterTouchesWhenObscured="false"
android:focusableInTouchMode="true"
tools:focusableInTouchMode="false"
android:clickable="true"
tools:ignore="RtlHardcoded" />
<ListView
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:ignore="NestedScrolling"
android:id="@+id/list_birthday"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
</RelativeLayout>
的manifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="premprakash.birthdayreminder">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".BirthdayReminderActivity">
<intent-filter>
<action android:name="android.intent.action.BIRTHDAYREMINDER" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".ReminderTypesActivity">
<intent-filter>
<action android:name="android.intent.action.REMINDERTYPES" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".BirthdayFormActivity">
<intent-filter>
<action android:name="android.intent.action.BIRTHDAYFORM" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".AnniversaryFormActivity">
<intent-filter>
<action android:name="android.intent.action.ANNIVERSARYFORM" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".OthersFormActivity">
<intent-filter>
<action android:name="android.intent.action.OTHERSFORM" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
CustomCursorAdapterBirthday.java
package premprakash.birthdayreminder;
import android.content.Context;
import android.database.Cursor;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CursorAdapter;
import android.widget.TextView;
public class CustomCursorAdapterBirthday extends CursorAdapter {
public CustomCursorAdapterBirthday(Context context, Cursor c) {
super(context, c);
}
@Override
public View newView(Context context, Cursor cursor, ViewGroup parent) {
// when the view will be created for first time,
// we need to tell the adapters, how each item will look
LayoutInflater inflater = LayoutInflater.from(parent.getContext());
View retView = inflater.inflate(R.layout.activity_birthday_reminder, parent, false);
return retView;
}
@Override
public void bindView(View view, Context context, Cursor cursor) {
// here we are setting our data
// that means, take the data from the cursor and put it in views
TextView textViewName = (TextView) view.findViewById(R.id.name);
textViewName.setText(cursor.getString(cursor.getColumnIndex(cursor.getColumnName(1))));
TextView textViewDate = (TextView) view.findViewById(R.id.date);
textViewDate.setText(cursor.getString(cursor.getColumnIndex(cursor.getColumnName(2))));
TextView textViewSetalarm = (TextView) view.findViewById(R.id.setalarm);
textViewSetalarm.setText(cursor.getString(cursor.getColumnIndex(cursor.getColumnName(3))));
TextView textViewDate1 = (TextView) view.findViewById(R.id.date);
textViewDate1.setText(cursor.getString(cursor.getColumnIndex(cursor.getColumnName(4))));
TextView textViewTime = (TextView) view.findViewById(R.id.time);
textViewTime.setText(cursor.getString(cursor.getColumnIndex(cursor.getColumnName(5))));
}
}
BirthdayDatabaseHelper.java
package premprakash.birthdayreminder;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class BirthdayDatabaseHelper {
private static final String TAG = BirthdayDatabaseHelper.class.getSimpleName();
// database configuration
// if you want the onUpgrade to run then change the database_version
private static final int DATABASE_VERSION = 4;
private static final String DATABASE_NAME = "birthdaydatabase.db";
// table configuration
private static final String TABLE_NAME = "birthday"; // Table name
private static final String BIRTHDAY_TABLE_COLUMN_ID = "_id"; // a column named "_id" is required for cursor
private static final String BIRTHDAY_TABLE_COLUMN_NAME = "name";
private static final String BIRTHDAY_TABLE_COLUMN_DATE = "date";
private static final String BIRTHDAY_TABLE_COLUMN_SETALARM = "setalarm";
private static final String BIRTHDAY_TABLE_COLUMN_DATE1 = "date1";
private static final String BIRTHDAY_TABLE_COLUMN_TIME = "time";
private DatabaseOpenHelper openHelper;
private SQLiteDatabase database;
// this is a wrapper class. that means, from outside world, anyone will communicate with PersonDatabaseHelper,
// but under the hood actually DatabaseOpenHelper class will perform database CRUD operations
public BirthdayDatabaseHelper(Context Context) {
openHelper = new DatabaseOpenHelper(Context);
database = openHelper.getWritableDatabase();
}
public void insertBirthday (String Name, String Date, String Setalarm, String Date1, String Time) {
// we are using ContentValues to avoid sql format errors
ContentValues contentValues = new ContentValues();
contentValues.put(BIRTHDAY_TABLE_COLUMN_NAME, Name);
contentValues.put(BIRTHDAY_TABLE_COLUMN_DATE, Date);
contentValues.put(BIRTHDAY_TABLE_COLUMN_SETALARM, Setalarm);
contentValues.put(BIRTHDAY_TABLE_COLUMN_DATE1, Date1);
contentValues.put(BIRTHDAY_TABLE_COLUMN_TIME, Time);
database.insert(TABLE_NAME, null, contentValues);
}
public Cursor getAllBirthday () {
String buildSQL = "SELECT * FROM " + TABLE_NAME;
Log.d(TAG, "getAllBirthday SQL: " + buildSQL);
System.out.println("HKJHDSFKJDhf");
return database.rawQuery(buildSQL, null);
}
// this DatabaseOpenHelper class will actually be used to perform database related operation
private class DatabaseOpenHelper extends SQLiteOpenHelper {
public DatabaseOpenHelper(Context Context) {
super(Context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
// Create your tables here
String buildSQL = "CREATE TABLE " + TABLE_NAME + "( " + BIRTHDAY_TABLE_COLUMN_ID + " INTEGER PRIMARY KEY, " + BIRTHDAY_TABLE_COLUMN_NAME + " TEXT, " + BIRTHDAY_TABLE_COLUMN_DATE + " TEXT, " + BIRTHDAY_TABLE_COLUMN_SETALARM + " TEXT, " + BIRTHDAY_TABLE_COLUMN_DATE1 + " TEXT, " + BIRTHDAY_TABLE_COLUMN_TIME + " TEXT )";
Log.d(TAG, "onCreate SQL: " + buildSQL);
sqLiteDatabase.execSQL(buildSQL);
}
@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int oldVersion, int newVersion) {
// Database schema upgrade code goes here
String buildSQL = "DROP TABLE IF EXISTS " + TABLE_NAME;
Log.d(TAG, "onUpgrade SQL: " + buildSQL);
sqLiteDatabase.execSQL(buildSQL); // drop previous table
onCreate(sqLiteDatabase); // create the table from the beginning
}
}
}
答案 0 :(得分：0)
这是一个标准的空指针异常，即您正在尝试访问不存在的对象。在您的情况下，您的活动无法找到您的ListView，因此findViewById已返回空对象。
我怀疑这一行是BirthdayReminderActivity.java中的问题：
setContentView(R.layout.activity_birthday_reminder);
它与您提供的xml文件（birthdayreminderactivity.xml）的名称不同，因此您的活动正在膨胀名为activity_birthday_reminder的文件。尝试将其更改为：
setContentView(R.layout.birthdayreminderactivity);
答案 1 :(得分：0)
您正在使用linearlayout的ID而不是布局文件名
设置contentview setContentView(R.layout.birthdayreminderactivity);