我想从sqlite中检索数据并在android中的文本视图中显示。我在下面编写了一些代码，但它显示了空指针异常。请帮我解决这个空指针异常。在此先感谢这是我的代码
package com.developer.milanandroid;
import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class AdminSettings extends Activity {
public LoginDataBaseAdapter logindatabase_adapter_child;
Button fetch_database;
TextView text_fetched_database_results;
String username,password;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.admin_settings);
fetch_database = (Button)findViewById(R.id.Button_Fetch_Database);
text_fetched_database_results = (TextView)findViewById(R.id.textView_fetched_database_results);
fetch_database.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Cursor c = logindatabase_adapter_child.db.rawQuery("select * from MilanloginRegistration", null);
text_fetched_database_results.setText("");
c.moveToFirst();
do{
username = c.getString(c.getColumnIndex("USERNAME"));
password = c.getString(1);
text_fetched_database_results.append("USERNAME::-->"+username+"PASSWORD::-->"+password+"\n");
}while(c.moveToNext());
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.admin_settings, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
这是我的login_database_adapter：
package com.developer.milanandroid;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.view.View;
public class LoginDataBaseAdapter
{
AdminSettings admin_settings_child;
static final String DATABASE_NAME = "MilanloginRegistration.db";
static final int DATABASE_VERSION = 1;
public static final int NAME_COLUMN = 1;
// TODO: Create public field for each column in your table.
// SQL Statement to create a new database.
static final String DATABASE_CREATE = "create table "+"MilanLoginregistration"+
"( " +"ID"+" integer primary key autoincrement,"+"USERNAME text UNIQUE,PASSWORD text); ";
// Variable to hold the database instance
public SQLiteDatabase db;
// Context of the application using the database.
private final Context context;
// Database open/upgrade helper
private DataBaseHelper dbHelper;
public LoginDataBaseAdapter(Context _context)
{
context = _context;
dbHelper = new DataBaseHelper(context, DATABASE_NAME, null, DATABASE_VERSION);
}
public LoginDataBaseAdapter open() throws SQLException
{
db = dbHelper.getWritableDatabase();
return this;
}
public void close()
{
db.close();
}
public SQLiteDatabase getDatabaseInstance()
{
return db;
}
public void insertEntry(String username,String password)
{
ContentValues newValues = new ContentValues();
newValues.put("USERNAME",username);
newValues.put("PASSWORD",password);
// Insert the row into your table
db.insert("MilanLoginregistration",null,newValues);
///Toast.makeText(context, "Reminder Is Successfully Saved", Toast.LENGTH_LONG).show();
}
public int deleteEntry(String username,String password)
{
//String id=String.valueOf(ID);
String where="USERNAME=?";
int numberOFEntriesDeleted= db.delete("MilanLoginregistration", where, new String[]{username,password}) ;
// Toast.makeText(context, "Number fo Entry Deleted Successfully : "+numberOFEntriesDeleted, Toast.LENGTH_LONG).show();
return numberOFEntriesDeleted;
}
public String getSinlgeEntry(String userName)
{
Cursor cursor=db.query("MilanLoginregistration", null, " USERNAME=?", new String[]{userName}, null, null, null);
if(cursor.getCount()<1) // UserName Not Exist
{
cursor.close();
return "NOT EXIST";
}
cursor.moveToFirst();
String password= cursor.getString(cursor.getColumnIndex("PASSWORD"));
cursor.close();
return password;
}
public String checkSinlgeEntry(String userName)
{
Cursor cursor=db.query("MilanLoginregistration", null, " USERNAME=?", new String[]{userName}, null, null, null);
if(cursor.getCount()>=1) // UserName Exist
{
cursor.close();
return "NOT EXIST";
}
cursor.close();
return "";
}
public void updateEntry(String user_name,String pasword)
{
// Define the updated row content.
ContentValues updatedValues = new ContentValues();
// Assign values for each row.
updatedValues.put("USERNAME", user_name);
updatedValues.put("PASSWORD",pasword);
String where="USERNAME = ?";
db.update("MilanLoginregistration",updatedValues, where, new String[]{user_name});
}
/*public void Display(View v){
Cursor c = db.rawQuery("select * from MilanloginRegistration", null);
admin_settings_child.text_fetched_database_results.setText("");
c.moveToFirst();
do{
String username = c.getString(c.getColumnIndex("USERNAME"));
String password = c.getString(1);
admin_settings_child.text_fetched_database_results.append("USERNAME::-->"+username+"PASSWORD::-->"+password+"\n");
}while(c.moveToNext());
}*/
}
这是我的logcat
04-27 13:26:17.040: E/AndroidRuntime(23436): FATAL EXCEPTION: main
04-27 13:26:17.040: E/AndroidRuntime(23436): Process: com.developer.milanandroid, PID: 23436
04-27 13:26:17.040: E/AndroidRuntime(23436): java.lang.NullPointerException
04-27 13:26:17.040: E/AndroidRuntime(23436): at com.developer.milanandroid.AdminSettings$1.onClick(AdminSettings.java:28)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.View.performClick(View.java:4438)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.View.onKeyUp(View.java:8241)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.widget.TextView.onKeyUp(TextView.java:5682)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.KeyEvent.dispatch(KeyEvent.java:2664)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.View.dispatchKeyEvent(View.java:7665)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1408)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1408)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1408)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1408)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1408)
04-27 13:26:17.040: E/AndroidRuntime(23436): at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchKeyEvent(PhoneWindow.java:2035)
04-27 13:26:17.040: E/AndroidRuntime(23436): at com.android.internal.policy.impl.PhoneWindow.superDispatchKeyEvent(PhoneWindow.java:1505)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.app.Activity.dispatchKeyEvent(Activity.java:2418)
04-27 13:26:17.040: E/AndroidRuntime(23436): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1962)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:3852)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3826)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3399)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3449)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3418)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3525)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3426)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3582)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3399)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3449)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3418)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3426)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3399)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3449)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3418)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3558)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:3718)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2010)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1704)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1695)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:1987)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.os.MessageQueue.nativePollOnce(Native Method)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.os.MessageQueue.next(MessageQueue.java:138)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.os.Looper.loop(Looper.java:123)
04-27 13:26:17.040: E/AndroidRuntime(23436): at android.app.ActivityThread.main(ActivityThread.java:5017)
04-27 13:26:17.040: E/AndroidRuntime(23436): at java.lang.reflect.Method.invokeNative(Native Method)
04-27 13:26:17.040: E/AndroidRuntime(23436): at java.lang.reflect.Method.invoke(Method.java:515)
04-27 13:26:17.040: E/AndroidRuntime(23436): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-27 13:26:17.040: E/AndroidRuntime(23436): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-27 13:26:17.040: E/AndroidRuntime(23436): at dalvik.system.NativeStart.main(Native Method)
04-27 13:27:20.620: I/Process(23436): Sending signal. PID: 23436 SIG: 9
04-27 13:27:20.960: D/libEGL(23654): loaded /system/lib/egl/libEGL_VIVANTE.so
04-27 13:27:20.980: D/libEGL(23654): loaded /system/lib/egl/libGLESv1_CM_VIVANTE.so
04-27 13:27:21.070: D/libEGL(23654): loaded /system/lib/egl/libGLESv2_VIVANTE.so
04-27 13:27:21.160: D/OpenGLRenderer(23654): Enabling debug mode 0
答案 0 :(得分：0)
您已宣布
logindatabase_adapter_child但是在使用之前你没有实例化NullPointerException。因此，您将获得setContentView()。
在logindatabase_adapter_child = new LoginDataBaseAdapter(this);
SQLite This是了解有关Europe/Amsterdam
答案 1 :(得分：0)
您的logindatabase_adapter_child ArrayAdapter未初始化并在此处抛出异常：
Cursor c = logindatabase_adapter_child.db.rawQuery("select * from MilanloginRegistration", null);
....你必须先使用它才能使用它。