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我想知道是否有人可以帮助我这个。Android游标返回null
我打算在我的应用程序中使用Sqlite3，之前我从来没有使用过它，所以我想制作一个单独的项目来测试它，以确保我能够把它记下来。
我遇到了这个问题，我插入几个文本值到单个列表中，这似乎很好。在查询表中的所有名称并通过光标移动以获取信息后，会发生此问题。
我测试这种方式是通过有几个TextViews和一个按钮。当按下按钮时，TextView将其文本设置为光标第一位置的值，然后将调用moveToNext()，并且下一个TextView将更新等。
第一个TextView始终正确更新，但是只要在第一次调用moveToNext()后尝试设置下一个，我会得到一个空例外。
有没有办法在调试器中看到Cursor的内容？我似乎无法弄清楚。
任何想法？下面的代码：
MainActivity.java
package com.firsttread.anthony.databasetest;
import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import com.firsttread.anthony.databasetest.DBContract.DBInfo;
public class MainActivity extends AppCompatActivity {
private Button testButton;
private TextView text1, text2, text3, text4;
private SQLiteDatabase db;
private DatabaseHelper DBHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
testButton = (Button) findViewById(R.id.button);
text1 = (TextView) findViewById(R.id.textView);
text1 = (TextView) findViewById(R.id.textView2);
text1 = (TextView) findViewById(R.id.textView3);
text1 = (TextView) findViewById(R.id.textView4);
DBHelper = DatabaseHelper.getInstance(this);
db = DBHelper.getWritableDatabase();
Log.d("MainActivity", "getWritable successful");
ContentValues sessionValues1 = new ContentValues();
sessionValues1.put(DBInfo.COLUMN_SESSION_NAME, "Sleep");
ContentValues sessionValues2 = new ContentValues();
sessionValues2.put(DBInfo.COLUMN_SESSION_NAME, "Work");
ContentValues sessionValues3 = new ContentValues();
sessionValues3.put(DBInfo.COLUMN_SESSION_NAME, "Reading");
ContentValues sessionValues4 = new ContentValues();
sessionValues4.put(DBInfo.COLUMN_SESSION_NAME, "Walking");
long newRowId1, newRowId2, newRowId3, newRowId4;
//newRowId1 = db.insert(DBInfo.TABLE_SESSIONS, null, sessionValues1);
//newRowId2 = db.insert(DBInfo.TABLE_SESSIONS, null, sessionValues2);
//newRowId3 = db.insert(DBInfo.TABLE_SESSIONS, null, sessionValues3);
//newRowId4 = db.insert(DBInfo.TABLE_SESSIONS, null, sessionValues4);
String[] columns = {DBInfo.COLUMN_SESSION_NAME};
//DBHelper.deleteDB(db);
final Cursor c = db.query(
DBInfo.TABLE_SESSIONS,
columns,
null,
null,
null,
null,
null,
null
);
testButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
c.moveToFirst();
text1.setText(c.getString(c.getColumnIndex(DBInfo.COLUMN_SESSION_NAME)));
c.moveToNext();
text2.setText(c.getString(c.getColumnIndex(DBInfo.COLUMN_SESSION_NAME)));
c.moveToNext();
text3.setText(c.getString(c.getColumnIndex(DBInfo.COLUMN_SESSION_NAME)));
c.moveToNext();
text4.setText(c.getString(c.getColumnIndex(DBInfo.COLUMN_SESSION_NAME)));
}
});
}
@Override
protected void onStop() {
super.onStop();
db.close();
}
}
DataBaseHelper.java
package com.firsttread.anthony.databasetest;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
import com.firsttread.anthony.databasetest.DBContract.DBInfo;
public class DatabaseHelper extends SQLiteOpenHelper {
private static final String DATABASE_NAME = "SoundScope.db";
private static final int DATABASE_VERSION = 1;
private static final String SQL_CREATE_TABLES =
"CREATE TABLE " + DBInfo.TABLE_SESSIONS + " (" +
DBInfo.COLUMN_SESSION_NAME + " TEXT NOT NULL PRIMARY KEY);" +
"CREATE TABLE " + DBInfo.TABLE_SOUND_INFO + " (" +
DBInfo._ID + " INTEGER PRIMARY KEY, " +
DBInfo.COLUMN_SESSION_NAME + " TEXT NOT NULL, " +
DBInfo.COLUMN_SOUND + " TEXT NOT NULL, " +
DBInfo.COLUMN_VOLUME + " REAL NOT NULL); " +
"CREATE TABLE " + DBInfo.TABLE_SOUNDS + " (" +
DBInfo.COLUMN_SOUND_NAME + " TEXT NOT NULL PRIMARY KEY, " +
DBInfo.COLUMN_RAW + " INTEGER NOT NULL, " +
DBInfo.COLUMN_DRAWABLE + " INTEGER NOT NULL);" ;
private static final String SQL_DELETE_TABLES =
"DROP TABLE IF EXISTS " + DBInfo.TABLE_SOUNDS + ";" +
"DROP TABLE IF EXISTS " + DBInfo.TABLE_SOUND_INFO + ";" +
"DROP TABLE IF EXISTS " + DBInfo.TABLE_SESSIONS + ";" ;
private static DatabaseHelper mInstance;
public static synchronized DatabaseHelper getInstance(Context context){
if(mInstance == null){
mInstance = new DatabaseHelper(context.getApplicationContext());
Log.d("DatabaseHelper","DatabaseHelper instance successful");
}
return mInstance;
}
//private to ensure singleton
private DatabaseHelper(Context context){
super(context,DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(SQL_CREATE_TABLES);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public void deleteDB(SQLiteDatabase db){
db.delete(DBInfo.TABLE_SESSIONS,null,null);
//db.delete(DBInfo.TABLE_SOUND_INFO,null,null);
//db.delete(DBInfo.TABLE_SOUNDS,null,null);
db.execSQL(SQL_DELETE_TABLES);
}
}
DBContract.java
package com.firsttread.anthony.databasetest;
import android.provider.BaseColumns;
public final class DBContract {
public DBContract(){}
public static abstract class DBInfo implements BaseColumns{
/*
*
* database schema
*
* */
//sessions table
public static final String TABLE_SESSIONS = "sessions";
public static final String COLUMN_SESSION_NAME = "name";
//sound_info
//uses BaseColumns _ID
public static final String TABLE_SOUND_INFO = "sound_info";
public static final String COLUMN_SESSION = "session";
public static final String COLUMN_SOUND = "sound";
public static final String COLUMN_VOLUME = "volume";
//soundS
public static final String TABLE_SOUNDS = "sounds";
public static final String COLUMN_SOUND_NAME = "name";
public static final String COLUMN_RAW = "raw";
public static final String COLUMN_DRAWABLE = "drawable";
}
}
以及错误的堆栈跟踪：
01-21 11:17:12.319 9149-9149/? E/Zygote: MountEmulatedStorage()
01-21 11:17:12.319 9149-9149/? E/Zygote: v2
01-21 11:17:12.319 9149-9149/? I/libpersona: KNOX_SDCARD checking this for 10075
01-21 11:17:12.319 9149-9149/? I/libpersona: KNOX_SDCARD not a persona
01-21 11:17:12.319 9149-9149/? I/SELinux: Function: selinux_compare_spd_ram, SPD-policy is existed. and_ver=SEPF_SAMSUNG-SM-N910A_5.1.1 ver=38
01-21 11:17:12.319 9149-9149/? I/SELinux: Function: selinux_compare_spd_ram , priority [1] , priority version is VE=SEPF_SAMSUNG-SM-N910A_5.1.1_0038
01-21 11:17:12.319 9149-9149/? E/Zygote: accessInfo : 0
01-21 11:17:12.319 9149-9149/? E/SELinux: [DEBUG] get_category: variable seinfo: default sensitivity: NULL, cateogry: NULL
01-21 11:17:12.319 9149-9149/? I/art: Late-enabling -Xcheck:jni
01-21 11:17:12.359 9149-9149/? D/TimaKeyStoreProvider: TimaSignature is unavailable
01-21 11:17:12.359 9149-9149/? D/ActivityThread: Added TimaKeyStore provider
01-21 11:17:12.449 9149-9149/com.firsttread.anthony.databasetest D/SecWifiDisplayUtil: Metadata value : none
01-21 11:17:12.489 9149-9149/com.firsttread.anthony.databasetest D/PhoneWindow: *FMB* installDecor mIsFloating : false
01-21 11:17:12.489 9149-9149/com.firsttread.anthony.databasetest D/PhoneWindow: *FMB* installDecor flags : -2139029248
01-21 11:17:12.549 9149-9149/com.firsttread.anthony.databasetest D/DatabaseHelper: DatabaseHelper instance successful
01-21 11:17:12.559 9149-9149/com.firsttread.anthony.databasetest D/MainActivity: getWritable successful
01-21 11:17:12.569 9149-9196/com.firsttread.anthony.databasetest D/OpenGLRenderer: Use EGL_SWAP_BEHAVIOR_PRESERVED: true
01-21 11:17:12.579 9149-9149/com.firsttread.anthony.databasetest D/PhoneWindow: *FMB* isFloatingMenuEnabled mFloatingMenuBtn : null
01-21 11:17:12.579 9149-9149/com.firsttread.anthony.databasetest D/PhoneWindow: *FMB* isFloatingMenuEnabled return false
01-21 11:17:12.599 9149-9149/com.firsttread.anthony.databasetest D/SRIB_DCS: log_dcs ThreadedRenderer::initialize entered!
01-21 11:17:12.609 9149-9196/com.firsttread.anthony.databasetest I/Adreno: EGLInit: QTI Build: 07/16/15, 126f54a, If3804f16ae
01-21 11:17:12.619 9149-9196/com.firsttread.anthony.databasetest I/OpenGLRenderer: Initialized EGL, version 1.4
01-21 11:17:12.629 9149-9196/com.firsttread.anthony.databasetest D/OpenGLRenderer: Get maximum texture size. GL_MAX_TEXTURE_SIZE is 16384
01-21 11:17:12.629 9149-9196/com.firsttread.anthony.databasetest D/OpenGLRenderer: Enabling debug mode 0
01-21 11:17:12.699 9149-9149/com.firsttread.anthony.databasetest I/Timeline: Timeline: Activity_idle id: [email protected] time:177267508
01-21 11:17:15.329 9149-9149/com.firsttread.anthony.databasetest D/ViewRootImpl: ViewPostImeInputStage ACTION_DOWN
01-21 11:17:15.429 9149-9149/com.firsttread.anthony.databasetest D/AndroidRuntime: Shutting down VM
01-21 11:17:15.429 9149-9149/com.firsttread.anthony.databasetest E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.firsttread.anthony.databasetest, PID: 9149
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.firsttread.anthony.databasetest.MainActivity$1.onClick(MainActivity.java:87)
at android.view.View.performClick(View.java:5242)
at android.widget.TextView.performClick(TextView.java:10530)
at android.view.View$PerformClick.run(View.java:21185)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6872)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
谢谢。
您的CREATE_TABLES和DELETE_TABLES错误。您不能通过用分号分隔它们来执行多个SQL命令。您必须一次执行SQL命令。 –
我认为moveToNext会返回一个布尔值，您可以检查。还DatabaseUtils.dumpCursorToString（）帮助很多 –
@ HrundiV.Bakshi这不是问题。多SQL命令似乎工作正常。问题在于，当通过id查找时，我将所有TextView都设置为text1。它解决了这个问题。 –