我正在尝试为我的应用添加一项功能，允许用户查看他们所玩游戏的所有游戏分数的平均分数。
为了做到这一点，我需要找到数据库中一列中所有值的总和。
这是我目前的尝试：
查询方法：
/**
* Method that gives the the total of all
* Average Meditation levels in the DB
* @return
*/
public Cursor getTotalOfAllMedLevels(){
SQLiteDatabase db = this.getWritableDatabase();
String query = "SELECT SUM(avgmeditation) FROM " + TABLE_SCORE;
Cursor c = db.rawQuery(query, null);
//Add in the movetofirst etc here? see SO
c.moveToFirst();
c.getInt(0);
return c;
}
尝试实现它，将光标设置为等于textview中的返回值：
public void displayAverageOfAllMedValues（）{
//Setting cursor to return value of the method?
Cursor c = db.getTotalOfAllMedLevels();
if( c != null && c.moveToFirst() )
{
avgMed.setText("" + c.getInt( 0 ) );
}
}
这只是在我知道不正确的那一刻给出了答案0，我做错了什么？
表格结构（供参考）：
public class DatabaseHelper extends SQLiteOpenHelper {
// Database Version
private static final int DATABASE_VERSION = 10;
// Database Name
private final static String DATABASE_NAME = "MeditationDatabase";
// Contacts table name
private static final String TABLE_SCORE = "scores";
// Contacts Table Columns names
private static final String COL_SESSION = "sessionid";
private static final String COL_GAMETITLE = "game";
private static final String COL_NAME = "name";
private static final String COL_MED = "avgmeditation";
private static final String COL_MAX = "maxmeditation";
private static final String COL_AVGATT = "avgattention";
private static final String COL_MAXATT = "maxattention";
private static final String COL_SCORE = "score";
private static final String COL_DATE = "date";
/**
* Constructor
*
* @param context
*/
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
/**
* Method that creates the database
*/
@Override
public void onCreate(SQLiteDatabase db) {
//VERY IMPORTANT: ALWAYS CHECK THAT THERE ARE SPACES AND COMMAS IN CORRECT PLACE IN CODE BELOW:
String CREATE_TABLE_SCORE = "CREATE TABLE " + TABLE_SCORE + "(" + COL_SESSION
+ " STRING PRIMARY KEY, " + COL_GAMETITLE + " STRING, " + COL_NAME + " STRING, " + COL_MED + " INTEGER, "
+ COL_MAX + " INTEGER, " + COL_AVGATT + " INTEGER, " + COL_MAXATT + " INTEGER, " + COL_SCORE + " INTEGER, " + COL_DATE + " STRING " + ")";
db.execSQL(CREATE_TABLE_SCORE);
}
编辑：
尝试实现以下代码时，我得到一个与displayAverageOfAllAttValues（）方法相关的NullPointerException。
更新活动：
public class thirdActivity extends ActionBarActivity {
TextView count;
TextView avgMed;
TextView avgAtt;
DatabaseHelper db = new DatabaseHelper(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.summary);
initialiseVars();
displayAllResults();
}
public void initialiseVars(){
count= (TextView) findViewById(R.id.tvSummary1);
avgMed= (TextView) findViewById(R.id.tvSummary2);
avgAtt= (TextView) findViewById(R.id.tvSummary3);
}
//Facade method that then calls all the rest
public void displayAllResults(){
displayNumberofGamesPlayed();
displayAverageOfAllMedValues();
displayAverageOfAllAttValues();
}
public void displayNumberofGamesPlayed(){
//show how many games have been played
int totalGamesPlayed = db.getTotalGamesPlayed();
count.setText("games played " + totalGamesPlayed);
}
public void displayAverageOfAllMedValues() {
//Setting cursor to return value of the method?
int i = db.getTotalOfAllAvgMedLevels();
avgMed.setText("Total of med" + i );
}
public void displayAverageOfAllAttValues() {
//Setting cursor to return value of the method?
int i = db.getTotalOfAllAvgAttLevels();
avgAtt.setText("Total of att" + i );
}
}
databaseHelper中的displayAverageOfAllAttValues（）方法：
public int getTotalOfAllAvgAttLevels(){
SQLiteDatabase db = this.getWritableDatabase();
String query = "SELECT SUM(avgattention) FROM " + TABLE_SCORE;
Cursor c = db.rawQuery(query, null);
c.moveToFirst();
int i=c.getInt(0);
return i;
}
Logcat错误报告：
08-08 00:36:45.400: E/AndroidRuntime(10219): FATAL EXCEPTION: main
08-08 00:36:45.400: E/AndroidRuntime(10219): Process: com.example.brianapp, PID: 10219
08-08 00:36:45.400: E/AndroidRuntime(10219): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.brianapp/com.example.brianapp.thirdActivity}: java.lang.NullPointerException
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.ActivityThread.startActivityNow(ActivityThread.java:2140)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:135)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:347)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:749)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.widget.TabHost.setCurrentTab(TabHost.java:413)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:154)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:625)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.view.View.performClick(View.java:4633)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.view.View$PerformClick.run(View.java:19330)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.os.Handler.handleCallback(Handler.java:733)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.os.Handler.dispatchMessage(Handler.java:95)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.os.Looper.loop(Looper.java:157)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.ActivityThread.main(ActivityThread.java:5356)
08-08 00:36:45.400: E/AndroidRuntime(10219): at java.lang.reflect.Method.invokeNative(Native Method)
08-08 00:36:45.400: E/AndroidRuntime(10219): at java.lang.reflect.Method.invoke(Method.java:515)
08-08 00:36:45.400: E/AndroidRuntime(10219): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
08-08 00:36:45.400: E/AndroidRuntime(10219): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
08-08 00:36:45.400: E/AndroidRuntime(10219): at dalvik.system.NativeStart.main(Native Method)
08-08 00:36:45.400: E/AndroidRuntime(10219): Caused by: java.lang.NullPointerException
08-08 00:36:45.400: E/AndroidRuntime(10219): at com.example.brianapp.thirdActivity.displayAverageOfAllAttValues(thirdActivity.java:78)
08-08 00:36:45.400: E/AndroidRuntime(10219): at com.example.brianapp.thirdActivity.displayAllResults(thirdActivity.java:55)
08-08 00:36:45.400: E/AndroidRuntime(10219): at com.example.brianapp.thirdActivity.onCreate(thirdActivity.java:34)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.Activity.performCreate(Activity.java:5426)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
08-08 00:36:45.400: E/AndroidRuntime(10219): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
08-08 00:36:45.400: E/AndroidRuntime(10219): ... 18 more
答案 0 :(得分：1)
代码中似乎有一些错误。如果进行以下更改，请参阅
public int getTotalOfAllMedLevels(){
SQLiteDatabase db = this.getWritableDatabase();
String query = "SELECT SUM(avgmeditation) FROM " + TABLE_SCORE;
Cursor c = db.rawQuery(query, null);
//Add in the movetofirst etc here? see SO
c.moveToFirst();
int i=c.getInt(0);
return i;
}
public void displayAverageOfAllMedValues() {
//Setting cursor to return value of the method?
int i = db.getTotalOfAllMedLevels();
avgMed.setText("" +i );
}
答案 1 :(得分：0)
在以下方法中：
public void displayAverageOfAllAttValues() {
//Setting cursor to return value of the method?
int i = db.getTotalOfAllAvgAttLevels();
avgAtt.setText("Total of att" + i );
}
为db和avgAtt变量设置日志并检查哪一个为null并相应地修复问题。
请参阅我的回答here，了解调试问题的方法