如何解決零點異常

Question

我想建立一個基於數據庫的應用程序。但是當我按下搜索按鈕時，應用程序意外停止。現在我該怎麼辦......

我logcat的消息是：

12-09 18:05:52.177 1552-1552/com.example.bank E/AndroidRuntime﹕ FATAL EXCEPTION: main 
java.lang.NullPointerException 
     at com.example.bank.search$1.onClick(search.java:44) 
     at android.view.View.performClick(View.java:4204) 
     at android.view.View$PerformClick.run(View.java:17355) 
     at android.os.Handler.handleCallback(Handler.java:725) 
     at android.os.Handler.dispatchMessage(Handler.java:92) 
     at android.os.Looper.loop(Looper.java:137) 
     at android.app.ActivityThread.main(ActivityThread.java:5041) 
     at java.lang.reflect.Method.invokeNative(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:511) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
     at dalvik.system.NativeStart.main(Native Method) 

和search.java是：

public class search extends AppCompatActivity { 
    DatabaseOperations myDB; 
    Button search; 



protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.search); 

    search = (Button) findViewById(R.id.btn_search); 
    all_data(); 
} 

public void showMessage(String title, String message) 
{ 
    AlertDialog.Builder builder = new AlertDialog.Builder(this); 
    builder.setCancelable(true); 
    builder.setTitle(title); 
    builder.setMessage(message); 
    builder.show(); 


} 

public void all_data() 
{ 
    search.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View view) { 

      Cursor res = myDB.getAllData(); 
      if (res.getCount() == 0) 
      { 
       //message 
       showMessage("ERROR", "Nothig is Found"); 
       return; 
      } 

      else 
      { 
       StringBuffer buffer = new StringBuffer(); 
       while (res.moveToNext()) 
       { 
        buffer.append("EMAIL : "+res.getString(0)+"\n"); 
        buffer.append("FULL_NAME : "+res.getString(1)+"\n\n"); 


       } 

       //show all data 
       showMessage("Data", buffer.toString()); 
      } 

     } 
    }); 

} 




} 

DataBaseOperations.java：

public class DatabaseOperations extends SQLiteOpenHelper{ 
public static final int database_version = 1; 
public String CREATE_QUERY = "CREATE TABLE " + TableData.TableInfo.TABLE_NAME+"("+ TableData.TableInfo.EMAIL+" TEXT,"+ TableData.TableInfo.FULL_NAME+" TEXT,"+ TableData.TableInfo.USER_NAME+" TEXT,"+ TableData.TableInfo.USER_PASS+" TEXT,"+ TableData.TableInfo.DISTRICT+" TEXT,"+ TableData.TableInfo.CITY+" TEXT,"+ TableData.TableInfo.VARSITY+" TEXT,"+ TableData.TableInfo.BLOOD_GROUP+" TEXT,"+ TableData.TableInfo.LAST_DATE+" TEXT,"+ TableData.TableInfo.PHONE_NUM+" TEXT);" ; 


public DatabaseOperations(Context context) { 
    /* 
    some change is made here 
    check it for problem... 
    */ 

    super(context, TableData.TableInfo.DATABASE_NAME, null, database_version); 
    Log.d("Database operations", "Database Created"); 
} 

@Override 
public void onCreate(SQLiteDatabase sdb) { 

    sdb.execSQL(CREATE_QUERY); 
    Log.d("Database operations", "Table Created"); 
} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 

} 

public void putInformation(DatabaseOperations dop, String email, String full_name, String user_name, String user_pass, String district, String city, String varsity, String blood_group, String last_date, String ph_num) 
{ 
    SQLiteDatabase SQ = dop.getWritableDatabase(); 
    ContentValues cv = new ContentValues(); 
    cv.put(TableData.TableInfo.EMAIL, email); 
    cv.put(TableData.TableInfo.FULL_NAME, full_name); 
    cv.put(TableData.TableInfo.USER_NAME, user_name); 
    cv.put(TableData.TableInfo.USER_PASS, user_pass); 
    cv.put(TableData.TableInfo.DISTRICT, district); 
    cv.put(TableData.TableInfo.CITY, city); 
    cv.put(TableData.TableInfo.VARSITY, varsity); 
    cv.put(TableData.TableInfo.BLOOD_GROUP, blood_group); 
    cv.put(TableData.TableInfo.LAST_DATE, last_date); 
    cv.put(TableData.TableInfo.PHONE_NUM, ph_num); 


    long k = SQ.insert(TableData.TableInfo.TABLE_NAME, null, cv); 


    Log.d("Database operations", "One Raw Inserted"); 
} 


public Cursor getInformation(DatabaseOperations dop) 
{ 
    SQLiteDatabase SQ = dop.getReadableDatabase(); 
    String[] columns = {TableData.TableInfo.USER_NAME, TableData.TableInfo.USER_PASS}; 
    Cursor CR = SQ.query(TableData.TableInfo.TABLE_NAME, columns, null, null, null, null, null); 
    return CR; 

} 

public Cursor getAllData() 
{ 
    SQLiteDatabase db = this.getWritableDatabase(); 
    Cursor res = db.rawQuery("Select * from "+TableData.TableInfo.TABLE_NAME, null); 
    return res; 

} 

} 

幫助我解決這個問題。 在此先感謝

Answer 1

在您提供的代碼myDB永遠不會被初始化。因此Cursor res = myDB.getAllData(); thows一個例外。