當應用程序重新啓動時，爲什麼不從SQLite數據庫中檢索值

Question

我正在研究一個我必須存儲聯繫人信息的android應用程序。該應用程序存儲聯繫人並正確檢索它們，除非直到我重新啓動應用程序。重新啓動應用程序後，當我嘗試檢索值時，程序意外崩潰。如果我重新輸入值或插入新值，然後嘗試檢索值，則將檢索所有聯繫人，包括以前的聯繫人。

任何人都可以解釋這個問題的原因和設備解決方案嗎？ 在此先感謝。

下面是聯繫人類：

public class Contact { 
    //private variables 
    int _id; 
    String _name; 
    String _phone_number; 

    // Empty constructor 
    public Contact(){ 

    } 
    // constructor 
    public Contact(int id, String name, String _phone_number){ 
     this._id = id; 
     this._name = name; 
     this._phone_number = _phone_number; 
    } 

    // constructor 
    public Contact(String name, String _phone_number){ 
     this._name = name; 
     this._phone_number = _phone_number; 
    } 
    // getting ID 
    public int getID(){ 
     return this._id; 
    } 

    // setting id 
    public void setID(int id){ 
     this._id = id; 
    } 

    // getting name 
    public String getName(){ 
     return this._name; 
    } 

    // setting name 
    public void setName(String name){ 
     this._name = name; 
    } 

    // getting phone number 
    public String getPhoneNumber(){ 
     return this._phone_number; 
    } 

    // setting phone number 
    public void setPhoneNumber(String phone_number){ 
     this._phone_number = phone_number; 
    } 
} 

，這裏是數據庫處理程序類：

public class DatabaseHandler extends SQLiteOpenHelper{ 

    // Database Version 
    private static final int DATABASE_VERSION = 1; 

    // Database Name 
    private static final String DATABASE_NAME = "contactsManager"; 

    // Contacts table name 
    private static final String TABLE_CONTACTS = "contacts"; 

    // Contacts Table Columns names 
    private static final String KEY_ID = "id"; 
    private static final String KEY_NAME = "name"; 
    private static final String KEY_PH_NO = "phone_number"; 

    public DatabaseHandler(Context context) { 
     super(context, DATABASE_NAME, null, DATABASE_VERSION); 
    } 

    // Creating Tables 
    @Override 
    public void onCreate(SQLiteDatabase db) { 
     String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "(" 
       + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT," 
       + KEY_PH_NO + " TEXT" + ")"; 
     db.execSQL(CREATE_CONTACTS_TABLE); 
    } 

    // Upgrading database 
    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
     // Drop older table if existed 
     db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS); 

     // Create tables again 
     onCreate(db); 
    } 

    public void addContact(Contact contact) { 
     SQLiteDatabase db = this.getWritableDatabase(); 

     ContentValues values = new ContentValues(); 
     values.put(KEY_NAME, contact.getName()); // Contact Name 
     values.put(KEY_PH_NO, contact.getPhoneNumber()); // Contact Phone Number 

     // Inserting Row 
     db.insert(TABLE_CONTACTS, null, values); 
     db.close(); // Closing database connection 
    } 

    public void delete() { 
     SQLiteDatabase db = this.getWritableDatabase(); 
     db.execSQL("delete * from "+ TABLE_CONTACTS); 
     db.close(); // Closing database connection 
    } 

    // Getting All Contacts 
    public List<Contact> getAllContacts() { 
     List<Contact> contactList = new ArrayList<Contact>(); 
     // Select All Query 
     String selectQuery = "SELECT * FROM " + TABLE_CONTACTS; 

     SQLiteDatabase db = this.getWritableDatabase(); 
     Cursor cursor = db.rawQuery(selectQuery, null); 

     // looping through all rows and adding to list 
     if (cursor.moveToFirst()) { 
      do { 
       Contact contact = new Contact(); 
       contact.setID(Integer.parseInt(cursor.getString(0))); 
       contact.setName(cursor.getString(1)); 
       contact.setPhoneNumber(cursor.getString(2)); 
       // Adding contact to list 
       contactList.add(contact); 
      } while (cursor.moveToNext()); 
     } 

     // return contact list 
     return contactList; 
    } 
} 

而這裏的logcat：

01-13 18:46:36.734: D/dalvikvm(1166): GC_EXTERNAL_ALLOC freed 70K, 50% free 2692K/5379K, external 1859K/2108K, paused 20ms 
    01-13 18:46:38.687: D/AndroidRuntime(1166): Shutting down VM 
    01-13 18:46:38.687: W/dalvikvm(1166): threadid=1: thread exiting with uncaught exception (group=0x40015578) 
    01-13 18:46:38.687: E/AndroidRuntime(1166): FATAL EXCEPTION: main 
    01-13 18:46:38.687: E/AndroidRuntime(1166): java.lang.NullPointerException 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at com.example.tourguide_final.TourGuide$1.onClick(TourGuide.java:75) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:159) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at android.os.Handler.dispatchMessage(Handler.java:99) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at android.os.Looper.loop(Looper.java:123) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at android.app.ActivityThread.main(ActivityThread.java:3687) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at java.lang.reflect.Method.invokeNative(Native Method) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at java.lang.reflect.Method.invoke(Method.java:507) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 
    01-13 18:46:38.687: E/AndroidRuntime(1166):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

請瀏覽此鏈接處理運行時更改，這可能會幫助您 http://developer.android.com/guide/topics/resources/runtime-changes.html

Answer 2

您在TourGuide $ 1.onClick線75上有NPE。如果您使用的是Eclipse，只需雙擊日誌中的這一行，它就會將您帶到問題行。從目測來看，你可能會發現哪個變量沒有被正確初始化。

Answer 3

1.) Remove void delete() method .What it is doing at the end of the program it is deleting data from the database .Whwn you restart your program ,coompiler doesn't get any data and throws exception 

2.)OR What you can do after restarting 1st call void delete method then insert method and then at last call select query method