在一个类中设置值并从Java中的另一个类中检索

Question

当用户在应用程序启动时输入它时，我从我的Main类中设置了一个ID。考虑到我将它设置在名为GetSet.java的类中，该类也有一个方法在被调用时返回它。所以我需要这个第一类的id集合，名为MySQLitehelper.java，这样我就可以根据id集合检索数据，而不是硬编码它。这将使我的应用动态而不是静态。这是我的代码：

/---- MainActivity.java -----/

public class MainActivity extends Activity { 

EditText etGWid; 
Button OKbtn; 

public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE"; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 


    etGWid = (EditText)findViewById(R.id.etGWID); 
    OKbtn = (Button)findViewById(R.id.OKbtn); 

    final GetSet obj_getset = new GetSet(); 
    //MySQLitehelper db = new MySQLitehelper(null); 

    final SharedPreferences app_preferences = PreferenceManager.getDefaultSharedPreferences(this); 
      // Create a shared preferences variable using the SharedPreferenceManager for storing GWids 

    SharedPreferences.Editor editor = app_preferences.edit(); // We also need a shared preferences editor to handle the shared preference requests 
    // Call the edit method (library function) to editor variable can edit the key, values. 

    editor.putInt("47688507", 47688507); // put in the shared preferences values of user GWids and give them a key for retrieval purposes 
    editor.putInt("1234567", 1234567); 
    editor.putInt("7654321", 7654321); 

    editor.commit(); //commit is necessary to save the shared preferences 



    OKbtn.setOnClickListener(new View.OnClickListener() { 


     @Override 
     public void onClick(View arg0) { 
      // TODO Auto-generated method stub 

      String gwidCheck = etGWid.getText().toString(); //get the value user enters for GWid 

      if(app_preferences.contains(gwidCheck))  // this will check the stored shared preferences and compare with the value entered 
      { 
       Context context = getApplicationContext(); 
       CharSequence text = "Login Successfull"; 
       int duration = Toast.LENGTH_SHORT;        //If it exists, then create a toast message for success 



       //etGWid.setText(""); // make the textbox empty 
       long setid = Long.parseLong(gwidCheck); // take the string gwid and convert to long 
       obj_getset.setId(setid); // set the gwid entered 



       Toast toast = Toast.makeText(context, text, duration); 
       toast.show(); 
       intentfunction(); 
      } 
      else 
      { 
       Context context = getApplicationContext(); 
       CharSequence text = "Login Failed";      // If doesnt exist, create a toast for fail 
       int duration = Toast.LENGTH_SHORT; 

       Toast toast = Toast.makeText(context, text, duration); 
       toast.show(); 
      } 

     } 
    }); 
} 


private void intentfunction() 
{ 
    Intent intent = new Intent(this, SelectOptions.class); 
    //editText = (EditText) findViewById(R.id.editText1); 
    //editText = new EditText(this); 
    etGWid.setText(""); //set the edit text to blank 
    String message = "TestHello"; 
    intent.putExtra(EXTRA_MESSAGE, message); 
    startActivity(intent); 

} 
@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.activity_main, menu); 
    return true; 
} 

} 

/*-----GetSet.java----*/ 

public class GetSet { 

long gwid=0; 

public void setId(long id) 
{ 
    gwid = id; 
} 

public long getId() 
{ 
    return gwid; 
} 

} 

/--- MySQLitehelper.java ---/

public class MySQLitehelper { 

//public static final String TABLE_COMMENTS = "comments"; 
    public static final String COLUMN_ID = "GWid"; 
    public static final String COLUMN_DATE = "DateGWU"; 
    public static final String COLUMN_LOCATION = "location"; 
    public static final String COLUMN_TIME = "time"; 

    public static final String TABLE_NAME = "UPDTable"; 

    private static final String DATABASE_NAME = "UPDdb_version6"; 
    private static final int DATABASE_VERSION = 6; 

    private final Context context; 
    GetSet getset = new GetSet(); 
    public void GetIdForGwid(GetSet get) 
    { 
    getset=get; 
    } 

    // Database creation sql statement 
    private static final String DATABASE_CREATE = "CREATE TABLE " + TABLE_NAME + 
           " (" +COLUMN_ID+ " integer," + COLUMN_DATE + " VARCHAR," + 
           COLUMN_LOCATION+" VARCHAR," +COLUMN_TIME +" VARCHAR);"; 

// private static final String DATABASE_INSERT = "INSERT INTO " +TABLE_NAME + 
//            " Values (47688507,'DEC-07-2012','MARVIN 203','20:00');"; 

    private static final String DATABASE_INSERT = "INSERT INTO " +TABLE_NAME + " (" +COLUMN_ID+ " ," + COLUMN_DATE + "," + 
      COLUMN_LOCATION+" ," +COLUMN_TIME +")" + 
          " Values (47688507,'DEC-07-2012','MARVIN 203','20:00');"; 



    DatabaseHelper dbhelper; 
    SQLiteDatabase db; 




public MySQLitehelper(Context ctx) 
    { 
     this.context = ctx; 
     dbhelper = new DatabaseHelper(ctx); 
    } 



private static class DatabaseHelper extends SQLiteOpenHelper { 

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

@Override 
public void onCreate(SQLiteDatabase db) { 
    // TODO Auto-generated method stub 
    db.execSQL(DATABASE_CREATE);   //execute create table 
    db.execSQL(DATABASE_INSERT);   //execute insert query 
    db.execSQL("INSERT INTO " +TABLE_NAME + " (" +COLUMN_ID+ " ," + COLUMN_DATE + "," +COLUMN_LOCATION+" ," +COLUMN_TIME +")" +" Values (47688507,'DEC-22-2012','OLD MAIN','23:00');"); 
    db.execSQL("INSERT INTO " +TABLE_NAME + " (" +COLUMN_ID+ " ," + COLUMN_DATE + "," +COLUMN_LOCATION+" ," +COLUMN_TIME +")" +" Values (1234567,'DEC-14-2012','FUNGER','12:00');"); 
    db.execSQL("INSERT INTO " +TABLE_NAME + " (" +COLUMN_ID+ " ," + COLUMN_DATE + "," +COLUMN_LOCATION+" ," +COLUMN_TIME +")" +" Values (7654321,'DEC-29-2012','GELMAN','22:00');"); 
    db.execSQL("INSERT INTO " +TABLE_NAME + " (" +COLUMN_ID+ " ," + COLUMN_DATE + "," +COLUMN_LOCATION+" ," +COLUMN_TIME +")" +" Values (47688507,'DEC-12-2012','IVORY','23:00');"); 
} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    // TODO Auto-generated method stub 
    Log.w(MySQLitehelper.class.getName(), 
      "Upgrading database from version " + oldVersion + " to " 
       + newVersion + ", which will destroy all old data"); 
     db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME); 
     onCreate(db); 
    } 
} 


// open the DB 
public MySQLitehelper open() throws SQLException 
{ 
    System.out.println("Inside open function"); 
    db = dbhelper.getWritableDatabase(); 
    return this; 
} 

public void close() 
{ 
    dbhelper.close(); 
} 



public void insertRecord(long gwid, String date, String location, String time) 
    { 
      ContentValues initialValues = new ContentValues(); 
      initialValues.put(COLUMN_ID, gwid); 
      initialValues.put(COLUMN_DATE, date); 
      initialValues.put(COLUMN_LOCATION, location); 
      initialValues.put(COLUMN_TIME, time); 
      db.insert(TABLE_NAME, null, initialValues); 
    } 

public Cursor getAllrows()  //function to get all rows in the DB. Testing initially. 
{ 

    Cursor cur = db.rawQuery("SELECT * FROM "+TABLE_NAME, null); 
    return cur; 
} 

public Cursor getRecord() throws SQLException 
{ 
     Cursor mCursor = 
     db.query(true, TABLE_NAME, new String[] {COLUMN_ID, 
     COLUMN_DATE, COLUMN_LOCATION, COLUMN_TIME}, 
     COLUMN_ID + "= 47688507", null, null, null, null, null); //HARDCODED. Please make it dynamic 
     if (mCursor != null) 
     { 
      mCursor.moveToFirst(); 
     } 
return mCursor; 
} 

} 

我在上面的类的getRecord（）方法中检索它的底部，而不是COLUMN_ID + "= 47688507我希望从GetSet类（getId方法）中获得设置值并将其替换为plac e的47688507。有什么建议么 ？

Answer 1

我想，你应该能够使用你的实例变量obj_getset设置id与

obj_getset.setid(mCursor.COLUMN_ID)

在getRecord函数然后检索它以同样的方式，当你需要

obj_getset.getID()

Answer 2

• 使用全局单例存储和检索data。当你这样做时，确保单例不包含对你的活动或上下文的引用。
• 由于mysqlitehelper具有对上下文的引用，因此您还可以从SharedPreferences中检索持久数据。
-

Answer 3

阅读“单例模式”，这是一种创建持久对象以保存信息的方法。

但是，请记住，一旦您的活动调用了onPause()，系统就可以完全销毁您的应用程序，并在以后重新构建。这意味着即使是单例类也不能保证您的数据将在您的应用程序重新启动时存在。您必须始终准备好从您的onCreate（）方法中的永久存储中恢复状态。

以下是我如何使用单例类实现全局首选项持有者：简而言之，在任何有上下文的地方，都可以调用Prefs.getPrefs（）来获取首选项对象。在第一次调用时，它将从持久存储中读取首选项。在所有后续调用中，它只是返回指向已经加载的对象的指针。如果您的应用程序被系统中止并重新启动，则getPrefs（）会根据需要无缝地重新读取首选项。

/** 
* This is a singleton class to hold preferences. 
*/ 
public class Prefs { 
    private static volatile Prefs instance = null; 

    // The values held by this class 
    String identity = null; 
    String apiUrl = "http://example.com/cgi-bin/api"; 
    int interval = 3600; // when there's no activity 

    /** 
    * Return the single instance of this class, creating it 
    * if necessary. This method is thread-safe. 
    */ 
    public static Prefs getPrefs(Context ctx) { 
     if (instance == null) { 
      synchronized(Prefs.class) { 
       if (instance == null) { 
        instance = new Prefs(); 
        instance.readPrefs(ctx); 
       } 
      } 
     } 
     return instance; 
    } 

    /** 
    * Re-read all preferences (you never need to call this explicitly) 
    */ 
    private void readPrefs(Context ctx) { 
     try { 
      SharedPreferences sp = 
       PreferenceManager.getDefaultSharedPreferences(ctx); 
      identity = sp.getString("identity", identity); 
      apiUrl = sp.getString("apiUrl", apiUrl); 
      interval = sp.getInt("interval", interval); 
     } catch (Exception e) { 
      Log.e(TAG, "exception reading preferences: " + e, e); 
      // TODO: report it 
     } 
    } 

    /** 
    * Save preferences; you can call this from onPause() 
    */ 
    public void savePrefs(Context ctx) { 
     try { 
      SharedPreferences.Editor sp = 
       PreferenceManager.getDefaultSharedPreferences(ctx).edit(); 
      sp.putString("identity", identity); 
      sp.putString("apiUrl", apiUrl); 
      sp.putInt("interval", interval); 
      sp.commit(); 
     } catch (Exception e) { 
      Log.e(TAG, "exception reading preferences: " + e, e); 
      // TODO: report it 
     } 
    } 

    /** 
    * Save preferences to a bundle. You don't really need to implement 
    * this, but it can make start-up go faster. 
    * Call this from onSaveInstanceState() 
    */ 
    public void onSaveInstanceState(Bundle state) { 
     state.putString("identity", identity); 
     state.putString("apiUrl", apiUrl); 
     state.putInt("interval", interval); 
    } 

    /** 
    * Recall preferences from a bundle. You don't really need to implement 
    * this, but it can make start-up go faster. 
    * Call this from onCreate() 
    */ 
    public void restoreInstanceState(Bundle state) { 
     identity = state.getString("identity"); 
     apiUrl = state.getString("apiUrl"); 
     interval = state.getInt("interval"); 
    } 
}