我NullPointerException異常，而我試圖從SQLite數據庫中獲取數據運行此應用程序

Question

這是我的主要活動 我不明白什麼是錯了會on.while我嘗試運行此，DDMS顯示NullPointerException.Please幫助我找出錯誤。

public class MainActivity extends ActionBarActivity implements OnClickListener { 

    private Button btnAdd, btnView; 
    private TextView txtView; 
    private EditText etName; 

    private DBHelper dbHelper; 
    // private osList oslist; 
    private ArrayList<osList> allList; 
    private osList olist; 

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

     btnAdd = (Button) findViewById(R.id.btnAdd); 
     btnView = (Button) findViewById(R.id.btnView); 
     txtView = (TextView) findViewById(R.id.txtView); 
     etName = (EditText) findViewById(R.id.etName); 
     btnAdd.setOnClickListener(this); 
     btnView.setOnClickListener(this); 
    } 

    private void setCustomActionBar() { 
     // TODO Auto-generated method stub 
     ActionBar actionBar = getSupportActionBar(); 
     // actionBar.setBackgroundDrawable(new ColorDrawable(
     // com.arifxdroid.listviewdemo.R.color.my_color)); 
     // actionBar.setDisplayOptions(ActionBar.DISPLAY_SHOW_CUSTOM); 
     // View view = getLayoutInflater() 
     // .inflate(R.layout.custom_actionbar, null); 
     // actionBar.setCustomView(view, new ActionBar.LayoutParams(
     // ActionBar.LayoutParams.MATCH_PARENT, 
     // ActionBar.LayoutParams.MATCH_PARENT)); 
     actionBar.setBackgroundDrawable(new ColorDrawable(R.color.my_color)); 

    } 

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

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      return true; 
     } else if (id == R.id.action_add) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

    @Override 
    public void onClick(View v) { 

     switch (v.getId()) { 

     case R.id.btnAdd: 

      String givenName = etName.getText().toString(); 
      olist = new osList(givenName); 
      long inserted = dbHelper.insertOS(olist); 
      if (inserted == -1) { 
       txtView.setText("Data can't be inserted"); 
      } else { 
       Toast.makeText(getApplicationContext(), "sucessfully added", 
         Toast.LENGTH_SHORT).show(); 
      } 
      break; 

     case R.id.btnView: 

      allList = dbHelper.getosList(); 
      if (allList != null || allList.size() == 0) { 
       // txtView.setText("NO list found"); 
       Toast.makeText(getApplicationContext(), "No List found", 
         Toast.LENGTH_SHORT).show(); 
      } else { 
       for (osList l : allList) { 
        txtView.append("\n" + l.toString()); 
       } 
      } 
      break; 

     default: 
      break; 
     } 

    } 

} 

這是SQLite的輔助類

public class DBHelper extends SQLiteOpenHelper { 

    public static final String DB_NAME = "ListOfOS"; 
    public static final int VERSION = 1; 

    public static final String TABLE_NAME = "allOS"; 
    public static final String ID_FIELD = "_ID"; 
    public static final String NAME_FIELD = "osName"; 
    public static final String TABLE_SQL = "CREATE TABLE " + TABLE_NAME + " (" 
      + ID_FIELD + " INTEGER PRIMARY KEY AUTOINCREMENT, " + NAME_FIELD 
      + " TEXT)"; 

    public DBHelper(Context context) { 
     super(context, DB_NAME, null, VERSION); 
     // TODO Auto-generated constructor stub 
    } 

    @Override 
    public void onCreate(SQLiteDatabase db) { 
     // TODO Auto-generated method stub 
     db.execSQL(TABLE_SQL); 

    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
     // TODO Auto-generated method stub 

    } 

    public long insertOS(osList oslist) { 
     SQLiteDatabase db = this.getWritableDatabase(); 
     ContentValues value = new ContentValues(); 

     value.put(NAME_FIELD, oslist.getOsName()); 
     long inserted = db.insert(TABLE_NAME, "", value); 

     db.close(); 
     return inserted; 
    } 

    public ArrayList<osList> getosList() { 

     SQLiteDatabase db = this.getReadableDatabase(); 
     ArrayList<osList> all = new ArrayList<osList>(); 
     Cursor cursor = db.query(TABLE_SQL, null, null, null, null, null, null); 
     if (cursor != null) { 
      if (cursor.getCount() > 0) { 
       cursor.moveToFirst(); 
       do { 
        String name = cursor.getString(cursor 
          .getColumnIndex(NAME_FIELD)); 
        osList list = new osList(name); 
        all.add(list); 
       } while (cursor.moveToNext()); 
      } 
     } 
     cursor.close(); 
     db.close(); 
     return all; 
    } 

} 

Answer 1

需要初始化助手類，你還沒有完成它。

dbHelper = new DBHelper(getApplicationContext());