Android应用中怎么实现Activity之间的数据传递

public class MainActivity extends Activity {   private Button mainBtn=null;   private final static int REQUEST_CODE=1;         @Override   public void onCreate(Bundle savedInstanceState)   {     super.onCreate(savedInstanceState);     setContentView(R.layout.main);     mainBtn=(Button)findViewById(R.id.mainBtn);     mainBtn.setOnClickListener(listener);   }      private OnClickListener listener=new OnClickListener()   {     @Override     public void onClick(View v)     {       Intent intent=new Intent();       intent.setClass(MainActivity.this, SecondActivity.class);       intent.putExtra("str", "Intent Demo");       startActivityForResult(intent, REQUEST_CODE);            }        };    @Override   protected void onActivityResult(int requestCode, int resultCode, Intent data)   {     if (requestCode==REQUEST_CODE)     {       if (resultCode==SecondActivity.RESULT_CODE)       {         Bundle bundle=data.getExtras();         String str=bundle.getString("back");         Toast.makeText(MainActivity.this, str, Toast.LENGTH_LONG).show();       }     }   }       }