Android implements data transfer between activities based on Intent

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public class MainActivity extends Activity {
  private Button btn=null;
  public void onCreate(Bundle savedInstanceState)//The onCreate method is used to initialize the Activity instance object  {
    (savedInstanceState);//(savedInstanceState) is to call the onCreate method of its parent class Activity to realize the drawing work of the interface    setContentView(.activity_main);//The function of setContentView() is to load an interface    btn=(Button)findViewById();
    (listener);
  }
  private  listener=new ()
  {
    @Override
    public void onClick(View v)
    {
//     Intent intent=new Intent();
//     (Intent.ACTION_SENDTO);
//     (("smsto:5554"));
// ("sms_body", "Hello!");//sms_body cannot be replaced at will//     startActivity(intent);
      Intent intent=new Intent();
      (, );//Skip from one activity to another activity      ("str", "Intent Demo");//Add extra data to the intent, the key is "str", and the key value is "Intent Demo"      startActivity(intent);
    }
  };
  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(.activity_main, menu);
    return true;
  }
}

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public class SecondActivity extends Activity
{
  private TextView secondTxt;
  @Override
  protected void onCreate(Bundle savedInstanceState)
  {
    // TODO Auto-generated method stub
    (savedInstanceState);
    setContentView();
    Intent intent=getIntent();//getIntent retrieves the original intent contained in the project and assigns the retrieved intent to a variable of Intent of type intent    Bundle bundle=();//.getExtras() gets the extra data attached to the intent    String str=("str");//getString() returns the value of the specified key    secondTxt=(TextView)findViewById();// Use TextView to display the value    (str);
  }
}