I have two Activitys, named App and Gallery.
App contains  frontend buttons and functionalities, Gallery lists the images in the sd card.
I want to open Gallery in onClick() inside App.
App.java
public class App extends Activity implements OnClickListener {
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
 // set a click listener on the yesno button
    Button Delete = (Button) findViewById(R.id.Delete);
    Delete.setOnClickListener(this);
    // set a click listener on the Upload button
    Button Upload = (Button) findViewById(R.id.Upload);
    Upload.setOnClickListener(this);
    Button Listvideo = (Button) findViewById(R.id.Listvideo);
    Listvideo.setOnClickListener(this);
}
public void onClick(View view) {
    // which button is clicked?
    // the Toast button
    // the delete button is clicked
    if (view == findViewById(R.id.Delete)) {
        // prepare the alert box
        AlertDialog.Builder alertbox = new AlertDialog.Builder(this);
        // set the message to display
        alertbox.setMessage("Do u  want to Delete!");
        // set a positive/yes button and create a listener
        alertbox.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
            // do something when the button is clicked
            public void onClick(DialogInterface arg0, int arg1) {
                Toast.makeText(getApplicationContext(), "'Yes' button clicked", Toast.LENGTH_SHORT).show();
            }
        });
        // set a negative/no button and create a listener
        alertbox.setNegativeButton("No", new DialogInterface.OnClickListener() {
            // do something when the button is clicked
            public void onClick(DialogInterface arg0, int arg1) {
                Toast.makeText(getApplicationContext(), "'No' button clicked", Toast.LENGTH_SHORT).show();
            }
        });
        // display box
        alertbox.show();
    }
    // the delete button is clicked
    // the delete button is clicked
    if (view == findViewById(R.id.Upload)) {
          ProgressDialog dialog = new ProgressDialog(this);
          // make the progress bar cancelable
          dialog.setCancelable(true);
          // set a message text
          dialog.setMessage("Uploading the video...");
          // show it
          dialog.show();
    }
    if (view == findViewById(R.id.Listvideo)) {
  }
}
}
 
     
     
    