How do I open an Activity in button's onClick()?

Question

I have two Activitys, named App and Gallery.

App contains frontend buttons and functionalities, Gallery

Answer 1

if your Gallery.java is an activity you could start it by calling startActivity(new Intent(this, Gallery.class));

Answer 2

Use an Intent

 Intent  intent  = new Intent(this,Gallery.class);
 intent.putExtra(<key>,<value>);
 startActivity(intent);