If you want restore your MainActivity then you should remove finish() from your code. Code
public void click_contact(View v)
{
Intent myIntent = new Intent(MainActivity.this, ContactActivity.class);
MainActivity.this.startActivity(myIntent);
overridePendingTransition (R.anim.slide_in_right, R.anim.slide_out_left);
}
If you want restart MainActivity on back in ContactActivity then you should start it in onBackPressed():
Code
@Override
public void onBackPressed()
{
Intent myIntent = new Intent(ContactActivity.this, MainActivity.class);
startActivity(myIntent);
super.onBackPressed();
}