For example a package being tested has a method:
package package.being.tested
....
public class MainActivity extends AppCompatActivity {
....
public Int myMehod(String s){
<do something>
}
How to call this method in tests?
Exactly as you would call this method in a non-test class.
@Test
public void myMethodTest() {
MainActivity mainActivity = new MainActivity();
Integer i = mainActivity.myMethod("A string");
}