Call a public method in the Activity class from another class?

Make that method as static so you can call without creating the class object

public static void Mymethod()
{}

and call like this way

MainActivity.Mymethod();

Why not just pass the activity to the constructor like

public class MyActivity extends Activity { 

   onCreate(){ 
        MyClass myobj=new MyClass(MyActivity.this);     
   } 

   public void myMethod(){

   } 
} 

//HELPER CLASS IN A SEPARATE FILE     
public class MyClass{ 
    public MyClass(MyActivity act) { 
        act.myMethod();
    } 
}