Von MainActivity auf Var aus anderer Klasse zugreifen.

public class MainActivity extends FragmentActivity {

private int number;
private MyClass myClass;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.sample_main);
     
        myClass = new myClass();
        myClass.number = 1;
        number = 1;

    }

    public int getNumber () {
        number = number + 1;
        myClass.number = myClass.number + 1;
        
        return number;

}

class MyClass {

        public int number;

}