String FormName;
boolean formState;

public void openForm(JFrame formName){


    if (formState == false){

    formName.setVisible(true);

    formState = true;

    }
    else{

    JOptionPane.showMessageDialog(formName, "Form - " + formName.getTitle()+" is already open");

    }
}

open_Class cl = new open_Class();



    cl.openForm(JFrame2); // Jframe2 throw an error
}