How to write a method that accepts a class type as a parameter, and uses it to cast some objects

Question

I am working on a Java 8 desktop application using JavaFX 8.
I have this method in the MainApp class (the one that extends the Application class).

public void showUserLayout() {
    try {
        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(MainApp.class.getResource("view/userLayout.fxml"));
        AnchorPane userPane = (AnchorPane) loader.load();

        rootAnchorPane.getChildren().clear();
        rootAnchorPane.getChildren().add(userPane);

        userLayoutController controller = loader.getController();
        controller.setMainApp(this);
    } catch (IOException e) {
        // Handle Exception
    }
}

and I am using the same code for each layout I want to load.

Is there any way to create a method that accepts the class type as a parameter and does the exact same job, for example:

public void genericLayoutLoader(String fxmlFilename, Class rootFXMLElement, Class fxmlController) {
    try {
        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(MainApp.class.getResource(fxmlFilename));
        // Not sure for the Object below
        Object chooseUserAndInterval = (rootFXMLElement) loader.load();
        // rootAnchorPane is same for every layout
        rootAnchorPane.getChildren().clear();
        rootAnchorPane.getChildren().add((rootFXMLElement) chooseUserAndInterval);

        Object controller = (fxmlController) loader.getController();
        ((fxmlController)controller).setMainApp(this);
    } catch (IOException e) {
        // Handle Exception
    }
}

I would use it like this:

public void showUserLayout() {
    genericLayoutLoader("view/userLayout.fxml", AnchorPane, userLayoutController);  
}

Is there any way to achieve this behavior?

Can't you solve it by letting all of them implement some common interface? — Dakshinamurthy Karra, Aug 24 '15 at 12:22
Do you mean something like [in this answer](http://stackoverflow.com/questions/14524751/cast-object-to-generic-type-for-returning/14524815#14524815)? — SpaceTrucker, Aug 24 '15 at 12:24
@KDM The `showSomeLayout` methods are defined all in the MainApp class. So, with that in mind, I don't think this can be done, right? — Chris, Aug 24 '15 at 12:36
@Chris not sure without seeing more code. I am also handicapped because I don't know much of JavaFx. See my answer below. Might be useful. — Dakshinamurthy Karra, Aug 24 '15 at 12:37

Dakshinamurthy Karra · Accepted Answer · 2015-08-24T12:58:56.563

1

If you want to stick to using Classes as parameters, your code should look somewhat like this:

    public void genericLayoutLoader(String fxmlFilename, Class rootFXMLElement, Class fxmlController) {
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(MainApp.class.getResource(fxmlFilename));
            // Not sure for the Object below
            Object chooseUserAndInterval = loader.load();
            // rootAnchorPane is same for every layout
            rootAnchorPane.getChildren().clear();
            rootAnchorPane.getChildren().add(chooseUserAndInterval);

            Object controller = loader.getController();
            fxmlController.getMethod("setMainApp", new Class[] { MainApp.class }).invoke(controller, this);
        } catch (IOException e) {
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (SecurityException e) {
            e.printStackTrace();
        }
    }

    public void showUserLayout() {
        genericLayoutLoader("view/userLayout.fxml", AnchorPane.class, Controller.class);  
    }

But, I still suggest to try to solve this using interfaces if possible.

edited Aug 24 '15 at 12:58

answered Aug 24 '15 at 12:36

Dakshinamurthy Karra

5,353
1
17
28

I was trying this as I've posted the question, but could not get past calling the setMainApp method. I've tried your way, and I got `java.lang.NoSuchMethodException: com.nomad.view.userLayoutController.setMainApp()`. Could you point me as to how would I do this using interfaces? – Chris Aug 24 '15 at 12:42
What is the signature for `setMainApp`? – Dakshinamurthy Karra Aug 24 '15 at 12:46
It's `public void setMainApp(MainApp mainApp)` - thank you for your time! – Chris Aug 24 '15 at 12:57
Updated the `getMethod` call. See whether it helps. – Dakshinamurthy Karra Aug 24 '15 at 13:02
It works just fine now, thank you! But could you point me to an example with your recommendation on using interfaces? If that's a good practice, I'd like use it. – Chris Aug 24 '15 at 13:11
@jovit.royeca answer is more like it. But without seeing more code - won't be able to provide better answer. – Dakshinamurthy Karra Aug 24 '15 at 13:13
3

`Class.getMethod` is a varargs method. So there is no need for a `new Class[]`… – Holger Aug 24 '15 at 13:39
@Holger true. When I posted code first, I don't know the type, left it as place holder. Invoke I used varargs version. – Dakshinamurthy Karra Aug 24 '15 at 14:45

score 0 · Answer 2 · answered Aug 24 '15 at 12:28

You can use an interface as a parameter in the method. Then pass in implementations of the interface as a parameter when you call the method.

interface Loader {
    void load();
}

interface Controller {
    void setMainApp(Object mainApp);
}

Here is a class that implements the loader interface:

class AnchorPane implements Loader {
    void load() {
        System.out.println("Loading...");
    }
}

And here is a class that implements the Controller interface:

class UserLayoutController implements Controller {
    void setMainApp(Object mainApp) {
        System.out.println("Setting main app...");
    }
}

Change the method definition to this:

public void genericLayoutLoader(String fxmlFilename, Loader loader, Controller controller) {
    try {
        FXMLLoader fxmlLoader = new FXMLLoader();
        fxmlLoader.setLocation(MainApp.class.getResource(fxmlFilename));

        // rootAnchorPane is same for every layout
        rootAnchorPane.getChildren().clear();
        rootAnchorPane.getChildren().add(loader);

        controller.setMainApp(this);
    } catch (IOException e) {
        // Handle Exception
    }
}

Then call it somewhere in your code:

genericLayoutLoader("view/userLayout.fxml", new AnchorPane(), new UserLayoutController());

1. In JavaFX, the AnchorPane class is provided, and I am not extending it, instead just using it as a member. So the `loader interface` would be implemented in each controller? In that case, that means a new load method, for each controller. I want to avoid creating methods that do the same thing, only with different objects. 2. I think that this line `rootAnchorPane.getChildren().add(loader);` isn't equivalent to passing the actual AnchorPane object, after I've loaded it from the FXML Loader. I might be missing sth with this notation though, as I am not that experienced with Java yet. — Chris, Aug 24 '15 at 13:36
[2-continue] Also, the loader interface, has a method that returns nothing, so how would that `.add(loader)` work? — Chris, Aug 24 '15 at 13:37

score 0 · Answer 3 · edited May 23 '17 at 12:22

1) For the Type of object which you are not sure, consider using a wildcard.

Refer to this: When to use generic methods and when to use wild-card?

2) Once you get the object, now you can use the instance Of operator to check the type of object and perform steps specific to this object like:-

if(object instanceof AnchorPane){
//do this
} else if(object instance of  BorderPane){
//do this
}

How to write a method that accepts a class type as a parameter, and uses it to cast some objects

3 Answers3