Question

I am trying to follow this JavaFX 2 guide:

http://docs.oracle.com/javafx/2/ui_controls/table-view.htm#CJAGAAEE

I use Scala instead of Java and it looks like this for me:

<TableView fx:id="test">
     <columns>
         <TableColumn prefWidth="75.0" text="Message" />
     </columns>
</TableView>

And code:

val c = test.getColumns.get(0) // Get the first column.
c.setCellValueFactory(new PropertyValueFactory[Foo, _]("message")) // Foo is my model with a single SimpleStringProperty called "message".

val observableFoos = FXCollections.observableList(foos)
test.setItems(observableFoos)

The problem I have is that the setCellValueFactory line causes:

error: class type required but
javafx.scene.control.cell.PropertyValueFactory[com.myapp.models.Foo,
_] found c.setCellValueFactory(new PropertyValueFactoryFoo, _)

I don’t understand how I am supposed to use this method. If I replace _ with String, then I get:

error: type mismatch; found :
javafx.scene.control.cell.PropertyValueFactory[com.myapp.models.Foo,String]
required:
javafx.util.Callback[javafx.scene.control.TableColumn.CellDataFeatures[com.myapp.models.Foo,?0],javafx.beans.value.ObservableValue[?0]]
where type ?0 c.setCellValueFactory(new PropertyValueFactoryFoo,
String)

I can confirm that everything works fine if I remove the setCellValueFactory line — I just don’t see any content in the table — just blank rows as expected..

Answer 1

TL;DR: It is easy in java to silently bypass type safety with respect to type parameters.
Scala won’t let you do that, short of performing an explicit cast.
In addition, using Table.getColumns to retrieve the column means that the we lose the type of cell.
Solutions: cast your TableColumn instance to the proper type, or instantiate the column by hand.

The first problem lies in the signature of Table.getColumns: it returns a ObservableList[TableColumn[S,_]] (see http://docs.oracle.com/javafx/2/api/javafx/scene/control/TableView.html#getColumns()).
So your c val is typed as TableColumn[Foo,_].
This means that you lose the type of the cell content (denoted by the second type parameter).

The only solution here is to properly type the column (c) using a cast:

val c = test.getColumns.get(0).asInstanceOf[TableColumn[Foo, String]]
c.setCellValueFactory(new PropertyValueFactory[Foo, String]("message"))

This is quite logical: you have a list of columns where each column can contain objects of a different type. We would face the same dilemma with a scala List containing objects of different types, all unknown a priori: only a cast (or a match on the type, which is a cast in disguise) will let you get back the a specific type at compile time.

Note also that in many JavaFx examples around the web, people don’t retrieve the columns through Table.getColumns. Instead, they instantiate them by hand and then call setCellValueFactory on it right after.
Doing the same would solve your problem (without any need for a cast), as you will yourself specify the type parameters:

val c = new TableColumn[Foo, String] // See ma, no cast!
c.setCellValueFactory(new PropertyValueFactory[Foo, String]("message"))

Now, you might look at some java examples and notice that they don’t actually provide any type parameter when instantiating their TableColumn:

TableColumn c  = new TableColumn();    
c.setCellValueFactory( new PropertyValueFactory<Foo,String>("name"));

And indeed it does compile. How so? Well the sad truth is that the above code is not safer than doing a cast, as it relies on java’s backward compatibility for pre-generics aware classes.
In other words, because c is typed as just TableColumn and not TableColumn<?, String> by example, the java compiler treat it as a non generic class and not perform any type checking regarding the type parameters of TableColumn.
Contrast this to what happens if you explictly set the type of c to a generic type (but with an unknwon cell type):

TableColumn<Foo, ?> name  = new TableColumn("Name");
name.setCellValueFactory( new PropertyValueFactory<Room,String>("name"));

In this case the compiler emits a type mismatch error:

error: method setCellValueFactory in class TableColumn<S,T> cannot be applied to given types;
...

This is just like in scala when c is typed as TableColumn[Foo, _]