Cfrgtkky

So I am trying to create an ObservableMap that has String as a key and FXCollections.observableList as a value

    Map<String, ObservableList<String>> namesMap = new HashMap<>();

    ObservableMap<String, ObservableList<String>> names = FXCollections.observableMap(namesMap);

    names.addListener(new MapChangeListener<String, List<String>>() {

        @Override

        public void onChanged(MapChangeListener.Change<? extends String, ? extends List<String>> change) {

            System.out.println("Detected a change!");

            System.out.println(change.getValueAdded());

        }

    });



    names.put("1", FXCollections.observableArrayList().addListener(new ListChangeListener<String>(){

        @Override

        public void onChanged(ListChangeListener.Change<? extends String> c) {

            //to-do

        }



    }));

    names.get("1").add("One");

But it gives me this error at the line where I'm assigning listener to the value of the key 1. I put FXCollections.observableArrayList() as a value for the key 1 because when creating an observable list, that is the value that is assigned so I tried to set that as a value. Suggestions of the compiler didn't help. The reason why I am doing this is to create an observable map with values that I could track/observe if a change has happened.

Can anyone help me? Any help would be appreciated. Thanks!

The method ObservableList.addListener returns nothing (i.e. void). This means you are calling names.put("1", void) which is not valid.

You need to create the ObservableList and add the listener separately.

ObservableList<String> list = FXCollections.observableArrayList();

list.addListener(/* your listener */);

names.put("1", list);



// OR



// Less efficient since you have to perform a lookup

// in order to add the listener.

names.put("1", FXCollections.observableArrayList());

names.get("1").addListener(/* your listener */);

The other option is to add the ListChangeListener inside the MapChangeListener.

// Stored in variable so it can be removed if necessary

final ListChangeListener<String> listListener = listChange -> {

    // TODO: Implement...

};

names.addListener((MapChangeListener<String, ObservableList<String>>) mapChange -> {

    if (mapChange.wasAdded()) {

        mapChange.getValueAdded().addListener(listListener);

    } 

    if (mapChange.wasRemoved()) {

        mapChange.getValueRemoved().removeListener(listListener);

    }

});



// ListChangeListener will now be added automatically by the MapChangeListener

names.put("1", FXCollections.observableArrayList());