How can you invoke a method in Java class without using its class?

Question

In OOP, I learned that we can call a method by using its class.

For example:

Person person = new Person();
person.methodA(); // calling the method in person class

Based on javafx docs of ListView (link) , The getSelectionModel() is a method of a class ListView (okay working). But the selectedItemProperty() method is the class of SelectionModel (link)

How can you call a method selectedItemProperty() without using its SelectionModel class? And what class does this addListener method came from?

myListView.getSelectionModel().selectedItemProperty().addListener(new ChangeListener<TodoItem>() {
///  blah blah blah
}
});

Answer 1

MultipleSelectionModel ( link ) extends SelectionModel ( link ), in which the selectedItemProperty() method is implemented.

selectedItemProperty() returns a ReadOnlyObjectProperty ( link ) instance, which extends ObservableValue ( link ), from which the addListener() method is coming.

As for how you can call these methods- this is just how inheritance works. The subclass ( MultipleSelectionModel ) inherits all of the properties of its superclass ( SelectionModel ). Thus any methods defined in SelectionModel you can call against a MultipleSelectionModel instance.

OOP Concept for Beginners: What is Inheritance?

Answer 2

When you call ListView#getSelectionModel() you are getting an object which is an instance of SelectionModel —more specifically, an instance of MultipleSelectionModel . Since you now have an instance of MultipleSelectionModel you can invoke methods present in that class, including inherited methods. This means you can do the following:

listView.getSelectionModel().selectedItemProperty().addListener(yourChangeListener);

Which is equivalent to the following:

MultipleSelectionModel<T> sModel = listView.getSelectionModel();
ReadOnlyObjectProperty<T> selectedItemProp = sModel.selectedItemProperty();
selectedItemProp.addListener(yourChangeListener);

As you can see, you are using the class of every object involved. The former, known as method chaining, is just shorthand.