Private class as return type from public method

user_1038548 0
Viewed 5305

Why is this valid?

Foo.java

public class Foo {

    public Bar getBar() {
        return new Bar();
    }

    private class Bar {}

}

If Bar is private, how will users of this class use this method? Polymorphism can be used of course, but shouldn't this be invalid and the declaration should indicate this as returning an Object?

access-modifiers polymorphism java
8 Answers

I have a perfectly valid use case for this, and I'm glad this is allowed.

Let's stay you have a class that creates UI pieces. It accepts somekind of domain object and creates a piece of UI:

public Node createPersonUI(Person person) {
    BasePanel panel = new BasePanel();
    // ... setup panel with values ...
    return panel;
}

BasePanel is a subclass of Node and is some internal class that the caller has no business with, as this class determines how things will look.

Now, I found myself needing to re-use part of this class when I needed to support a new object, PersonalProfile that contains much more information, but also contains the basic Person data:

public Node createPersonalProfileUI(PersonalProfile profile) {
    BasePanel panel = new BasePanel();
    // ... setup panel with values ...
    return panel;
}

However, that code was partially duplicated, so I did:

public Node createPersonalProfileUI(PersonalProfile profile) {
    BasePanel panel = (BasePanel)createPerson(profile.getPerson());
    // ... only setup missing values ...
    return panel;
}

The cast however is a bit ridiculous -- changing it to return BasePanel not only works, but doesn't expose any functionality of my private class. Instead it only exposes the methods from any public classes it inherits from... brilliant!

Full code:

public BasePanel createPersonUI(Person person) {
    BasePanel panel = new BasePanel();
    // ... setup panel with values ...
    return panel;
}

public BasePanel createPersonalProfileUI(PersonalProfile profile) {
    BasePanel panel = createPerson(profile.getPerson());
    // ... only setup missing values ...
    return panel;
}

private class BasePanel extends Node {
}
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