When to use inner classes in Java for helper classes

Question

If I have for example a class along with a helper class to do some of its functionality, does it make sense to make it as an inner class.

    public class F         


        

Answer 1

Here are some uses of inner classes.

• Inner classes are used to get functionality which can get an object better than method.
• They can be used in the case when a set of multiple operations are required and chances of reusability are good inside the class and they will not be accessed but methods outside the outer class.
• Inner classes are made to achieve multiple inheritance also.
• Inner classes are used when they are useful in class context.
• They are used to separate logic inside classes.

So if you have some requirement matching above points than inner classes can be used. It is always better to make inner class private to prevent access from other classes. In your case use of inner classes is helpful to make code readable and separate logic in the outer class.

Answer 2

As per Oracle Docs, Simply Explained

Compelling reasons for using nested classes include the following:

It is a way of logically grouping classes that are only used in one place: If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. Nesting such "helper classes" makes their package more streamlined.

It increases encapsulation: Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared private. By hiding class B within class A, A's members can be declared private and B can access them. In addition, B itself can be hidden from the outside world.

It can lead to more readable and maintainable code: Nesting small classes within top-level classes places the code closer to where it is used.

Answer 3

Inner classes make sense when they are tiny and don't need names. Listeners in GUIs are classic examples where they make sense.

If the class is big and important, it should be named and placed in a separate file.

The listener classes in normal GUI examples do one tiny thing, usually just dispatch to some other function to do real work.

I also often use static nested classes (which are technically not inner classes) for classes which are only used in the context of another class - Map.Entry is a good example of this. It's only used in conjunction with a Map, so having the definition of Entry be a part of the Map interface makes organizational sense.

I don't generally have much use for other types of nested classes, like nonstatic member classes and local classes. But they do occasionally come in useful. For a good example of a legitimate use for member classes, see the source code for LinkedList.ListItr. This is a private inner class whose purpose is to provide an implementation of ListIterator for a LinkedList. To do this, it's useful to have access to the private data inside the LinkedList. To achieve this using only top-level classes, it would have been necessary to expose more public methods in LinkedList to allow the ListIterator to get at the underlying implementation of the LinkedList. Instead, using an inner class allows LinkedList to keep its implementation private, as it should be.

Answer 4

When to use inner classes?

1. Inner class does not need any additional or separate file to place.
2. Inner classes are comparatively small and tightly coupled with the “parent” class or “outer” class. Avoids lot of helper classes and fits to explain the OOD principle of containment.
3. All the code in the same file increases readability and increases performance (like online code). Their importance is growing in the coding and treated as a good practice.
4. Anonymous inner classes are very useful when you want to define callbacks on the fly.
5. An object of an inner class can access the implementation of the object that created it- INCLUDING private data.
6. Inner classes are capable of accessing the behavior of their parent class including private.

Answer 5

If you think that FooHelper will not at all be useful for other classes than Foo, then it makes sense to make it as private inner class of Foo. One example of this kind of design can be found in HashMap where it defines a private inner class KeySet

Otherwise having it as a private instance looks good.