Inner Class in Java, when to use?

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11

Sometimes I find class codes with Inner class, such as:

class ClasseExterna {

    private int a = 10;
    // ...

    class InnerClass {  

        public void accessOuter() {
            System.out.println("Outra classe  " + a);
        }

        // ...
    }
}

I always wonder:

Why and when to use Inner class? I have not yet come across a situation where there was no alternative.

java oop

2 answers

8


Usually the purpose of a Inner class is to separate functionalities to make code more organized, or when specific functionality requires a separate class, but is too specific to create a separate class.

One example I can cite is a class that implements Iterable and is declared the Iterator as a Inner class.

An example of this case can be seen in the class java.util.ArrayList (I removed the implementation to not get too long):

public Iterator<E> iterator() {
    return new Itr();
}

private class Itr implements Iterator<E> {
    //...
}

3

There is always an alternative to the inner class.

In general, we use an internal class when you need to access non-public /methods attributes.

The official documentation extract :

Use a non-static nested class (or Inner class) if you require access to an enclosing instance’s non-public Fields and methods. Use the Static nested class if you don’t require this access.

Obviously, we don’t create an inner class, if you know, it may be necessary elsewhere.

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