Accessing inner class from static outer util function

Question

I have a class structure roughly like this:

final public class Util {
    private Util() {
        throw new AssertionError(); // there is not supposed to exist an instance
    }

    public static DataElem getData() {
        return new Util().new DataElem();
    }

    public class DataElem {
        // class definition
    }
}

The code for correctly generating an instance of the inner class was taken from this thread. But I didn't like that every time an inner class instance gets made, an instance from the outer one gets made first. And since I put the AssertionError into its constructor, it doesn't work.

Do I have to carry a dummy-instance around just to create instances from the inner class? Can't I make something like Util.DataElem work?

Add `static` to the class header for `DataElem`? – awksp Jul 09 '14 at 18:32 — awksp, Jul 09 '14 at 18:32

jmj · Accepted Answer · 2014-07-09T18:39:08.653

3

You can make your inner class static

final public class Util {
    private Util() {
        throw new AssertionError(); // there is not supposed to exist an instance
    }

    public static DataElem getData() {
        return  new Util.DataElem();
    }

    private static class DataElem {
        private DataElem(){} // keep private if you just want elements to be created via Factory method
        // class definition
    }
}

and then initialize it like

new Util.DataElem();

edited Jul 09 '14 at 18:39

answered Jul 09 '14 at 18:33

jmj

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what about this `return new DateElem()` – Braj Jul 09 '14 at 18:34
@Braj yes what about it ? – jmj Jul 09 '14 at 18:35
1

You should remove the `=` in `return = new Util().new DataElem();`. – Jashaszun Jul 09 '14 at 18:36
1

Simply return `new DateElem()` there is no need to add the `Util` class name because it's already inside the `Util` class. – Braj Jul 09 '14 at 18:37
@JigarJoshi ... no it wasn't. The `=` is still an error. – Jashaszun Jul 09 '14 at 18:39
... Ok, now the problem is that you have to have a `new` before `DataElem();`. That line has to be either `return new Util().new DataElem();` or `return new DataElem();`. – Jashaszun Jul 09 '14 at 18:42
@Jasha this is perfectly fine – jmj Jul 09 '14 at 18:42
Sorry for taking so long to respond. But I cannot make the inner class static, as I mean to have several different instances of them. – Arne Jul 09 '14 at 19:21
Certainly you can have multiple instance of inner static class – jmj Jul 09 '14 at 19:22

Accessing inner class from static outer util function

1 Answers1