Add an object of unknown class to ArrayList with reflection

Question

So I have an ArrayList of objects of unknown type extending Entity (my own class; not to be confused java.swing.text.html.parser.Entity) for reasons unrelated to this problem.

private ArrayList<? extends Entity> listedEntities = new ArrayList<>();

This is in a class that in its constructor receives a String and assigns it to the field entityClass.

At some point I am executing the following piece of code:

    Class<?> clazz = Class.forName(this.entityClass);
    Object newEntity = clazz.newInstance();

    listedEntities.add(newEntity);

Naturally, this doesn't work as the ArrayList is expecting an object of a class extending Entity and newEntity is of Object. So first thing was to try casting.

listedEntities.add(Class.forName(this.entityClass).cast(newEntity));

Also doesn't work. And after a few minutes not finding an answer I am here typing this.

The exception:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    The method add(capture#8-of ? extends Entity) in the type ArrayList<capture#8-of ? extends Entity> is not applicable for the arguments (capture#9-of ?)

Is there a good reason that you are using String for entityClass instead of a Class? — Radiodef
– Radiodef, Commented Jan 15, 2017 at 20:57

Radiodef · Accepted Answer · 2017-01-15 21:15:34Z

4

A simple solution is like this:

Change listedEntities to ArrayList<Entity> instead of ArrayList<? extends Entity>. It doesn't look like you have a particular reason to use the wildcard and the wildcard prohibits you from adding to the list.

Change your construction to this:

Class<?> clazz = Class.forName(this.entityClass);
Entity newEntity = (Entity) clazz.newInstance();

listedEntities.add(newEntity);

This will solve the compilation error and it's type-safe in that the explicit cast will throw an exception if somehow this.entityClass is not some subtype of Entity.

A fully-generic version as I believe @RC. hints at in the comments would be like this:

class Example<E extends Entity> {
    private List<E> listedEntities = new ArrayList<>();
    private Class<E> entityClass;

    Example(Class<E> entityClass) {
        this.entityClass = entityClass;
    }

    void method()
    throws IllegalAccessException, InstantiationException {
        E newEntity = entityClass.newInstance();
        listedEntities.add(newEntity);
    }
}

That is more 'proper'; however, whether you can use something like it or not depends on your specific needs.

As a side note, the fact that your error is thrown as an exception worries me a little bit. You should use a proper IDE which will flag compilation errors like that in the editor so you don't have to run the program to find them.

answered Jan 15, 2017 at 21:15

Radiodef

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6 Comments

Hristo Valchev Hristov Over a year ago

Regarding the side note: I'm using Eclipse Neon 2.0 but the exception is an exception due to having the following entry point - public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { public void run() { new Primary(); }}); }

Radiodef Over a year ago

Well that seems fine. I think it should still flag the error during compilation but unfortunately I don't know enough about Eclipse to say why it might not in this case.

Hristo Valchev Hristov Over a year ago

Regarding the proposed solution: It doesn't fulfil my needs to use an ArrayList<Entity>. Actually that was the initial version before using reflection at all. I need to have objects of the same class and that class needs to extend Entity. The fully generic version also doesn't do me good for one reason: The class I am in is defined as Lister<E> and it never is the same as the class the ArrayList holds.

Radiodef Over a year ago

If you've lost the type of the list you assign to listedEntities, you could convert it to ArrayList<Entity> by copying it e.g. listedEntities = new ArrayList<Entity>(theSourceList). For the generic version, unless I'm misunderstanding, you could just add a second type variable like Entity<E, T extends Entity>. It's a bit hard to offer a better suggestion without knowing more about those restrictions. I already emphasized this briefly, but the wildcard will simply have to go if you want to add to listedEntities. See e.g. stackoverflow.com/q/2723397/2891664.

Hristo Valchev Hristov Over a year ago

No, of course, I could add a second type variable, but the thing is at the moment of initialisation of the variable of class Lister I still don't know the class extending Entity that will be required. As I said, your answer was helpful and it pushed me towards resolving the problem. Thanks.

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