Java Child referenced type inside Parent array

Question

For the following question,

Suppose the object referenced in a cell of

ArrayList<Polygon> pentagonGroup

is of type Pentagon. If you later reference that object, what type will it have?

What would the answer be?

I tried playing around in a IDE and it seems like child objects even when placed into a Parent list are identified as a Child when doing the following operation.

Child c = new Child();
ArrayList<Parent> pa = new ArrayList<Parent>();
pa.add(c);
Class cls = pa.get(0).getClass();
System.out.println(cls.getName());

This seems kind of odd when one is unable to use Child object specific methods without downcasting it back to an Child.

System.out.println(pa.get(0).getChildMessage()); // invalid
System.out.println(((Child)pa.get(0)).getChildMessage()); // valid

Given my results the answer to the first question would be (Pentagon). Would that be correct?

EDIT:

So I'm actually still not sure about the very first question asked above; Would the answer be Polygon or would it be Pentagon?

Ok, so when the child's methods are "locked" away until being casted as a Child again; that is a separate concern? — michael16574
– michael16574, Commented Aug 2, 2018 at 15:55

xingbin · Accepted Answer · 2018-08-02 15:56:23Z

2

At runtime, you can get the type of pa.get(0):

Class cls = pa.get(0).getClass();
System.out.println(cls.getName());

While at compile stage, the compiler only knows pa.get(0) is Parent, so you can not call pa.get(0).getChildMessage() without down casting. It's for type safe.

In fact, you can casting any object to Child to make it compile,

Object o = new Object();
System.out.println(o.getChildMessage());

but it will throw casting exeception at run time.

answered Aug 2, 2018 at 15:56

xingbin

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4 Comments

michael16574 Over a year ago

So would the answer to the first question actually technically be Polygon instead of Pentagon at the compile stage? And then when during run time it would be a Pentagon?

michael16574 Over a year ago

Also, would that "any object" be limited to objects that aren't parents of the child? I tried downcasting a parent to child but that obviously didn't work.

kumesana Over a year ago

@michael16574 Polygon at compile time and Pentagon at runtime is a rather good way to put it. At compile time, the type of your objects won't be anything else than what you declared, and you declared the list as ArrayList<Polygon>, so the stuff it contains are of type Polygon. At runtime the objects actually exist and are being handled, and they have a class rather than just a declared type. This class get be inspected by calling getClass().

kumesana Over a year ago

@michael16574 downcasting a parent to a child can work if it so happens the runtime object going through downcasting is of a class that is a subtype of the type you're downcasting to.

Alan · Accepted Answer · 2018-08-02 16:03:06Z

0

Assuming Child extends Parent, the ArrayList is composed of Parent objects, as far as the compiler is concerned. So when you get a value from the list, it is a Parent. In your case, it is also a Child, but it might not be in the general case.

Remember java is not like groovy with dynamic binding, so if the Parent doesn't have the getChildMessage() method, it has no method it can assign to. So you have the option of either defining a getChildMessage() method in Parent (maybe return an empty string or throw an exception) and override it in Child, or downcasting as you did, but then make sure it is a Child first with instanceof, e.g.

answered Aug 2, 2018 at 16:03

Alan

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