Polymorphism With Data Structures

Question

Possible Duplicate:
Implementations and Collections

My super class is Exam. One of my sub classes is Analytical.

This is a valid within my ide

Exam object = new Analytical();

However this is not.

ArrayList<Exam> object = new ArrayList<Analytical>();

The error is from new to the end of the statement and it says "Change type of object to ArrayList. So my question is how can I properly utilize polymorphis with this data structure. Thank you.

http://stackoverflow.com/questions/7098402/implementations-and-collections/ — zw324, Oct 01 '12 at 17:42
[Also worth looking at](http://stackoverflow.com/questions/12604477/use-of-in-collection-generics/12605337#12605337) — Brian, Oct 01 '12 at 20:56

score 1 · Accepted Answer · answered Oct 01 '12 at 17:43

1

You cannot have different parameterized types on two sides of generic type declaration..

So, you must use the same type on both side..

It should be: -

ArrayList<? extends Exam> object = new ArrayList<Analytical>();

or

ArrayList<Exam> object = new ArrayList<Exam>();

or

ArrayList<? extends Exam> object = new ArrayList<Exam>();

Polymorphism, does not apply on the parameterized type..

answered Oct 01 '12 at 17:43

Rohit Jain

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Option 1 is still giving me an error from new to the end of the statement. – gmustudent Oct 01 '12 at 17:49
Howcum?? Exam is your base class right?? And Analytical your subclass?? Then it should compile.. What have you written, can you post it here? – Rohit Jain Oct 01 '12 at 17:52
nevermind i'm good. thank you – gmustudent Oct 01 '12 at 17:57
@gmustudent.. Did it worked?? – Rohit Jain Oct 01 '12 at 17:58
yep. thank you. learned something new today! – gmustudent Oct 01 '12 at 17:59
@gmustudent.. Then its good.. And you're welcome.. :) – Rohit Jain Oct 01 '12 at 18:01

score 0 · Answer 2 · answered Oct 01 '12 at 17:42

0

You need to change it to something like:

ArrayList<? extends Exam> object = new ArrayList<Exam>();

Inheritance in generics is little different from our regular understanding.

answered Oct 01 '12 at 17:42

kosa

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1

Or `ArrayList extends Exam> object = new ArrayList();` – Gilberto Torrezan Oct 01 '12 at 17:43
Question. How will java know that I want the object type to be the Analytical subclass and not the MultipleChoice subclass for example. In other words, how can I specify the object type I desire. – gmustudent Oct 01 '12 at 17:48
1

If you want to keep only Analytical, not another type, then ArrayList object = new ArrayList(); – kosa Oct 01 '12 at 17:49

score 0 · Answer 3 · answered Oct 01 '12 at 17:55

Collections are checked only during Compilation time not during the Runtime, so this measure has been taken, so that by mistake a wrong type doesn't get into the collection...

ArrayList<? extends Exam> object = new ArrayList<Analytical>();

or

ArrayList<? extends Exam> object = new ArrayList<Exam>();

Polymorphism With Data Structures

3 Answers3