How to iterate a list of superclass and accessing subclass properties?

Question

Editing my question because I realized better what I want:

Say we have List<A> with bunch of instances of classes extending A each with its own unique properties.

Like in this example:

class A has field int x, getter and setter.
class B extends A, and also has field int y with its getter and setter.
class C extends A, and also has field int z with its getter and setter.

I don't want to use downcasting and instanceof when iterating the list.

I read this post which was interesting: https://softwareengineering.stackexchange.com/questions/245263/how-to-avoid-downcasting

What is the right approach?

Thanks

Also it does make sense for a subclass to have more fields than its super class. Every time you write your own class with a field you’re doing that, because it extends Object which does not have that field. — cpp beginner, Apr 01 '18 at 19:37
Right but A b is better practice. that's why I ask if it makes sense — belostoky, Apr 01 '18 at 19:38
It’s better practice for interfaces, e.g. List x = new ArrayList<>() but for classes, I don’t think it is. — cpp beginner, Apr 01 '18 at 19:39
It's better practice if all you care about, in the rest of the code, is that the variable refers to an A. It's not the case here. You very rarely do `Object o = "hello"`, right? Because you want a variable of type String, not a variable of type Object. — JB Nizet, Apr 01 '18 at 19:42
and if its List with bunch of instances of classes extending A each with its own unique properties, then use `instanceof` every time I want to use one of these properties? — belostoky, Apr 01 '18 at 19:47
No. Use a polymorphic method of each object, or an adapter, or the visitor pattern. — JB Nizet, Apr 01 '18 at 19:54

score 0 · Answer 1 · answered Apr 01 '18 at 19:36

0

In most cases, you should be doing B b = new B() instead of A b = new B(). If you must use a field of type A, then downcasting is your best bet. Keep in mind, though, that it shouldn't happen very often.

answered Apr 01 '18 at 19:36

Sam

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Thanks for your answer, I changed my question a bit. – belostoky Apr 01 '18 at 20:27
This answer is wrong for the general case. The main rule is to code to the interface, to the abstraction, not to the concrete. Avoid coding to the concrete unless absolutely necessary. There are whole design patterns created to avoid doing this, such as the Visitor Pattern. – Hovercraft Full Of Eels Apr 01 '18 at 20:28

wes · Answer 2 · 2018-04-01T20:20:56.313

Yes, it does makes sense for a sub-class to have more fields than it's super.

If you find yourself having to downcast, then it likely means that you should instantiate the variable with the static class that you are down-casting to, in this case, do B b = new B() instead of A b = new B(). When you use A b = new B(), it means the context/function in which you want to use the variable expects an object of type A (i.e. expects to use functions that are declared in A), therefore if you are down-casting to B, you may need to review your design and ask yourself the data type that you want to work with in that context/function.

Thanks for your answer, I changed my question a bit. – belostoky Apr 01 '18 at 20:27 — belostoky, Apr 01 '18 at 20:27

How to iterate a list of superclass and accessing subclass properties?

2 Answers2