Store interface in array list java

Question

I am learning java. I am trying to use composite design pattern. I am trying to use following logic. ( Don't laugh I know it is very basic :) )

Item -> interface
Folder -> class
File -> class

In folder class, can I create an arraylist of Item to store files information?

ArrayList<Item> info = ArrayList<Item>();

Or should I use Folder Arraylist?

ArrayList<Folder> info = ArrayList<Folder>();

I don't know if interface can store real data as there is no variable just function definitions.

Thanks for helping a newbie :)

polygenelubricants · Accepted Answer · 2010-03-14 00:05:38Z

5

You can do both (with some syntactic correction)

List<Item> info = new ArrayList<Item>();

With regards to this comment:

I don't know if interface can store real data as there is no variable just function definitions.

Interfaces do more than provide function definitions. Most importantly, they define a type. info, declared as above, is a list of objects of type Item. Those objects can most certainly store data.

As an example, consider the following:

interface Item { ... }
class Folder implements Item { ... }

Item it = new Folder();

Now, it refers to an instance of Folder, which is an Item.

edited Mar 14, 2010 at 0:05

answered Mar 14, 2010 at 0:00

polygenelubricants

385k129 gold badges568 silver badges627 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user238384 Over a year ago

can interface store all variable information that is in Folder class??

polygenelubricants Over a year ago

Interfaces can only define static fields, which is probably not what you meant. In actuality, inheriting instance variables don't make sense with interfaces; you want to inherit the type, not the implementation.

Benjamin Ortuzar · Accepted Answer · 2010-03-14 00:16:03Z

2

An interface can't store data. An interface is a Type that you can use as a contract that your Files and Folder classes will implement. Since your Files and Folder classes both implement the Item interface, you can add files and folder objects to the List that accepts the Item type.

List<Item> info = new ArrayList<Item>()

edited Mar 14, 2010 at 0:16

answered Mar 14, 2010 at 0:09

Benjamin Ortuzar

7,8416 gold badges44 silver badges46 bronze badges

Comments

Michael Koval · Accepted Answer · 2010-03-14 00:02:39Z

1

As long as Folder and File both implement Item, this will work fine. Be aware, however, that you will only have access to the properties that are defined in the Item interface: not those specific to Folder or specific to File.

From what you've described, it sounds like a reasonable way to approach the problem, although it's hard to say for sure without knowing more about the use of this data structure.

answered Mar 14, 2010 at 0:02

Michael Koval

8,4075 gold badges44 silver badges53 bronze badges

Comments

Carl Manaster · Accepted Answer · 2010-03-14 00:02:49Z

1

You certainly can create an ArrayList of Items. It is not a problem that Item, as an interface, doesn't have instance variables. The only Items that can be created, however, are not "just" Items - they will be instances of some class that implements the Item interface. So, for instance, if Folder and File both implement the Item interface, you can put both Folders and Files into that ArrayList. But the ArrayList would be declared just to hold Items - and, since they implement that interface, both Folders and Files qualify - they are Items.

answered Mar 14, 2010 at 0:02

Carl Manaster

40.5k17 gold badges109 silver badges158 bronze badges

Collectives™ on Stack Overflow

Store interface in array list java

4 Answers 4

2 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related