In Java is there a way to name an ArrayList inside the ArrayList?

Question

When I've needed a way of easily identifying an ArrayList inside an ArrayList, I would just put an identifier in the 0 index, then print out all the zero indexes with a loop. Now I'm wondering if there is a way to make the nested ArrayLists have names.

for example:

ListOfLists.add( new ArrayList<String>() );
ListOfLists.get(ListOfLists.size() -1).add("someIdentifierHumansCanRead");

would there be some way to make ListOfLists.add(new ArrayList<String>()) be ListOfLists.add(ArrayList someIdentifierHumansCanRead = new ArrayList<String>)?

Or would this just be completely absurd?

Konstantin Pribluda · Accepted Answer · 2012-10-21 09:03:43Z

4

You are speaking of map where all the array lists are keyed by strings - but you will lose iteration order. Alternatively you can store keys in array list and array lists in maps.

answered Oct 21, 2012 at 9:03

Konstantin Pribluda

12.3k1 gold badge33 silver badges38 bronze badges

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

5 Comments

assylias Over a year ago

"you will lose iteration order" => it depends on the map implementation. LinkedHashMap preserves insertion order for example.

Krumelur Over a year ago

@assylias +1, I was just going to suggest LinkedHashMap

assylias Over a year ago

@VincenzoSanchez My main comment is that retrieving a NamedList by name with your approach will be an O(n) operation and will require looping over all the lists, whereas with the Map approach, it becomes an O(1) operation.

Juvanis Over a year ago

@assylias but what if the user wants to keep 2 lists with the same name? e.g. dogs,cats,cats,horses,cats. the op doesn't have to merge these "cats" lists? in that case a map will fail. am i wrong?

assylias Over a year ago

@VincenzoSanchez Yes you are right, the Map approach assumes that the names are unique identifiers, which might not be the case - the question is farily vague to be honest. Or he could store the lists in a Map<String, List<List<...>>>, where each name points to all the lists with that name.

Juvanis · Accepted Answer · 2012-10-21 09:10:12Z

1

Implement your own ArrayList class and define an identifier property for it:

class NamedList<T> extends ArrayList 
{
    private String name;

    public NamedList(String name) {
        this.name = name;
    }

    // Getters/Setters and other stuff for identifier, etc.
}

ArrayList<NamedList<String>> superList = new ArrayList<NamedList<String>>();
superList.add(new NamedList<String>("babangida"));

answered Oct 21, 2012 at 9:10

Juvanis

26k5 gold badges75 silver badges88 bronze badges

1 Comment

Frank Pavageau Over a year ago

@VincenzoSanchez I'd say "see mine" :-) To add unrelated functionality, you should use composition, not inherit directly from ArrayList. What if you now need 2 separate lists in the structure? It also hides implementation details, such as using an ArrayList or a LinkedList. See the Liskov substitution principle.

Community · Accepted Answer · 2017-05-23 10:34:54Z

If you need to "name" your ArrayList, then you really want to create a meaningful data structure instead of using hacks. This is OO programming, after all:

public void MyNamedBean {
    private String name;
    private final List<String> list = new ArrayList<String>();

    public MyNamedBean(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    // If you don't need to change the name, make the field
    // final and remove this method.
    public void setName(String name) {
        this.name = name;
    }

    // It should have a more meaningful name.
    public List<String> getList() {
        return list;
    }
}

Update, based on my comments on another answer: it's an OO design issue. If it's a list with a name, it's not just a list, which means you shouldn't directly extend a List implementation, but instead create a meaningful class. What if you need at some point a second list, or another "name" (a human-readable label, for example)? You can read on the Liskov substitution principle about when to use inheritance and when to use composition. See also Prefer composition over inheritance.

Deepak Bala · Accepted Answer · 2012-10-21 09:07:03Z

0

If you meant references you cant try this

ArrayList someIdentifierHumansCanRead = new ArrayList<String>();
ListOfLists.add(someIdentifierHumansCanRead)

Or you can use a Map<String,List<String>> if you want to retrieve the arraylist using an identifier.

map.put("id",new ArrayList<String>());

The map can be a HashMap / LinkedHashMap / TreeMap etc. The iteration order will depend on the chosen implementation.

answered Oct 21, 2012 at 9:07

Deepak Bala

11.2k2 gold badges40 silver badges49 bronze badges

2 Comments

Elderwyrm Over a year ago

I'll have to look into this. I'm not sure I've ever tried to implement a map in Java.

Deepak Bala Over a year ago

You don't have to implement a Map. Jave provides one for you. Try new HashMap<String,List<String>>();

Collectives™ on Stack Overflow

In Java is there a way to name an ArrayList inside the ArrayList?

4 Answers 4

5 Comments

1 Comment

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

5 Comments

1 Comment

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related