CODEWITHSUNDEEP
javacollections
keo
keo

Reputation: 591

Best way to get value from Collection by index

What is the best way to get value from java.util.Collection by index?

Upvotes: 59

Views: 218508

Answers (11)

gouessej
gouessej

Reputation: 4085

If your Collection is a List, simply cast it as a List and call get(final int index). Otherwise, it might make sense to consider finding the nth element in an ordered set, for example if it's a LinkedHashSet respecting insertion order (keep in mind that it's possible to create such an instance not respecting insertion order), you can use Collection.stream().skip(index).limit(1).findFirst().orElse(null).

Upvotes: 0

Soni K
Soni K

Reputation: 166

You can get the value from collection using for-each loop or using iterator interface. For a Collection c for (<ElementType> elem: c) System.out.println(elem); or Using Iterator Interface

 Iterator it = c.iterator(); 
        while (it.hasNext()) 
        System.out.println(it.next());

Upvotes: -1

akarnokd
akarnokd

Reputation: 70007

I agree with Matthew Flaschen's answer and just wanted to show examples of the options for the case you cannot switch to List (because a library returns you a Collection):

List list = new ArrayList(theCollection);
list.get(5);

Or

Object[] list2 = theCollection.toArray();
doSomethingWith(list[2]);

If you know what generics is I can provide samples for that too.

Edit: It's another question what the intent and semantics of the original collection is.

Upvotes: 27

butterchicken
butterchicken

Reputation: 13883

You shouldn't. a Collection avoids talking about indexes specifically because it might not make sense for the specific collection. For example, a List implies some form of ordering, but a Set does not.

Collection<String> myCollection = new HashSet<String>();
myCollection.add("Hello");
myCollection.add("World");

for (String elem : myCollection) {
    System.out.println("elem = " + elem);
}

System.out.println("myCollection.toArray()[0] = " + myCollection.toArray()[0]);

gives me:

elem = World
elem = Hello
myCollection.toArray()[0] = World

whilst:

myCollection = new ArrayList<String>();
myCollection.add("Hello");
myCollection.add("World");

for (String elem : myCollection) {
    System.out.println("elem = " + elem);
}

System.out.println("myCollection.toArray()[0] = " + myCollection.toArray()[0]);

gives me:

elem = Hello
elem = World
myCollection.toArray()[0] = Hello

Why do you want to do this? Could you not just iterate over the collection?

Upvotes: 61

Bhushan Bhangale
Bhushan Bhangale

Reputation: 10987

Convert the collection into an array by using function 

Object[] toArray(Object[] a)

Upvotes: 3

dfa
dfa

Reputation: 116372

you definitively want a List:

The List interface provides four methods for positional (indexed) access to list elements. Lists (like Java arrays) are zero based.

Also

Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a > list is typically preferable to indexing through it if the caller does not know the implementation.

If you need the index in order to modify your collection you should note that List provides a special ListIterator that allow you to get the index:

List<String> names = Arrays.asList("Davide", "Francesco", "Angelocola");
ListIterator<String> i = names.listIterator();

while (i.hasNext()) {
    System.out.format("[%d] %s\n", i.nextIndex(), i.next());
}

Upvotes: 1

LifesAway
LifesAway

Reputation: 159

It would be just as convenient to simply convert your collection into a list whenever it updates. But if you are initializing, this will suffice:

for(String i : collectionlist){
    arraylist.add(i);
    whateverIntID = arraylist.indexOf(i);
}

Be open-minded.

Upvotes: 1

apksatheesh
apksatheesh

Reputation: 1

use for each loop...

ArrayList<Character> al = new ArrayList<>();    
String input="hello";

for (int i = 0; i < input.length(); i++){
    al.add(input.charAt(i));
}

for (Character ch : al) {               
    System.Out.println(ch);             
}

Upvotes: 0

JodaStephen
JodaStephen

Reputation: 63405

I agree that this is generally a bad idea. However, Commons Collections had a nice routine for getting the value by index if you really need to:

CollectionUtils.get(collection, index)

Upvotes: 11

Aaron Digulla
Aaron Digulla

Reputation: 328724

You must either wrap your collection in a list (new ArrayList(c)) or use c.toArray() since Collections have no notion of "index" or "order".

Upvotes: 6

Matthew Flaschen
Matthew Flaschen

Reputation: 284927

In general, there is no good way, as Collections are not guaranteed to have fixed indices. Yes, you can iterate through them, which is how toArray (and other functions) work. But the iteration order isn't necessarily fixed, and if you're trying to index into a general Collection, you're probably doing something wrong. It would make more sense to index into a List.

Upvotes: 19

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