Best way to get value from Collection by index

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

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);             
}

Answer 4

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()); 

Answer 5

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?

Answer 6

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());
}