Java, How Iterate through Collection?

Question

so I was given an interface where one method I need to implement gives me a Collection and wants me to "addAll" the data in the collection to my object. I'm still not sure what exactly a collection is. Is it an array list? I don't belive it is, but can I use a for each loop for each piece of data in the collection? Or is there another way to iterate through the collect accessing all of the value.

Collection implements the `Iterator` interface, doesn't it? Couldn't you use that to **Iterate** through? — SteveK, Apr 12 '15 at 02:28
An `ArrayList` is a [collection](https://docs.oracle.com/javase/tutorial/collections/). You should google "java collections"; google helps in situations like this. — Vince, Apr 12 '15 at 02:28
possible duplicate of [Java: Best way to iterate through an Collection (here ArrayList)](http://stackoverflow.com/questions/5228687/java-best-way-to-iterate-through-an-collection-here-arraylist) — John, Apr 12 '15 at 02:29
@Steve `interface Collection extends Iterable` but more less yes to what you say. ; ) — Radiodef, Apr 12 '15 at 03:56

score 5 · Answer 1 · answered Apr 12 '15 at 02:31

5

Iterating a Collection<? extends E> can be done with an Iterator (and you can get one with Collection.iterator() which can iterate the Collection) like

public static <E> void iterateWithIterator(Collection<? extends E> coll) {
    Iterator<? extends E> iter = coll.iterator();
    while (iter.hasNext()) {
        E item = iter.next();
        // do something with the item.
    }
}

or, with Java 5+, with a for-each loop like

public static <E> void forEachIterate(Collection<? extends E> coll) {
    for (E item : coll) {
        // do something with the item.
    }
}

answered Apr 12 '15 at 02:31

Elliott Frisch

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For some reason in my case I get a compile error: error: incompatible types: Class extends E> cannot be converted to E. Any thoughts? Android/Java-7 – TahoeWolverine Jun 06 '18 at 20:06
PECS. Producer Extends, Consumer Super. Where you consume the collection, it should probably be `Collection super E> coll` – Elliott Frisch Jun 06 '18 at 20:07

Jean-François Savard · Accepted Answer · 2015-04-12T02:32:31.367

3

From the documentation of Collection :

The root interface in the collection hierarchy. A collection represents a group of objects, known as its elements. Some collections allow duplicate elements and others do not. Some are ordered and others unordered.

You can iterate over it with for or for-each loop or using an Iterator.

The most common type of collections are :

edited Apr 12 '15 at 02:32

answered Apr 12 '15 at 02:29

Jean-François Savard

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Java, How Iterate through Collection?

2 Answers2