Develop Reference

What's the difference between returning an Iterator and an unmodifiable list?

From what I understand, both are used to return unmodifiable objects, but why would one be used over the other?
Edit: My mistake, Iterators are not unmodifiable. But I'm still unclear on why an Iterator might be used instead of a List.

Return a List when you need a list; return an Iterator when you need an iterator. They are completely different data structures that serve different purposes.
As an aside, an Iterator might be able to modify the underlying collection through the remove() method (which removes the last item returned by next()).
Edit (in response to OP's edit): You would want to return an Iterator if you specifically wanted to prevent client code from seeing the underlying list, or you want to force sequential access to the elements. It's also required that you write an iterator() method that returns an Iterator when your class implements Iterable. There's also the interesting case of returning an Iterator that iterates through an undetermined collection of items, that might not even exist in memory until they are requested. (Think, for instance, of state space searching in an AI-based application.)

When would it be useful to implement Iterator but not Iterable?

When would it be useful to implement iterator without implementing iterable?
Or is implementing iterator simply a by product of implementing iterable?

These two are related but not the same.
A List is Iterable - you can get its Iterator. It is not an Iterator.
An Iterator is a single use class that can iterate along a Collection of objects using the hasNext and next methods.
An Iterable is a Collection class that returns an Iterator instance when then the iterator() method is called.
I would go as far as to say that I see no case where an Iterable should implements Iterator. And as a Iterator is single use I cannot see a case where Iterator should implements Iterable.

Generally speaking, you'd implement either an Iterable or an Iterator to hide the implementation of a collection from the code that's iterating over the collection. Either one would work well. The difference lies in how many times the collection can be traversed. An iterator can only traverse the collection once while the Iterable can traverse the collection many times. You can traverse the Iterable by asking it for an Iterator and you can do that multiple times.

They are very similar in semantics and use cases, but differ in implementation. Iterable is a simple factory for Iterators which can be used inside for loops. For this reason it's might be more convenient to implement an Iterable.
for(String s : getIterable()){
...
}
Versus:
Iterator<String> it = getIterator();
while(it.hasNext()){
String s = it.next();
...
}
However, in some case Iterator might not be re-instantiated, i.e. when you're running through a db request results, so in this case you can't make it Iterable without re-sending your request.

Why do collections in java not have a read only iterator concept?

The Enumeration interface of java has 2 methods
hasNext()
next()
Why doesn't Iterator extend Enumeration and add the remove method()?
Also, if all I want to do is loop over my collection, I can make do with an Enumeration (or an iterator if it extends enumeration) as loop statements only need an enumeration. I don't have to worry about the collection getting modified

Why doesn't Iterator extend Enumeration and add the remove() method?
From the documentation:
Iterator takes the place of Enumeration in the Java Collections Framework. Iterators differ from enumerations in two ways:
Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.
Method names have been improved.
I suspect the latter is the real reason for introducing a brand new interface instead of extending the existing one.
Also, if all I want to do is loop over my collection, I can make do with an Enumeration (or an iterator if it extends enumeration) as loop statements only need an enumeration. I don't have to worry about the collection getting modified
Well, if you don't want to modify the collection, then don't call remove(). If you really, really don't trust yourself, you could use Collections.unmodifiableCollection() et al to create a read-only wrapper.
Finally, it is worth noting that if you use a for-each loop to iterate over the collection, you don't have access to the remove() method anyway:
for (String s : str_list) {
...
}
Here, we are using the Iterator interface, but don't have access to the actual iterator object.

You could just obtain a java.util.Enumerator from your collection, via the java.util.Collections.enumeration() Method.
Anyway, if you don't want to modify your collection via its iterator, it's just a matter of not calling the remove() method.

According to http://www.journaldev.com/1330/java-collections-interview-questions-and-answers#iterator-vs-enumeration, "Enumeration is twice as fast as Iterator and uses very less memory. Enumeration is very basic and fits to basic needs."
So, I guess if you don't need remove(), then using Enumeration is more effective than using Iterator.

Need of Iterator class in Java?

The question might be pretty vague I know. But the reason I ask this is because the class must have been made with some thought in mind.
This question came into my mind while browsing through a few questions here on SO.
Consider the following code:
class A
{
private int myVar;
A(int varAsArg)
{
myVar = varAsArg;
}
public static void main(String args[])
{
List<A> myList = new LinkedList<A>();
myList.add(new A(1));
myList.add(new A(2));
myList.add(new A(3));
//I can iterate manually like this:
for(A obj : myList)
System.out.println(obj.myVar);
//Or I can use an Iterator as well:
for(Iterator<A> i = myList.iterator(); i.hasNext();)
{
A obj = i.next();
System.out.println(obj.myVar);
}
}
}
So as you can see from the above code, I have a substitute for iterating using a for loop, whereas, I could do the same using the Iterator class' hasNext() and next() method. Similarly there can be an example for the remove() method. And the experienced users had commented on the other answers to use the Iterator class instead of using the for loop to iterate through the List. Why?
What confuses me even more is that the Iterator class has only three methods. And the functionality of those can be achieved with writing a little different code as well.
Some people might argue that the functionality of many classes can be achieved by writing one's own code instead of using the class made for the purpose. Yes,true. But as I said, Iterator class has only three methods. So why go through the hassle of creating an extra class when the same job can be done with a simple block of code which is not way too complicated to understand either.
EDIT:
While I'm at it, since many of the answers say that I can't achieve the remove functionality without using Iterator,I would just like to know if the following is wrong, or will it have some undesirable result.
for(A obj : myList)
{
if(obj.myVar == 1)
myList.remove(obj);
}
Doesn't the above code snippet do the same thing as remove() ?

Iterator came long before the for statement that you show in the evolution of Java. So that's why it's there. Also if you want to remove something, using Iterator.remove() is the only way you can do it (you can't use the for statement for that).

First of all, the for-each construct actually uses the Iterator interface under the covers. It does not, however, expose the underlying Iterator instance to user code, so you can't call methods on it.
This means that there are some things that require explicit use of the Iterator interface, and cannot be achieved by using a for-each loop.
Removing the current element is one such use case.
For other ideas, see the ListIterator interface. It is a bidirectional iterator that supports inserting elements and changing the element under the cursor. None of this can be done with a for-each loop.
for(A obj : myList)
{
if(obj.myVar == 1)
myList.remove(obj);
}
Doesn't the above code snippet do the same thing as remove() ?
No, it does not. All standard containers that I know of will throw ConcurrentModificationException when you try to do this. Even if it were allowed to work, it is ambiguous (what if obj appears in the list twice?) and inefficient (for linked lists, it would require linear instead of constant time).

The foreach construct (for (X x: list)) actually uses Iterator as its implementation internally. You can feed it any Iterable as a source of elements.
And, as others already remarked: Iterator is longer in Java than foreach, and it provides remove().
Also: how else would you implement your own provider class (myList in your example)? You make it Iterable and implement a method that creates an Iterator.

For one thing, Iterator was created way before the foreach loop (shown in your code sample above) was introduced into Java. (The former came in Java2, the latter only in Java5).
Since Java5, indeed the foreach loop is the preferred idiom for the most common scenario (when you are iterating through a single Iterable at a time, in the default order, and do not need to remove or index elements). Note though that the foreach uses an iterator in the background for standard collection classes; in other words it is just syntactic sugar.

Iterator, listIterator both are used to allow different permission to user, like list iterator have 9 methods but iterator have only 3 methods, but have remove functionality which you can't achieve with for loop. Enumeration is another thing which is also used to give only read permissions.

Iterator is an implementation of the classical GoF design pattern. In that way you can achieve clear behaviour separation from the 'technical code' which iterates (the Iterator) and your business code.
Imagine you have to change the 'next' behaviour (say, by getting not the next element but the next EVEN element). If you rely only on for loops you will have to change manually every single for loop, in a way like this
for (int i; i < list.size(); i = i+2)
while if you use an Iterator you can simply override/rewrite the "next()" and "hasNext()" methods and the change will be visible everywhere in your application.

I think answer to your question is abstraction. Iterator is written because to abstract iterating over different set of collections.
Every collection has different methods to iterate over their elements. ArrayList has indexed access. Queues has poll and peek methods. Stack has pop and peek.
Usually you only need to iterate over elements so Iterator comes into play. You do not care about which type of Collection you need to iterate. You only call iterator() method and user Iterator object itself to do this.
If you ask why not put same methods on Collection interface and get rid of extra object creation. You need to know your current position in collection so you can not implement next method in Collection because you can not use it on different locations because every time you call next() method it will increment index (simplifying every collection has different implementation) so you will skip some objects if you use same collection at different places. Also if collection support concurrency than you can not write a multi-thread safe next() method in collection.
It is usually not safe to remove an object from collection iterating by other means than iterator. Iterator.remove() method is safest way to do it. For ArrayList example:
for(int i=0;i

How to get the Collection of an iterator in Java?

Often there is the need to setup an ArrayList<>. One of the constructors takes a collection, but there is no constructor that takes an iterator.
What if I have an iterator? Is there a way to "reach up" to the collection that offers the iterator in order to use the ArrayList<> constructor?
Specifically I have the iterator offered by PropertiesConfiguration.getKeys() which is part of org.apache.commons.

There's no such thing, an Iterator's Collection. An Iterator can be created independently of a Collection. It can be obtained from any Iterable, or you can even create a class implementing an iterator.
However, you can obtain an ArrayList from an Iterator by iterating it and adding its elements one by one:
Iterator<X> it = ...;
List<X> list = new ArrayList<X>();
while (it.hasNext()) {
list.add(it.next());
}
Note, however, that this cannot be done reliably for every possible iterator, since there's the possibility that an iterator will iterate forever, thus causing an infinite loop and most probably an OutOfMemoryError.
I'd suggest you take a look at Google Guava, an utility library from Google. It has a class called Lists, which allows you to do the following:
Iterator<X> it = ...;
List<X> list = Lists.newArrayList(it);
The library has tons of methods extremely useful for everyday Java coding. It contains mostly everything you want but cannot find in the standard Java 6 API.

There is no truly general way to do this, because in Java, Iterator is just an interface with three methods: next, hasNext and remove.
When you obtain an iterator, you use a factory method that gives you an instance of some class implementing the interface. If you know the specific class, you can look at its documentation or source to find if there is a way to find the collection it is iterating over.
But there are many such classes. In the case of ArrayList, look in the source code to see what kind of iterator you are getting.
EDIT:
Here is the source code for ArrayList in Open JDK 7: http://www.docjar.com/html/api/java/util/ArrayList.java.html
The iterator over an ArrayList is an instance of the private inner class Itr. This should not be too surprising. The iterator comes from a factory method, after all, so you're really not supposed to know what kind of iterator you are getting. In fact, if you were able to get the class of this instance and access its methods (say, via reflection) you would be doing a bad thing. This iterator is part of the (internal) implementation of the ArrayList class and (as #merryprankster points out) can change in the future.

I don't know if its possible, but in my opinion, a such function should not exist because an iterator may not come from a collection. For example, one could create an iterator which chains over multiple collections !