How to use Java Iterator to traverse and remove object from Collection

Java Iterator is used to iterate over Collections in Java. By Iteration, I mean, going over each element stored in collection and optionally performing some operation e.g. printing value of element, updating object or removing object from Collection. Iterator was not part of first Java release, and a similar class Enumeration was there to provide Iteration functionality. Iterator in Java was introduced form JDK 1.4 and it provides alternative to Enumeration, which is obsolete now days. Iterator is different to Enumeration in two main ways, first, Iterator allows programmer to remove elements from Collection during iteration. Second, names are shortened and improved in Iterator, by the way, you can see difference between Iterator and Enumeration for more differences. It's one of the frequently asked Java Interview question. Iterator is an interface and enhanced to support Generic from Java 1.5 release. Almost all popular collection implements Iterator, including ArrayList, LinkedList and HashSet. hasNext() method of Iterator is used as condition while Iterating, and next() method actually returns object, next in sequence maintained by Collection itself. In this Java programming tutorial, we will learn How to use Iterator in Java by coding Iterator example and iterating over ArrayList.

How to use Iterator in Java - Example

Using Iterator is probably simplest thing you will learn in Java programming. Every Collection, which implements Iterator interface, provides iterator() method which returns Iterator instance. This method comes from java.util.Iterator interface and return a type-safe Iterator. Now, In order to start iterating or navigating, we can use while loop and hasNext() method to check whether, there is more elements in Iterator or not. In each run of while loop, we get access to one element from Java Collection. In this example, we first print value of element and subsequently remove it from Collection. So, at the end of our iteration, Java collection should be empty. By the way, It's also worth knowing that there are two kinds of Iterator in Java, fail-safe and fail-fast. fail-safe Iterator doesn't throw ConcurrentModificationException during iteration while fail-fast does, if, Iterator realizes any structural change in Collection once Iteration begins. See difference between fail-fast and fail-safe Iterator, to know more about it.

Java Iterator Example

Without, delaying any more, here is our Java program to show, How to use Iterator in Java.

/**
 * Java program to show, How to use Iterator in Java.
 * Iterator provides a convenient way to access every element from Java Collections.
 * Iterator is also handy in removing elements from Collection.
 * @author java67
 */
public class IteratorHowTo {

    public static void main(String args[]) {
     
 
        //This is nice way to create List, but it's fixed lenght, don't supprot remove()
        List<String> androidVersions = Arrays.asList("Petit Four", "Cupcake", "Donut",
                                              "Eclair", "Froyo","Gingerbread",
                                              "Honeycomb", "Ice Cream Sandwich", "Jelly Bean");
       
        //this is bettern than adding individual elements - No?
        List<String> android = new ArrayList<String>(androidVersions);
       
       
        //Getting Iterator from List
        Iterator<String> iterator = android.iterator();
       
        System.out.println("Size of List before Iteratation " + android.size());
       
        //Using Iterator to iterate over List, acces object one by one
        while(iterator.hasNext()){
            String version = iterator.next();
            System.out.println(version);
            /* android.remove(version); */ //removing object from List - ConcurrentModificationException
            
            iterator.remove();  // you should be using Iterator's remove method
        }
       
        System.out.println("Size of list after removing object during Iteration : " + android.size());
    }
}

Output:
Size of List before Iteratation 9
Petit Four
Cupcake
Donut
Eclair
Froyo
Gingerbread
Honeycomb
Ice Cream Sandwich
Jelly Bean
Size of list after removing object during Iteration : 0

There are couple of interesting thing to note in this code. First thing is nice way of creating List in one line, This is best way to create List, if you know values in advance. Unfortunately, that List is fixed length List and doesn't support remove operation, calling remove() will result in "Exception in thread "main" java.lang.UnsupportedOperationException". By the way, it's important to remember that this is not a read only Collection, you can still modify existing elements by using set(index) method. Second interesting thing is using copy constructor of Collection, which is a nice way to copy elements from one Collection to another. Now last and probably most important thing to note is using iterator.remove() method to remove objects from List. Many programmer make mistake by calling List.remove() method instead of Iterator.remove() here, earlier will throw ConcurrentModificationException. You should only use remove() method from java.util.Iterator to remove elements from collection, while iterating.

That's all about Iterator in Java. Though we have seen a relative simple example of Java Iterator, it does demonstrate most common usage of Iterator. Just beware of ConcurrentModificationExcepton, while iterating over Java Collections in multi-threading environment. Though, most of Concurrent Collections such as ConcurrentHashMap and CopyOnWriteArrayList supports fail-safe Iterator, more popular ones like ArrayList or HashSet iterators are still fail-fast.

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