Whenever we talk about traversing the Collection first more thing comes in mind apart from for loop is to use an iterator. But the question comes what is an iterator and how it’s works. We will be discussing about Iterator in-depth in this post. Then what we are waiting for let’s get rolling.
Table of Contents
What is Iterator in Java?
Iterator helps to iterate or traverse over Java Collection. This is a cursor-based system used to traverse the element in Collection like List, Stack, Queue and etc. When we say Its cursor-based system then it means that Iterator uses the cursor to visit each of its elements and performs an operation on the visited element.
Iterator interface provide several methods that need to get implemented by the implementing class.
Iterator Methods in Java
Iterator provides several methods that will help ways to traverse the collection.
default void forEachRemaining(Consumer<? super E> action) | This method performs given action to all remaining elements of the collection until all elements are processed or Throws Exception |
boolean hasNext() | this method will return true if the iterator has more elements to traverse. |
E next() | next() the method moves the cursor to the next element and returns that element. |
default void remove() | remove() the method will remove an element at that curser from the collection on which iterator is called. |
Basic Iterator Example
private static void iterateList() {
List<String> list = new ArrayList<String>();
list.add("India");
list.add("US");
list.add("China");
list.add("America");
System.out.println("Intial Content of the list is: "+ list);
Iterator<String> iterator = list.iterator();
int count = 0;
while(iterator.hasNext()) {
if(count == 1) {
//Move the pointer from india to china
iterator.next();
//Remove China from the list.
iterator.remove();
}
System.out.println(iterator.next());
count++;
}
System.out.println("Content of the list is: "+ list);
}
Output of above code
Intial Content of the list is: [India, US, China, America] India China America Content of the list is: [India, China, America]
In the above code we saw that we created a list with
[India, US, China, America]
and with help of iterator that we retrieved from calling iterator() on created list. When we created our iterator, then we have something like this
Initially cursor is at 0 in above image.
When we do iterator.hasNext() then it check’s, does we have element at next position i.e at position 1
And if it found the element is there at next index then we perform iterator.next() which will do two task first it will move the cursor to next point i.e 1
and also returns the element at index 1 i.e India. That’s why to iterate over entire list we are using while loop to check availability of the next element as while(iterator.hasNext()) and if its there then move the pointer to next element using iterator.next(). Like this way we can move to next element until iterator.hasNext() returns false.
Advantages of using Iterator
- We can remove the elements while iterating over collection.
Limitations of Iterator
- The iterator does not add elements to collection.
- It does not traverse in backward direction.
- It does not do parallel processing as Streams or Spliterator
Difference Between Iterator and Enumeration
| Iterator | Enumeration |
|---|---|
| It was introduced in java 1.2. That’s why it not legacy | It was introduced in java 1.0. That’s why it is legacy |
| It support only reading the element from the collection | It support only reading and removal of the element from the collection |
In collection framework it is used to iterate over only Vector | In collection framework, it is used to iterate over all collection. |
IllegalStateException in Iterator
Whenever we call remove on empty list this exception will be throw as
private static void removeFromEmptyList() {
List<String> list = new ArrayList<String>();
Iterator<String> iterator = list.iterator();
iterator.remove();
}
Exception in thread "main" java.lang.IllegalStateException
at java.util.ArrayList$Itr.remove(ArrayList.java:872)
at com.techijournal.collection.TechiJournalIterator.removeFromEmptyList(TechiJournalIterator.java:18)
at com.techijournal.collection.TechiJournalIterator.main(TechiJournalIterator.java:12)NoSuchElementException Exception in Iterator
When iterator.next() on empty list or when their is not element at next index then this exception is thrown.
private static void nexttoEmptyList() {
List<String> list = new ArrayList<String>();
Iterator<String> iterator = list.iterator();
iterator.next();
}
Exception in thread "main" java.util.NoSuchElementException
at java.util.ArrayList$Itr.next(ArrayList.java:862)
at com.techijournal.collection.TechiJournalIterator.nexttoEmptyList(TechiJournalIterator.java:19)
at com.techijournal.collection.TechiJournalIterator.main(TechiJournalIterator.java:13)Thank’s Note
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