Enumeration, Iterator, ListIterator in Java

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Enumeration(I)

• It was introduced in Java 1.0

• It can only be used for legacy classes like Vector, Hashtable, Properties etc

• It is forward or uni-directional.

• elements() method is called on Collection object to get the enumeration

• hasMoreElements() to check if the enumeration has more elements

• nextElement() is used to get the next element

• It can be used to read the data only. Enumeration cannot be used to manipulate the data.

Program

public class VectorEnumerationExample {

	public static void main(String[] args) {

		// Creating a Vector

		Vector<String> vector = new Vector<String>();

		// Adding elements to Vector

		vector.add("John");

		vector.add("Mary");

		vector.add("Joseph");

		vector.add("Emma");

		// creating Enumeration object

		Enumeration<String> e = vector.elements();

		// checking if more elements are there in Enumeration

		while (e.hasMoreElements())

			// Printing the next element

			System.out.println(e.nextElement());

	}

}

Output

John
Mary
Joseph
Emma

Iterator(I)

• It was introduced in Java 1.2

• This is also called as universal iterator as it can be used on any class implementing the collection interface. People will say that it can be used on maps as well but that is not correct because it is executed on entrySet() which itself is a set i.e. a class implementing the collection class.

• It is also single or forward or uni-directional like enumeration.

• The method iterator() is called on Collection to get the iterator type object.

• hasNext() method is used to check if more elements are there in the iterator.

• next() method is used to get the next element.

• Iterator can remove an element by using remove() method.

Program

public class IteratorExample {

	public static void main(String[] args) {

		// Creating a List of String

		List<String> list = new ArrayList<String>();

		// Adding elements to List

		list.add("John");

		list.add("Mary");

		list.add("Joseph");

		list.add("Emma");

		// creating Iterator object

		Iterator<String> iterator = list.iterator();

		// Before iterating

		System.out.println(list);



		// checking if more elements are there in Iterator

		while (iterator.hasNext()) {

			String name = iterator.next();

			if (name.equals("Joseph")) {

				// removing element using remove() method

				iterator.remove();

			}

		}

		// After iterating and removing Joseph

		System.out.println(list);

	}

}

Output

[John, Mary, Joseph, Emma]
[John, Mary, Emma]

ListIterator

• It was introduced in Java 1.2

• This can only be used on classes implementing List interface like ArrayList, LinkedList etc.

• It is called as bi-directional iterator because it can iterate in both front and back directions.

• listIterator() method is called on List to get the ListIterator element.

• hasNext() method is used to check if more elements are there in forward direction

• hasPrevious() method is used to check if more elements are there in backward direction.

• next() method to get the next element.

• previous() method to get the previous element.

• nextIndex() method to get the integer index of next element.

• previousIndex() method to get the integer index of previous element.

• add() method to add an element after the current iterated element.

• remove() method to remove an element from the ListIterator.

• set() to replace an element with the current iterator element.

Program 1

public class ListIteratorExample {

	public static void main(String[] args) {

		// Creating a List of String

		List<String> list = new ArrayList<String>();

		// Adding elements to List

		list.add("John");

		list.add("Mary");

		list.add("Joseph");

		list.add("Emma");

		list.add("Chad");

		list.add("Kathy");

		list.add("Joel");

		list.add("Elle");

		// creating ListIterator object

		ListIterator<String> iterator = list.listIterator();

		System.out.println("Forward Direction iteration");

		// checking if more elements are there in Iterator

		while (iterator.hasNext()) {

			System.out.println(iterator.next());
		}

		System.out.println();
		System.out.println("Backward Direction iteration");

		// checking if more elements are there in Iterator

		while (iterator.hasPrevious()) {
			System.out.println(iterator.previous());
		}
	}
}

Output

Forward Direction iteration
John
Mary
Joseph
Emma
Chad
Kathy
Joel
Elle

Backward Direction iteration
Elle
Joel
Kathy
Chad
Emma
Joseph
Mary
John

Program 2

public class ListIteratorExample {
        public static void main(String[] args) {
                // Creating a List of String
                List<String> list = new ArrayList<String>();

                // Adding elements to List
                list.add("John");
                list.add("Mary");
                list.add("Joseph");
                list.add("Emma");
                list.add("Chad");
                list.add("Kathy");
                list.add("Joel");
                list.add("Elle");
                
                // creating ListIterator object
                ListIterator<String> listIterator = list.listIterator();

                System.out.println("Before: " + list);

                // checking if more elements are there in Iterator
                while (listIterator.hasNext()) {

                String name = listIterator.next();
                if (name.equals("Mary")) {
                        // removing Mary
                        listIterator.remove();
                }
                if (name.equals("Chad")) {
                        // adding Jt after Chad
                        listIterator.add("JT");
                }
                if (name.equals("Joel")) {
                        // replacing Joel with Tommy
                        listIterator.set("Tommy");
                        }
                }
                System.out.println();
                System.out.println("After: " + list);
         }
}

Output

Before: [John, Mary, Joseph, Emma, Chad, Kathy, Joel, Elle]
After: [John, Joseph, Emma, Chad, JT, Kathy, Tommy, Elle]

Enumeration v Iterator v ListIterator

	Enumeration(I)	Iterator(I)	ListIterator(I)
Introduced in	Java 1.0	Java 1.2	Java 1.2
Applicable on	Legacy Classes	Classes implementing Collection interface	Classes implementing List Interface
Direction	Forward/Uni-directional	Forward/Uni-directional	Bi-directional
Operation(s)	Read	Read and Remove	Read, Remove, Replace, Add
Method to get iterable object	elements();	iterator()	list.listIterator()
Methods	boolean hasMoreElements(); Object nextElement();	boolean hasNext(); Object next(); void remove();	boolean hasNext(); boolean hasPrevious(); Object next(); Object previous(); int nextIndex(); int previousIndex(); void remove(); void add(Object o); void set(Object o);

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