Iterators

Collection framework provides iterator interfaces like Enumeration, Iterator and ListIterator to iterate over the elements within a collection.

Enumeration is a legacy interface. While ListIterator is a child of Iterator specifically for lists.

Use cases and Trade Offs

Enumeration can only be used for legacy collections like Vector and Hashtable. Also, it supports only read operations.
Iterator is the most commonly used since it can be used for any Collection. It can be used to remove but not for replacing or adding new object.
Both Enumeration and Iterator are only forward direction cursor
ListIterator is the most powerful cursor which is bidirectional and has methods to remove, replace and add new objects. But it can only be used for List collections.

Initialization

 Enumeration e = v.elements(); //v is a vector
 Iterator itr = c.iterator(); //c is any collection object
 ListIterator litr = l.listIterator(); //l is any List object

But all the iterators mentioned above are interfaces.

How can we create objects for interfaces then? ⇒ We don't.

These methods being used to initialize the iterators have a class within them which implements the corresponding interface. The object of this class is then returned.

System.out.println(i.getClass().getName()); //i is an object of a iterator

Output:

vector$1              // if c is an Enumeration Object
vector$Itr            // if c is an Iterator Object
vector$ListItr        // if c is a List Iterator Object

vector$ means inner class of vector ( or whichever Collection class we're getting the object for)

1 means it is an anonymous inner class

Itr and ListItr are names of Inner Classes implementing Iterator and ListIterator respectively.

Methods

Enumeration

public boolean hasMoreElements()
public Object nextElement()

Iterator

public boolean hasNext()
public Object next()
public void remove()

List Iterator

Forward

public boolean hasNext()
public Object next()
public int nextIndex()

Extra capabilities

public void remove()
public void set(Object new) → replace
public void add(Object new)

Backward

public boolean hasPrevious()
public Object previous()
public int previousIndex()

The set() method of ListIterator interface is used to replace the last element which is returned by the next() or previous() along with the given element. The call can be added only if neither remove() nor add(E) method have been called.

Backlinks

Collection Framework (cs)