Iterator

It is common for a program to access the elements of an ArrayList one by one, in order. This can be done using a counting loop (as has been done so far this chapter). However, an Iterator object may also be used. To get an Iterator object for a ArrayList use this method:

Iterator iterator()  // Returns an iterator

An iterator object is used to visit the elements of the list one by one. This is sometimes more convenient than writing a loop. An iterator implements the Iterator<E> interface, andr has the following methods:

boolean hasNext()    // Return true if not all elements have been visited

E next()             // Returns the next element of the list. 

void remove()        // Remove from the list the element just returned by next()

Here is a program that prints out every element in the ArrayList:

import java.util.* ;
class IteratorExample
{
  public static void main ( String[] args)
  {
    ArrayList<String> names = new ArrayList<String>();

    names.add( "Amy" );    names.add( "Bob" ); 
    names.add( "Chris" );  names.add( "Deb" ); 
    names.add( "Elaine" ); names.add( "Frank" );
    names.add( "Gail" );   names.add( "Hal" );

    Iterator iter = names.iterator();

    while ( iter.hasNext() )
      System.out.println( iter.next() );

  }
}

There are several classes in the Java libraries that implement the Iterator<E> interface. Some classes are complicated, and a simple loop can't be used to access all their elements. For those classes, an iterator is very convenient.