Iterator Interface in Java Collection Framework

public interface Iterator

An iterator over a collection. Iterator takes the place of Enumeration in the Java collections framework. Iterators differ from enumerations in two ways:

• Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.
• Method names have been improved. 

Method Details

1) public boolean hasNext()

Returns true if the iteration has more elements. (In other words, returns true if next would return an element rather than throwing an exception.)

Returns:

true if the iterator has more elements.

2) public Object next()

Returns the next element in the iteration.

Returns:

the next element in the iteration.

Throws:

NoSuchElementException - iteration has no more elements.

3) public void remove()

Removes from the underlying collection the last element returned by the iterator (optional operation). This method can be called only once per call to next. The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.
      Throws:

UnsupportedOperationException - if the remove operation is not supported by this Iterator.

IllegalStateException - if the next method has not yet been called, or the remove method has already been called after the last call to the next method.

Let us discuss one example,how to use hasNext() and next()

import java.util.ArrayList;
import java.util.Iterator;

public class Main {

    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add("one");
        list.add("two");
        list.add("three");
        Iterator i = list.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }

}

The output is :

one
two
three

Let us see one more example,how to use remove() method

import java.util.ArrayList;
import java.util.Iterator;

public class Main {

    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add("one");
        list.add("two");
        list.add("three");
        Iterator i = list.iterator();
        while (i.hasNext()) {
            String value=(String) i.next();
            if(value.equals("one"))
            i.remove();
        }
        System.out.println(list);
    }

}

The output is

[two, three]

The element "one" is removed from the list.

java.lang.IllegalStateException

if the next method has not yet been called, or the remove method has already been called after the last call to the next method ,remove method throws an IllegalStateException.

import java.util.ArrayList;
import java.util.Iterator;

public class Main {

    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add("one");
        list.add("two");
        list.add("three");
        Iterator i = list.iterator();
        while (i.hasNext()) {
            i.remove();
            String value=(String) i.next();
            if(value.equals("one"))
            i.remove();
            i.remove();
        }
        System.out.println(list);
    }

}

The above marked remove() methods throw an exception,

Exception in thread "main" java.lang.IllegalStateException
at java.util.ArrayList$Itr.remove(ArrayList.java:767)
at Main.main(Main.java:13)

java.util.ConcurrentModificationException

if list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.

Please visit Difference between fail-fast Iterator vs fail-safe Iterator in Java to know more about ConcurrentModificationException.