Tutorials

Cursor

If we want to get object one by one from the collection then we should go for cursor. There are three types of cursor

Enumeration

Iterator

ListIterator

Enumeration(I)

We can use Enumeration to get object one by one from legacy collection object. We can create Enumeration object by using elements() of Vector class.

Program

import java.util.*;
public class EnumerationCursorDemo {
    public static void main(String[] args) {
        Vector v=new Vector();
        for(int i=0;i<=10;i++)
        {
            v.addElement(i);
        }
        System.out.println(v);
        Enumeration e=v.elements();
        while(e.hasMoreElements()) 
        {
            Integer in =(Integer)e.nextElement();
            if(in%2==0)
                System.out.println(in);
        }
        System.out.println(v);    
    }
} 

O/P

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
0
2
4
6
8
10
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Limitations of Enumeration

We can apply enumeration concept only for legacy classes and it is not an universal cursor.

We can get only read access and we can't perform remove operation.

To overcome above limitations, we should go for Iterator.

Iterator (I) - Universal Cursor

Iterator is a collection framework interface defined in java 1.2.It is a cursor object used to retrieve element from all collection type objects List, Set & Queue including Vector. It is an alternative of Enumeration object & replacement of "for" loop on collection objects.

By using Iterator we can perform both read/remove operation.

In collection interface, we have below method to obtain iterator object.

public Iterator iterator()

Program

import java.util.*;
public class IteratorCursorDemo {
    public static void main(String[] args) {
        ArrayList al=new ArrayList();
        for (int i = 1; i <= 10; i++) 
        {
            al.add(i);
        }
        System.out.println(al);
        Iterator itr=al.iterator();
        while (itr.hasNext()) 
        {
            Integer in= (Integer)itr.next();
            if(in%2==0)
                System.out.println(in);
            else
                itr.remove();
        }
        System.out.println(al);
    }
}

O/P

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
2
4
6
8
10
[2, 4, 6, 8, 10]

Limitations of Iterator

By using enumeration and iteration, we can always move towards forward direction & we can't move towards backward direction. These are single direction cursor.

By using iterator, we can perform only read & remove operation & we can't perform replacement and addition of new object.

To overcome above limitations, we should go for ListIterator.

ListIterator

It is a bidirectional cursor object used to retrieve elements only from list implemented collection objects- ArrayList, Vector, Stack, LinkedList. It is a sub interface of Iterator.

By using ListIterator, we can perform replacement & the addition of new objects in addition to read/remove operation.

Program

import java.util.*;
public class ListIteratorCursorDemo {
    public static void main(String[] args) {
        LinkedList ll=new LinkedList();
        ll.add("A");
        ll.add("B");
        ll.add("C");
        ll.add("D");
        System.out.println(ll);
        ListIterator ltr=ll.listIterator();
        while (ltr.hasNext()) 
        {
            String s=(String)ltr.next();
            if(s.equals("U"))
            ltr.remove();
            else if(s.equals("N"))
                 ltr.add("W");
            else if(s.equals("B"))
                ltr.set("M");
        }
        System.out.println(ll);
    }
}

O/P

[A, B, C, D]
[A, M, C, D]