You will frequently wish to repeat the collection’s pieces. You might want to show each element, for instance. Using an iterator—an object that implements either the Iterator or the ListIterator interface—is the simplest way to accomplish this.
You can cycle over a collection using an iterator, adding or removing elements as you go. ListIterator is an extension of the Iterator that enables both bidirectional list traversal and element manipulation.
Follow these steps to use an iterator to cycle through a collection’s contents:
- Call the iterator() method of the collection to get an iterator to the beginning of the collection.
- Establish a loop that calls hasNext ( ). Keep repeating the loop until hasNext() returns true.
- Get each element by calling next within the loop().
Methods Declared by Iterator:
| Sr.No. | Method & Description |
|---|---|
| 1 | boolean hasNext( )If there are more elements, it returns true. returns false if not. |
| 2 | Object next( )Returns the next element. If there is no next element, throws a NoSuchElementException. |
| 3 | void remove( )eliminates the present element. Throws an IllegalStateException if remove() is tried to be called without calling next first ( ). |
Methods Declared by ListIterator:
| Sr.No. | Method & Description |
|---|---|
| 1 | void add(Object obj)Inserts obj before the element that will be returned by the following call to next in the list ( ). |
| 2 | boolean hasNext( )Returns true if there is a next element. Otherwise, returns false. |
| 3 | boolean hasPrevious( )If there is a prior element, the function returns true. Returns false otherwise. |
| 4 | Object next( )Returns the next element. A NoSuchElementException is thrown if there is not a next element. |
| 5 | int nextIndex( )returns the next element’s index. Return the list’s length if there is no following element. |
| 6 | Object previous( )the preceding element is returned. If there isn’t a prior element, a NoSuchElementException is thrown. |
| 7 | int previousIndex( )returns the previous element’s index. Returns -1 if there isn’t a prior element. |
| 8 | void remove( )the current element is taken out of the list. If remove() is used before next() or previous(), an IllegalStateException is raised. |
| 9 | void set(Object obj)Adds obj to the currently selected element. This is the element returned by the most recent call to either next() or previous() ( ). |
Example:
import java.util.*;
public class Coderz {
public static void main(String args[]) {
// Create an array list
ArrayList al = new ArrayList();
// add elements to the array list
al.add("A");
al.add("E");
al.add("I");
al.add("O");
al.add("U");
// Use iterator to display contents of al
System.out.print("Original contents of al: ");
Iterator itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// Modify objects being iterated
ListIterator litr = al.listIterator();
while(litr.hasNext()) {
Object element = litr.next();
litr.set(element + "+");
}
System.out.print("Modified contents of al: ");
itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// Now, display the list backwards
System.out.print("Modified list backwards: ");
while(litr.hasPrevious()) {
Object element = litr.previous();
System.out.print(element + " ");
}
System.out.println();
}
}Output:
Original contents of al: A E I O U
Modified contents of al: A+ E+ I+ O+ U+
Modified list backwards: U+ O+ I+ E+ A+Note: also read about the Java.util.Collections Class
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Staying up to the mark is what defines me. Hi all! I’m Rabecca Fatima a keen learner, great enthusiast, ready to take new challenges as stepping stones towards flying colors.
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