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Java Iterator: Introduction
What is an Iterator?
In Java, an Iterator is an interface that provides methods to iterate over any Collection. It allows you to traverse through a collection and remove elements from the collection.
Why use an Iterator?
Iterators are used to process collections of objects in a standardized way. They provide a way to access elements without exposing the underlying representation.
import java.util.*;
class IteratorExample {
public static void main(String args[]) {
List list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
list.add("Cherry");
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Console Output:
Apple Banana Cherry
Removing Elements Using Iterator
Why Remove Elements with Iterator?
Removing elements from a collection while iterating can lead to ConcurrentModificationException. Using Iterator's remove method prevents this issue.
import java.util.*;
class IteratorRemoveExample {
public static void main(String args[]) {
List list = new ArrayList<>();
list.add("Dog");
list.add("Cat");
list.add("Rabbit");
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
String animal = iterator.next();
if ("Cat".equals(animal)) {
iterator.remove();
}
}
System.out.println(list);
}
}
Console Output:
[Dog, Rabbit]
Using Iterator with Set
Iterating Over a Set
Sets do not maintain an order of elements, but you can still use an Iterator to traverse them.
import java.util.*;
class SetIteratorExample {
public static void main(String args[]) {
Set set = new HashSet<>();
set.add("Red");
set.add("Green");
set.add("Blue");
Iterator iterator = set.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Console Output:
Red Green Blue
Iterator vs For-Each Loop
Comparison with For-Each Loop
While the for-each loop is simpler, it doesn't allow removal of elements during iteration. Iterator provides more control.
import java.util.*;
class IteratorVsForEach {
public static void main(String args[]) {
List list = new ArrayList<>();
list.add("X");
list.add("Y");
list.add("Z");
for (String s : list) {
System.out.println(s);
}
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Console Output:
X Y Z X Y Z
Using Iterator with Map
Iterating Over a Map
Maps require a different approach as they store key-value pairs. Use entrySet, keySet, or values for iteration.
import java.util.*;
class MapIteratorExample {
public static void main(String args[]) {
Map map = new HashMap<>();
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");
Iterator> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry entry = iterator.next();
System.out.println(entry.getKey() + " = " + entry.getValue());
}
}
}
Console Output:
1 = One 2 = Two 3 = Three
Iterator with LinkedList
Benefits with LinkedList
LinkedList allows bidirectional iteration using ListIterator, providing more flexibility.
import java.util.*;
class LinkedListIteratorExample {
public static void main(String args[]) {
LinkedList list = new LinkedList<>();
list.add("Monday");
list.add("Tuesday");
list.add("Wednesday");
ListIterator iterator = list.listIterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Console Output:
Monday Tuesday Wednesday
Custom Iterator Implementation
Creating a Custom Iterator
Implementing your own Iterator can be useful when you need custom traversal logic for your data structures.
import java.util.*;
class CustomIterator implements Iterator {
private int[] numbers;
private int index;
public CustomIterator(int[] numbers) {
this.numbers = numbers;
this.index = 0;
}
@Override
public boolean hasNext() {
return index < numbers.length;
}
@Override
public Integer next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return numbers[index++];
}
}
class CustomIteratorExample {
public static void main(String args[]) {
int[] numbers = {10, 20, 30};
CustomIterator iterator = new CustomIterator(numbers);
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Console Output:
10 20 30
Using Iterator for Concurrent Collections
Concurrent Collection Iteration
Iterators for concurrent collections like ConcurrentHashMap are fail-safe, meaning they don't throw ConcurrentModificationException.
import java.util.concurrent.*;
class ConcurrentIteratorExample {
public static void main(String args[]) {
ConcurrentHashMap map = new ConcurrentHashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);
Iterator iterator = map.keySet().iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Console Output:
A B C
Iterator Limitations
Understanding Iterator Limitations
Iterators can only move forward, and they do not support adding elements during iteration. For more complex operations, consider using ListIterator.
import java.util.*;
class IteratorLimitationsExample {
public static void main(String args[]) {
List list = new ArrayList<>();
list.add("Alpha");
list.add("Beta");
ListIterator listIterator = list.listIterator();
while (listIterator.hasNext()) {
System.out.println(listIterator.next());
listIterator.add("Gamma");
}
System.out.println(list);
}
}
Console Output:
Alpha Beta [Alpha, Gamma, Beta, Gamma]
