Iterators | Java Subject Material

Step 1 of 18

6.1 Introduction to Iteration

Learning objective

Define iteration and identify Java traversal mechanisms.

If a list contains Java, Python, and Database, iteration processes Java, then Python, then Database.

Traversal mechanisms

Iterator
ListIterator
Enumeration
Enhanced for loop
Traditional indexed for loop
forEach()

Check your understanding: What does iteration do?

It visits and processes collection elements one by one.

Step 2 of 18

6.2 Iterator Interface

Learning objective

Obtain an Iterator and use its four principal methods.

Obtaining an iterator

Iterator<String> iterator = names.iterator();

Iterator methods
Method	Description
hasNext()	Checks whether another element is available
next()	Returns the next element
remove()	Removes the last element returned by next()
forEachRemaining()	Processes all remaining elements

Check your understanding: Which method checks whether another element is available?

hasNext().

Step 3 of 18

6.3 Traversing with Iterator

Learning objective

Trace the iterator traversal cycle in a complete program.

IteratorDemo.java

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

public class IteratorDemo {
    public static void main(String[] args) {
        ArrayList<String> subjects = new ArrayList<>();
        subjects.add("Java");
        subjects.add("Python");
        subjects.add("Database");

        Iterator<String> iterator = subjects.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

Output

Java
Python
Database

Working process

iterator() creates the iterator
hasNext() checks availability
next() returns the element
The cycle repeats until traversal ends

Check your understanding: What is printed first by the example?

Java.

Step 4 of 18

6.4 Removing Elements Using Iterator

Learning objective

Remove matching elements safely during traversal.

IteratorRemoveDemo.java

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

public class IteratorRemoveDemo {
    public static void main(String[] args) {
        ArrayList<Integer> numbers =
                new ArrayList<>(Arrays.asList(10, 15, 20, 25));
        Iterator<Integer> iterator = numbers.iterator();
        while (iterator.hasNext()) {
            int number = iterator.next();
            if (number % 2 != 0) iterator.remove();
        }
        System.out.println(numbers);
    }
}

Output

[10, 20]

Check your understanding: Which values remain after removing odd numbers from 10, 15, 20, 25?

10 and 20.

Step 5 of 18

6.5 Concurrent Modification Problem

Learning objective

Distinguish direct structural modification from iterator-controlled removal.

Unsafe direct removal

while (iterator.hasNext()) {
    String name = iterator.next();
    if (name.equals("Amit")) names.remove(name); // unsafe
}

Safe iterator removal

while (iterator.hasNext()) {
    String name = iterator.next();
    if (name.equals("Amit")) iterator.remove();
}

Check your understanding: Does concurrent modification necessarily mean multiple threads?

No. Direct structural modification during iteration can cause it in a single thread.

Step 6 of 18

6.6 Limitations of Iterator

Learning objective

Identify when Iterator is insufficient.

Iterator limitations

Forward traversal only
No index information
Cannot add elements
Cannot directly replace elements
Can remove through remove()

Check your understanding: Which iterator should be selected for backward list traversal?

ListIterator.

Step 7 of 18

6.7 ListIterator Interface

Learning objective

Explain the list-specific capabilities of ListIterator.

Creating a ListIterator

ListIterator<String> iterator = names.listIterator();

Capabilities

Forward and backward traversal
Add and remove elements
Replace elements
Read next and previous indexes

Supported examples

ArrayList
LinkedList
Vector
Stack

Check your understanding: Can ListIterator be obtained directly from a Set?

No. It is designed for List implementations.

Step 8 of 18

6.8 Important ListIterator Methods

Learning objective

Match bidirectional and modification operations with ListIterator methods.

ListIterator methods
Method	Description
hasNext()	Checks for a next element
next()	Returns the next element
hasPrevious()	Checks for a previous element
previous()	Returns the previous element
nextIndex()	Index of the next element
previousIndex()	Index of the previous element
add(element)	Inserts an element
remove()	Removes the current element
set(element)	Replaces the current element

Check your understanding: Which method returns the preceding element?

previous().

Step 9 of 18

6.9 Forward and Backward Traversal

Learning objective

Move a ListIterator to the end and traverse back to the beginning.

ListIteratorDemo.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.ListIterator;

public class ListIteratorDemo {
    public static void main(String[] args) {
        ArrayList<String> subjects =
                new ArrayList<>(Arrays.asList("Java", "Python", "Database"));
        ListIterator<String> iterator = subjects.listIterator();

        System.out.println("Forward traversal:");
        while (iterator.hasNext()) System.out.println(iterator.next());
        System.out.println("Backward traversal:");
        while (iterator.hasPrevious()) System.out.println(iterator.previous());
    }
}

Output

Forward traversal:
Java
Python
Database
Backward traversal:
Database
Python
Java

Start directly at the end

ListIterator<String> iterator = subjects.listIterator(subjects.size());

Check your understanding: What is the first value printed during backward traversal?

Database.

Step 10 of 18

6.10 Modifying a List Using ListIterator

Learning objective

Insert and replace values during traversal.

ListIteratorModifyDemo.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.ListIterator;

public class ListIteratorModifyDemo {
    public static void main(String[] args) {
        ArrayList<String> subjects =
                new ArrayList<>(Arrays.asList("Java", "Python", "Database"));
        ListIterator<String> iterator = subjects.listIterator();
        while (iterator.hasNext()) {
            String subject = iterator.next();
            if (subject.equals("Python")) iterator.set("Advanced Python");
            if (subject.equals("Java")) iterator.add("Advanced Java");
        }
        System.out.println(subjects);
    }
}

Output

[Java, Advanced Java, Advanced Python, Database]

Check your understanding: Which method replaces Python with Advanced Python?

set().

Step 11 of 18

6.11 Difference Between Iterator and ListIterator

Learning objective

Compare scope, direction, modification, indexes, and creation methods.

Iterator and ListIterator
Iterator	ListIterator
Works with most collections	Works only with lists
Traverses forward	Traverses forward and backward
Can remove	Can add, remove, and replace
No index information	Provides next and previous indexes
Created by iterator()	Created by listIterator()

Check your understanding: Which one provides nextIndex() and previousIndex()?

ListIterator.

Step 12 of 18

6.12 Enumeration Interface

Learning objective

Traverse legacy collections with Enumeration.

Enumeration methods
Method	Description
hasMoreElements()	Checks whether another element exists
nextElement()	Returns the next element

EnumerationDemo.java

import java.util.Enumeration;
import java.util.Vector;

public class EnumerationDemo {
    public static void main(String[] args) {
        Vector<String> subjects = new Vector<>();
        subjects.add("Java"); subjects.add("Python"); subjects.add("Database");
        Enumeration<String> enumeration = subjects.elements();
        while (enumeration.hasMoreElements()) {
            System.out.println(enumeration.nextElement());
        }
    }
}

Output

Java
Python
Database

Check your understanding: Which method returns the next Enumeration element?

nextElement().

Step 13 of 18

6.13 Iterator versus Enumeration

Learning objective

Contrast modern Iterator behavior with legacy Enumeration.

Iterator and Enumeration
Iterator	Enumeration
Modern interface	Legacy interface
Most modern collections	Mainly legacy classes
hasNext()	hasMoreElements()
next()	nextElement()
Supports removal	No removal
Can be fail-fast	Usually not fail-fast

Check your understanding: Which cursor supports remove()?

Iterator.

Step 14 of 18

6.14 Enhanced For Loop

Learning objective

Use enhanced for for clear read-only traversal of arrays and Iterable collections.

List traversal

for (String subject : subjects) {
    System.out.println(subject);
}

Advantages

Simple and readable
No index management
Works with arrays and most collections

Limitations

No direct index
Forward traversal
Do not directly remove during traversal
Less modification control than ListIterator

Check your understanding: Does enhanced for directly provide an index?

No.

Step 15 of 18

6.15 Traditional For Loop

Learning objective

Use indexed traversal where position matters.

Indexed list traversal

for (int i = 0; i < subjects.size(); i++) {
    System.out.println(i + " : " + subjects.get(i));
}

Use it when

The index is required
Elements must be accessed or updated by position
The collection implements List

Check your understanding: Why is this loop unsuitable for Set?

Set does not provide index-based access.

Step 16 of 18

6.16 forEach Method

Learning objective

Process collection elements with a lambda or method reference.

Lambda form

subjects.forEach(subject -> System.out.println(subject));

Method-reference form

subjects.forEach(System.out::println);

Check your understanding: What method reference prints every element?

System.out::println.

Step 17 of 18

6.17 Traversal Method Comparison

Learning objective

Compare traversal direction, modification support, and suitable structures.

Traversal comparison
Method	Direction	Modification	Suitable for
Iterator	Forward	Remove	Most collections
ListIterator	Both	Add, remove, replace	Lists
Enumeration	Forward	None	Legacy classes
Enhanced for	Forward	Normally read-only	Arrays and collections
Traditional for	Either by index	Index-based	Lists
forEach()	Forward processing	Normally processing	Modern code

Check your understanding: Which method supports bidirectional traversal and replacement?

ListIterator.

Step 18 of 18

6.18 Selection Guide

Learning objective

Select a traversal mechanism for a concrete requirement.

Requirement and choice
Requirement	Recommended choice
Simple read-only traversal	Enhanced for
Safe removal while traversing	Iterator
Bidirectional list traversal	ListIterator
Add or replace during list traversal	ListIterator
Index required	Traditional for
Concise processing action	forEach()
Legacy Vector or Hashtable code	Enumeration

Check your understanding: Which mechanism safely removes matching elements from most collections during traversal?

Iterator.