Java Iterator Tutorial with Examples

Java Iterator tutorial with examples will help you to understand how to use the Java Iterator in an easy way. Iterator interface in Java is a member of the Java Collection Framework. Java Iterator is used to iterate over a collection to retrieve its elements.

Unlike Enumeration, the Iterator in Java allows us to remove elements from the underlying collection while iterating over the elements of the collection.

How to iterate over a Collection using the Iterator?

The Iterator in Java defines three main methods. The hasNext method returns boolean depending upon whether there are more elements to iterate over in the collection. The next method returns the next element of the collection. The remove method removes the current element from the underlying collection.

The iterator method is used to get the Iterator for the collection class, for example, implementations of Set and List interfaces. Here is how to iterate over a collection.

Get the iterator object for the collection by calling the iterator method.
Loop until the Iterator’s hasNext method returns true.
Inside the loop, call the next method of the Iterator to get the current element of the collection.

The below given code snippet shows how to iterate over an ArrayList using an iterator.

ArrayList aListDays = new ArrayList<String>();

aListDays.add("Sunday");

aListDays.add("Monday");

aListDays.add("Tuesday");

* get the iterator by using iterator method

* of ArrayList class

Iterator iterator = aListDays.iterator();

* loop through elements till iterator's

* hasNext method returns true.

while( iterator.hasNext() ){

* get the next element using

* next method

System.out.println( iterator.next() );

}

Output

Sunday

Monday

Tuesday

Important note:

Do not call the next method without first checking whether the collection has more elements using the hasNext method. Also, do not call the next method multiple times inside the loop to make sure that there will always an element. The next method will throw NoSuchElementException if there are no elements to iterate.

Consider below given code snippet.

import java.util.ArrayList;

import java.util.Iterator;

import java.util.List;

class Student{

private String name;

private int marks;

public Student(String name, int marks){

this.name = name;

this.marks = marks;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getMarks() {

return marks;

}

public void setMarks(int marks) {

this.marks = marks;

}

public class Demo {

public static void main(String args[]){

List<Student> listStudents = new ArrayList<Student>();

listStudents.add( new Student("Ryan", 70) );

listStudents.add( new Student("Tanya", 84) );

listStudents.add( new Student("Tina", 65) );

listStudents.add( new Student("John", 43) );

listStudents.add( new Student("Shawn", 58) );

Iterator<Student> iterator = listStudents.iterator();

while(iterator.hasNext()){

System.out.println( iterator.next().getName() + " " + iterator.next().getMarks() );

}

What do you think the output would be? If you think that the code will print names and marks for all the students, you are wrong. Here is the output of the code.

Ryna 84

Tina 43

Exception in thread "main" java.util.NoSuchElementException

at java.util.ArrayList$Itr.next(Unknown Source)

at Demo.main(Demo.java:45)

The code prints the name of the first student followed by the marks of the second student. Again, it prints the name of the third student and the marks of the fourth student. It is because we have called the next method twice in the loop. When you call the next method on the iterator object, it returns the next element of the collection. When the iterator is on the fifth element, we call the next method and since there are no more objects in the collection, the code throws NoSuchElementException.

Here is how the above code should be written.

Iterator<Student> iterator = listStudents.iterator();

Student student = null;

while(iterator.hasNext()){

student = iterator.next();

System.out.println( student.getName() + " " + student.getMarks() );

}

Output

Ryan 70

Tanya 84

Tina 65

John 43

Shawn 58

Also, there should be one next method call per hasNext method to avoid NoSuchElementException.

How to iterate over ArrayList in Java?

ArrayList<Integer> aListNumbers = new ArrayList<Integer>();

aListNumbers.add(1);

aListNumbers.add(2);

aListNumbers.add(3);

//get the Iterator using iterator method of ArrayList

Iterator<Integer> iterator = aListNumbers.iterator();

System.out.println("Iterate through ArrayList elements");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through ArrayList elements

Please visit the ArrayList tutorial to know more about ArrayList.

How to iterate over LinkedList in Java?

LinkedList<String> linkedListColors = new LinkedList<String>();

linkedListColors.add("Red");

linkedListColors.add("Green");

linkedListColors.add("Blue");

//iterator method of the LinkedList

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

System.out.println("Iterate through LinkedList elements");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through LinkedList elements

Red

Green

Blue

Please visit the LinkedList tutorial to know more about LinkedList.

How to iterate over TreeSet in Java?

TreeSet<Integer> tSetOddNumbers = new TreeSet<Integer>();

tSetOddNumbers.add(5);

tSetOddNumbers.add(1);

tSetOddNumbers.add(3);

//get the iterator using iterator method of TreeSet

Iterator<Integer> iterator = tSetOddNumbers.iterator();

System.out.println("Iterate through TreeSet elements");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through TreeSet elements

Please visit the TreeSet tutorial to know more about TreeSet.

How to iterate over HashSet in Java?

HashSet<Integer> hSetEvenNumbers = new HashSet<Integer>();

hSetEvenNumbers.add(6);

hSetEvenNumbers.add(2);

hSetEvenNumbers.add(4);

//get the iterator using iterator method of HashSet

Iterator<Integer> iterator = hSetEvenNumbers.iterator();

System.out.println("Iterate through HashSet elements");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through HashSet elements

Please visit the HashSet tutorial to know more about HashSet.

How to iterate over HashMap in Java?

Iterator through keys of HashMap

HashMap<Integer, String> hMapNumbers = new HashMap<Integer, String>();

hMapNumbers.put(1, "One");

hMapNumbers.put(2, "Two");

hMapNumbers.put(3, "Three");

* Iterate through all keys of HashMap

//get the key set using the keySet method

Set<Integer> keys = hMapNumbers.keySet();

//get the iterator using the iterator method

Iterator<Integer> iterator = keys.iterator();

System.out.println("Iterate through keys of HashMap");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through keys of HashMap

Iterator through values of HashMap

HashMap<Integer, String> hMapNumbers = new HashMap<Integer, String>();

hMapNumbers.put(1, "One");

hMapNumbers.put(2, "Two");

hMapNumbers.put(3, "Three");

* Iterate through all values of HashMap

//get all values of HashMap using the values method

Collection<String> values = hMapNumbers.values();

//get the iterator using the iterator method

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

System.out.println("Iterate through values of HashMap");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through values of HashMap

One

Two

Three

Iterate through key-value mappings of HashMap

HashMap<Integer, String> hMapNumbers = new HashMap<Integer, String>();

hMapNumbers.put(1, "One");

hMapNumbers.put(2, "Two");

hMapNumbers.put(3, "Three");

* Iterate through all key value mappings of HashMap

//get all mappings of HashMap using the entrySet method

Set<Map.Entry<Integer, String>> entrySet = hMapNumbers.entrySet();

//get the iterator using the iterator method

Iterator<Map.Entry<Integer, String>> iterator = entrySet.iterator();

System.out.println("Iterate through mappings of HashMap");

while( iterator.hasNext() ){

Map.Entry<Integer, String> entry = iterator.next();

System.out.println( entry.getKey() + " => " + entry.getValue() );

}

Output

Iterate through mappings of HashMap

1 => One

2 => Two

3 => Three

Please visit the HashMap tutorial to know more about HashMap.

How to iterate over an TreeMap in Java?

Iterator through keys of TreeMap

TreeMap<Integer, String> tMapOddNumbers = new TreeMap<Integer, String>();

tMapOddNumbers.put(1, "One");

tMapOddNumbers.put(3, "Three");

tMapOddNumbers.put(5, "Five");

* Iterate through all keys of TreeMap

//get the key set using the keySet method

Set<Integer> keys = tMapOddNumbers.keySet();

//get the iterator using the iterator method

Iterator<Integer> iterator = keys.iterator();

System.out.println("Iterate through keys of TreeMap");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through keys of TreeMap

Iterator through values of TreeMap

TreeMap<Integer, String> tMapOddNumbers = new TreeMap<Integer, String>();

tMapOddNumbers.put(1, "One");

tMapOddNumbers.put(3, "Three");

tMapOddNumbers.put(5, "Five");

* Iterate through all values of TreeMap

//get all values of TreeMap using the values method

Collection<String> values = tMapOddNumbers.values();

//get the iterator using the iterator method

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

System.out.println("Iterate through values of TreeMap");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through values of TreeMap

One

Three

Five

Iterate through key-value mappings of HashMap

TreeMap<Integer, String> tMapOddNumbers = new TreeMap<Integer, String>();

tMapOddNumbers.put(1, "One");

tMapOddNumbers.put(3, "Three");

tMapOddNumbers.put(5, "Five");

* Iterate through all key value mappings of TreeMap

//get all mappings of TreeMap using the entrySet method

Set<Map.Entry<Integer, String>> entrySet = tMapOddNumbers.entrySet();

//get the iterator using the iterator method

Iterator<Map.Entry<Integer, String>> iterator = entrySet.iterator();

System.out.println("Iterate through mappings of TreeMap");

while( iterator.hasNext() ){

Map.Entry<Integer, String> entry = iterator.next();

System.out.println( entry.getKey() + " => " + entry.getValue() );

}

Output

Iterate through mappings of TreeMap

1 => One

3 => Three

5 => Five

Please visit the TreeMap tutorial to know more about TreeMap.

How to iterate over an LinkedMap in Java?

Iterator through keys of LinkedHashMap

LinkedHashMap<Integer, String> lhMapEvenNumbers = new LinkedHashMap<Integer, String>();

lhMapEvenNumbers.put(2, "Two");

lhMapEvenNumbers.put(4, "Four");

lhMapEvenNumbers.put(6, "Six");

* Iterate through all keys of LinkedHashMap

//get the key set using the keySet method

Set<Integer> keys = lhMapEvenNumbers.keySet();

//get the iterator using the iterator method

Iterator<Integer> iterator = keys.iterator();

System.out.println("Iterate through keys of LinkedHashMap");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through keys of LinkedHashMap

Iterator through values of LinkedHashMap

LinkedHashMap<Integer, String> lhMapEvenNumbers = new LinkedHashMap<Integer, String>();

lhMapEvenNumbers.put(2, "Two");

lhMapEvenNumbers.put(4, "Four");

lhMapEvenNumbers.put(6, "Six");

* Iterate through all values of LinkedHashMap

//get all values of LinkedHashMap using the values method

Collection<String> values = lhMapEvenNumbers.values();

//get the iterator using the iterator method

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

System.out.println("Iterate through values of LinkedHashMap");

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Iterate through values of LinkedHashMap

Two

Four

Six

Iterate through key-value mappings of LinkedHashMap

LinkedHashMap<Integer, String> lhMapEvenNumbers = new LinkedHashMap<Integer, String>();

lhMapEvenNumbers.put(2, "Two");

lhMapEvenNumbers.put(4, "Four");

lhMapEvenNumbers.put(6, "Six");

* Iterate through all key value mappings of LinkedHashMap

//get all mappings of LinkedHashMap using the entrySet method

Set<Map.Entry<Integer, String>> entrySet = lhMapEvenNumbers.entrySet();

//get the iterator using the iterator method

Iterator<Map.Entry<Integer, String>> iterator = entrySet.iterator();

System.out.println("Iterate through mappings of LinkedHashMap");

while( iterator.hasNext() ){

Map.Entry<Integer, String> entry = iterator.next();

System.out.println( entry.getKey() + " => " + entry.getValue() );

}

Output

Iterate through mappings of LinkedHashMap

2 => Two

4 => Four

6 => Six

Please visit the LinkedHashMap tutorial to know more about LinkedHashMap.

How to remove elements using the Iterator?

The remove method of the Iterator class removes the current element from the collection.

ArrayList<String> aListColors = new ArrayList<String>();

aListColors.add("Red");

aListColors.add("Green");

aListColors.add("Blue");

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

String color = null;

while(iterator.hasNext()){

color = iterator.next();

if(color.equalsIgnoreCase("green")){

//remove element from collection using remove method

iterator.remove();

}

System.out.println("ArrayList contains: ");

System.out.println(aListColors);

Output

1 2	ArrayList contains: [Red, Blue]

Note: The remove method throws IllegalStateException if,

The remove method is called before the first call of the next method.
The remove method is called the second time for the same element.

Also remove method is not supported by all the collections. If you call the remove method on a collection which does not support the remove operation, it will throw UnsupportedOperationException.

For example, the list obtained by converting an array object using the asList method of the Arrays class does not support the remove operation.

String[] strColors = {"Red", "Green", "Blue"};

List<String> listColors = Arrays.asList(strColors);

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

String color = null;

while(iterator.hasNext()){

color = iterator.next();

if(color.equalsIgnoreCase("green")){

iterator.remove();

}

Output

Exception in thread "main" java.lang.UnsupportedOperationException

at java.util.AbstractList.remove(Unknown Source)

at java.util.AbstractList$Itr.remove(Unknown Source)

at Demo.main(Demo.java:64)

Below given are additional Java iterator examples that discuss topics in more detail.

Java Iterator Examples

References:
Java Iterator interface Javadoc

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Rahim

I have a master's degree in computer science and over 18 years of experience designing and developing Java applications. I have worked with many fortune 500 companies as an eCommerce Architect. Follow me on LinkedIn and Facebook.