Iterator

Syntax

Iterator<Type> it = collection.iterator();

Examples

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

ArrayList<String> names = new ArrayList<>();
names.add("Fola");
names.add("Zain");
names.add("Jamal");

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

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

ArrayList<Integer> numbers = new ArrayList<>();
for (int i = 1; i <= 10; i++) numbers.add(i);

Iterator<Integer> it = numbers.iterator();
while (it.hasNext()) {
    int num = it.next();
    if (num % 2 == 0) {
        it.remove();  // safe removal - use it.remove(), not numbers.remove()
    }
}

System.out.println(numbers);  // [1, 3, 5, 7, 9]

import java.util.ArrayList;

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

try {
    for (int num : numbers) {
        if (num == 2) {
            numbers.remove(Integer.valueOf(2));  // modifying during for-each
        }
    }
} catch (java.util.ConcurrentModificationException e) {
    System.out.println("Caught: " + e.getClass().getSimpleName());
}

Best practices

• Use it.remove() through an Iterator when you need to delete elements while looping - never call collection.remove() directly inside a for-each loop
• Use a for-each loop for simple read-only iteration - it is more concise than manually managing an Iterator
• Use a ListIterator (available on List) instead of a plain Iterator when you also need to traverse backward or modify elements in place
• Remember an Iterator becomes invalid ("fails fast") if the underlying collection is structurally modified by anything other than the iterator itself

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation