How to Iterate List in Java
In Java, List is is an interface of the Collection framework. It provides us to maintain the ordered collection of objects. The implementation classes of List interface are ArrayList, LinkedList, Stack, and Vector. The ArrayList and LinkedList are widely used in Java. In this section, we will learn how to iterate a List in Java. Throughout this section, we will use ArrayList.
Java for Loop
- Basic for Loop
- Enhanced for Loop
Java Iterators
- Iterator
- ListIterator
Java forEach Method
- Iterable.forEach[]
- Stream.forEach[]
1. Introduction
Iterating over the elements of a list is one of the most common tasks in a program.
In this tutorial, we're going to review different ways to do this in Java. We'll be focusing on iterating through the list in order, though going in reverseis simple, too.
How to iterate over a Java list?
Often, you will want to cycle through the elements in a collection. For example, you might want to display each element.
The easiest way to do this is to employ an iterator, which is an object that implements either the Iterator or the ListIterator interface.
Iterator enables you to cycle through a collection, obtaining or removing elements. ListIterator extends Iterator to allow bidirectional traversal of a list and the modification of elements.
Before you can access a collection through an iterator, you must obtain one. Each of the collection classes provides an iterator[] method that returns an iterator to the start of the collection. By using this iterator object, you can access each element in the collection, one element at a time.
In general, to use an iterator to cycle through the contents of a collection, follow these steps −
- Obtain an iterator to the start of the collection by calling the collection's iterator[] method.
- Set up a loop that makes a call to hasNext[]. Have the loop iterate as long as hasNext[] returns true.
- Within the loop, obtain each element by calling next[].
1. Iterate ArrayList with ‘for loop’
Java program to iterate through an ArrayList of objects using the standard for loop.
ArrayList namesList = new ArrayList[Arrays.asList[ "alex", "brian", "charles"] ]; for[int i = 0; i < namesList.size[]; i++] { System.out.println[namesList.get[i]]; }2. Iterate ArrayList with ‘for-each loop’
Java program to iterate through an ArrayList of objects using for-each loop.
ArrayList namesList = new ArrayList[Arrays.asList[ "alex", "brian", "charles"] ]; for[String name : namesList] { System.out.println[name]; }3 Examples of looping through a List in Java
The other two methods like Iterator and enhanced for loop can be used along with any Collection class like HashSet, TreeSet, LinkedHashSet, etc. They are actually the standard way to iterate through the collection in Java.
The traditional for loop approach is only possible and efficient because of the index-based nature of the List interface, but if you use this with a linked list then you will get worse performance because in LinkedListaccessing an element with an index is O[n] operation rather than O[1] operation. This is also the fundamental difference between an ArrayList and LinkedList in Java.
The new method of iterating over a list using the forEach[] method is only available in Java SE 8 and I recommend you to read this article to learn how to loop through a list using the forEach[]method in Java 8 before you start using them.
Btw, Java SE 8 is full of exciting and more powerful features and it's essential for a Java developer to learn those features, given it's the most popular Java version. If you are interested, you should follow a good Java 8 course likeWhat's New in Java 8 on Pluralsight to learn quickly.
Alternatively, you can always choose the one from this list of good Java 8 books, earlier published by me.
How to loop over a List in Java
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
/**
*
* Java program to demonstrate different ways to loop,
* iterate or traverse List in Java.
* first examples show how to loop List
* using advanced Java 5 for loop
* use of traditional for loop for traversing over
*
* @author Javin Paul
*/
public class ListLoopExample{
public static void main[String args[]]{
//First example to iterate List in Java using Iterator
List languages = Arrays.asList["Java",
"C++", "Scala", "Groovy"];
//Getting Iterator from List in Java
Iterator iterator = languages.iterator[];
System.out.println["Iterating List in Java
using Iterator "];
//Iterating through all elements of List
while [iterator.hasNext[]] {
System.out.printf["Current element in List
is %s %n", iterator.next[]];
}
//Second example of Iterating over List in Java
// using a foreach loop
System.out.println["Looping List in Java using a
foreach loop"];
for [String city : languages] {
System.out.println["List Element: " + city];
}
//Third example of Looping List using traditional for loop
for[int i =0; i