How an iterate object can be used to iterate a list in Java?

How to iterate over a Java list?

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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 −

1. Obtain an iterator to the start of the collection by calling the collection's iterator() method.
2. Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
3. 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

Here is a sample Java program that demonstrates How to loop through aList in Java in three different ways, Iterator, for-each loop, and traditional for loop. This technique can be used to loop throughArrayList or any other index-based List implementation like Vector.

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

Here is our complete Java program to show you all three ways to loop over a list in Java. You can use these methods to loop over any List like ArrayList, LinkedList, or even Vector in Java.

package example;

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.

* There are three examples in this program,
* first examples show how to loop List

* using Iterator, Second Example shows Looping over List
* using advanced Java 5 for loop

* while third and last examples demonstrate
* use of traditional for loop for traversing over

* a List in Java.
*
* @author Javin Paul
*/
public class ListLoopExample{

public static void main(String args[]){

//First example to iterate List in Java using Iterator
List<String> languages = Arrays.asList("Java",
"C++", "Scala", "Groovy");


//Getting Iterator from List in Java
Iterator<String> 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<languages.size(); i++){
System.out.printf("programming language #%d in
List is : %s %n", i, languages.get(i) );
}


}
}

Output:
Iterating List in Java using Iterator
The current element in List is London
The current element in List is Tokyo
The current element in List is NewYork
The current element in List in Mumbai
Looping List in Java using a foreach loop
List Element: London
List Element: Tokyo
List Element: NewYork
List Element: Mumbai
City #0 in List is: London
City #1 in List is: Tokyo
City #2 in List is: NewYork
City #3 in List is: Mumbai

That’s all on How to iterate or loop over aList in Java. In this Java tutorial, we have seen an example of all three ways of looping List in Java. the first example demonstrates the use of Iterator with List, the second use for loop for looping over List, and the third example uses traditional old for a loop. All this technique can be applied to any index-based List implementation including ArrayList and Vector in Java.

Further Learning
TheComplete Java Masterclass
Java Fundamentals: Collections
Java Programming Interview Exposed by Markham

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