Iterate List in reverse order in Java

This post will discuss various ways to iterate a list in reverse order in Java without actually reversing the list.

1. Using `ListIterator`

The List interface provides a special iterator, called ListIterator, that allows bidirectional access. We can call the List.listIterator(index) method to get a ListIterator over the list elements starting from the specified position in the list. Since we need to start from the last element, the starting index would be equal to the list’s size.

import java.util.Arrays;

import java.util.List;

import java.util.ListIterator;

class Main

{

// Java program to iterate list in reverse order

public static void main(String[] args)

{

List<String> list = Arrays.asList("C", "C++", "Java");

// use `ListIterator` to iterate list in reverse order

ListIterator<String> itr = list.listIterator(list.size());

// `hasPrevious()` returns true if the list has a previous element

while (itr.hasPrevious()) {

System.out.println(itr.previous());

}

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2. Using Apache Common Collections

Apache Common provides ReverseListIterator that we can use to iterate list in reverse order.

The first call to next() will return the last element from the list, the next call to next() will return the previous element, and so on. hasNext() method checks if there is another element.

import org.apache.commons.collections4.iterators.ReverseListIterator;

import java.util.Arrays;

import java.util.List;

class Main

{

// Java program to iterate list in reverse order

public static void main(String[] args)

{

List<String> list = Arrays.asList("C", "C++", "Java");

ReverseListIterator itr = new ReverseListIterator(list);

while (itr.hasNext()) {

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

}

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3. Using for-loop

We know that List is an ordered collection, so we can access elements by their index (position) in the list using a for-loop. Please note we can’t use enhanced for-loop here to iterate from the end at the beginning of a List as it uses an Iterator behind the scenes, and Iterator is not bidirectional.

import java.util.List;

import java.util.ArrayList;

import java.util.Arrays;

class Main

{

// Java program to iterate list in reverse order

public static void main(String[] args)

{

List<String> list = Arrays.asList("C", "C++", "Java");

// print list backward using for-loop

for (int i = list.size() - 1; i >= 0 ; i--) {

System.out.println(list.get(i));

}

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4. Java 8 – Using streams

In Java 8, we can:

Get stream using List.stream().
Accumulate elements of this stream into a LinkedList using Stream.collect().
Get an iterator over the elements in the LinkedList in reverse sequential order using LinkedList.descendingIterator() method.
Perform the print operation on elements of the list using Iterator.forEachRemaining() until all elements have been processed. We can either provide method reference System.out::println or pass lambda expression S -> System.out.println(S) to Iterator.forEachRemaining().

The following program demonstrates it:

import java.util.Arrays;

import java.util.LinkedList;

import java.util.List;

class Main

{

// Java program to iterate list in reverse order

public static void main(String[] args)

{

List<String> list = Arrays.asList("C", "C++", "Java");

new LinkedList<>(list) // LinkedList

.descendingIterator() // Iterator

.forEachRemaining(System.out::println);

}

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We can also write a collector that collects elements in the reversed order, as shown below:

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

import java.util.stream.Collectors;

class Main

{

// Java program to iterate list in reverse order

public static void main(String[] args)

{

List<String> list = Arrays.asList("C", "C++", "Java");

list.stream()

.collect(Collectors.collectingAndThen(Collectors.toList(),

lst -> {

Collections.reverse(lst);

return lst.stream();

}

))

.forEach(System.out::println);

}

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That’s all about iterating over a List in reverse order in Java.

Related Post:

Iterate over a List in Java

Iterate List in reverse order in Java

1. Using ListIterator

2. Using Apache Common Collections

3. Using for-loop

4. Java 8 – Using streams

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1. Using `ListIterator`