Monday, July 26, 2021

How to Reverse an Array in Java - Integer and String Array Example

This Java tip is about, how to reverse an array in Java, mostly primitive types e.g. int, long, double and String arrays. Despite of Java’s rich Collection API, use of array is quite common, but standard JDK doesn’t have great utility classes for Java. It’s difficult to convert between Java Collection e.g. List, Set to primitive arrays. Java’s array utility class java.util.Arrays, though offers some the critical functionalities like comparing arrays in Java and support to print arrays, It lacks a lot of common features, such as combining two arrays and reverse primitive or object array. 


Thankfully, Apache commons-lang, an open-source library from Apache software foundation offers one interesting class ArrayUtils, which can be used in conjunction with java.util.Arrays class to play with both primitive and object array in Java. 

 This API offers conveniently overloaded method to reverse different kinds of array in Java e.g. int, double, float, log or Object arrays. On a similar note, you can also write your own utility method to reverse Array in Java, and this is even a good programming question. 

 For production usage, I personally prefer tried and tested library methods instead of reinventing the wheel. Apache commons-lang fits the bill, as it offers other convenient API to complement JDK. In this Java tutorial, we will reverse int and String array in Java using ArrayUtils to show How to reverse primitive and object array in Java.


Reverse int and String array in Java - Example

Java program to reverse int and String array in Java with ExampleHere is the code example to reverse any array in Java. We have declared two arrays, iArray which is an int array and sArray which stores String objects. We have also included commons-lang-2.6.jar to use org.apache.commons.lang.ArrayUtils class to reverse Array in Java. As discussed in our last post How to print array element in Java, We are using Arrays.toString() to print content of array.



import java.util.Arrays;
import org.apache.commons.lang.ArrayUtils;

/**
 *
 * Java program to reverse array using Apache commons ArrayUtils class.
 * In this example we will reverse both int array and String array to
 * show how to reverse both primitive and object array in Java.
 *
 * @author
 */
public class ReverseArrayExample {

    public static void main(String args[]) {

        int[] iArray = new int[] {101,102,103,104,105};
        String[] sArray = new String[] {"one", "two", "three", "four", "five"};
       
       
        // reverse int array using Apache commons ArrayUtils.reverse() method
        System.out.println("Original int array : " + Arrays.toString(iArray));
        ArrayUtils.reverse(iArray);
        System.out.println("reversed int array : " + Arrays.toString(iArray));
       
        // reverse String array using ArrayUtis class
        System.out.println("Original String array : " + Arrays.toString(sArray));
        ArrayUtils.reverse(sArray);
        System.out.println("reversed String array in Java : " 
                             + Arrays.toString(sArray));        
    }   
   
}

Output:
Original int array : [101, 102, 103, 104, 105]
reversed int array : [105, 104, 103, 102, 101]
Original String array : [one, two, three, four, five]
reversed String array in Java : [five, four, three, two, one]

That's all on How to reverse Array in Java. In this Java program,  we have seen examples to reverse String and int array using Apache commons ArrayUtils class. By the way, there are a couple of other ways to reverse array as well, e.g. by converting Array to List and then using Collections.reverse() method or by using brute force and reverse array using an algorithm. It depends upon, which method you like. If you are doing practice and learning Java, try to do this exercise using loops.

Related Java Programming tutorials from Javarevisited Blog
Difference between LinkedList and ArrayList in Java
How to sort Hashtable and HashMap by values
How to remove duplicate elements from ArrayList
How to convert ArrayList to HashSet in Java
4 ways to iterate over Map in Java with examples

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17 comments:

  1. SARAL SAXENAMarch 22, 2013 at 9:46 AM

    Hi Javin gr8 article but if we go by the way of java..then this thing can also be achieved..

    You can reverse an array like this:

    public void reverse(Object [] a){
    for(int i = 0; i < a.length / 2; i++){
    Object temp = a[i]; // swap using temporary storage
    a[i] = a[a.length - i - 1];
    a[a.length - i - 1] = temp;
    }
    }
    It's worthy to note that it doesn't matter if the array length is an odd number, as the median value will remain unchanged. I have to admit that I haven't tested this but it should work.

    For now please check the following program..http://ideone.com/3qqRAm

    ReplyDelete
  2. AnonymousMay 12, 2014 at 2:44 PM

    disappointed. i expected that you would provide algorithm for this. but you ended up in using open source library method.

    ReplyDelete
  3. himanshuJanuary 11, 2015 at 8:57 PM

    public void swap(int[] arr,int a,int b){

    int temp=arr[a];
    arr[a]=arr[b];
    arr[b]=temp;

    }
    public int[] reverseArray(int[] arr){
    int size=arr.length-1;

    for(int i=0;i<size;i++){

    swap(arr,i,size--);

    }

    return arr;
    }

    ReplyDelete
  4. UnknownMarch 18, 2015 at 6:27 AM


    /* let s1 and s2 be the two string that we have to compare.
    we'll insert every character of the first string into a hashmap.
    the character will be the key, whereas, the frequency of the character will be the value */


    Hashmap hm = new Hashmap();

    count = 1;
    for (i=0; i1)
    {
    val--;
    hm.put(j,val);
    }
    else
    {
    hm.remove(j);
    }
    else
    {
    System.out.println("Not anagrams");
    }
    }

    if (hm.isEmpty())
    {
    System.out.println("Anagrams");
    }
    else
    {
    System.out.println("Not anagrams");
    }

    ReplyDelete
  5. AnonymousNovember 24, 2015 at 9:34 AM

    Disappointing...... Was looking for an algorithm.

    ReplyDelete
  6. UnknownMarch 13, 2017 at 9:18 AM

    Agree, bad answer.
    It's better to write something like this:
    void reverse(int[] arr) {
    for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
    int tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
    }
    }

    ReplyDelete
  7. AnonymousJune 7, 2017 at 4:41 PM

    Agree, bad answer.
    It's better to write something like this:
    void reverse(int[] arr) {
    for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
    int tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
    }
    }
    this doesn't work
    Read more: http://javarevisited.blogspot.com/2013/03/how-to-reverse-array-in-java-int-String-array-example.html#ixzz4jMYu3XQ5

    ReplyDelete
  8. UnknownSeptember 30, 2018 at 1:28 PM

    public class Nevermind
    {

    public static void main (String[] args) {

    int[] array = {1,2,3,4,5,6,7,8,9,10};

    for( int i : array ){ System.out.println("Before: "+i); }

    array = reverse(array);

    for( int i : array ){ System.out.println("After: "+i); }

    }

    static int[] reverse (int[] a) {

    int index = a.length-1;

    int first = 0;

    for (int i = first; i < index; i++){

    int temp = a[index];

    a[index] = a[first];
    a[first] = temp;

    first += 1;
    index -= 1;
    }


    return a;
    }

    }

    ReplyDelete
  9. AnonymousJanuary 23, 2020 at 9:15 PM

    public class SwapInPlace {
    public static int[] reverseInPlace(int arr[]) {
    int l=0;
    int high=arr.length-1;
    int mid=(l+high)/2;
    int len=high;
    for(int i=0;i<=mid;i++) {

    if(high%2==0) {
    if(i<mid) {

    int temp=arr[i];
    arr[i]=arr[len];
    arr[len]=temp;
    len--;
    }
    }

    if(high%2!=0) {
    int temp=arr[i];
    arr[i]=arr[len];
    arr[len]=temp;
    len--;
    }

    }
    return printArray(arr);

    }
    public static int[] printArray(int array[]) {
    for(int i=0;i<array.length;i++) {
    System.out.print(array[i]+" ");
    }
    System.out.println();
    return array;
    }
    public static void main(String args[]) {
    int array[]= {1,3,5,4,2};
    reverseInPlace(array);
    int array1[]= {1,2,3,4,5,6};
    reverseInPlace(array1);
    }
    }

    ReplyDelete
  10. KennedyMarch 6, 2020 at 11:33 PM

    /**
    * Program to reverse an array in place in Java.
    * Works both when array size is odd and even :)
    * @author: Kennedy
    */
    import java.util.Arrays;
    public class reverseArrayInPlace
    {
    public static int [] arr;
    public static int a =0;
    public static int b = 0;
    //method to swap two values
    public static void swap(int[] arr,int a,int b)
    {
    //swap two values a & b
    int temp=arr[a];
    arr[a]=arr[b];
    arr[b]=temp;
    }
    //method to reverse array values
    public static int[] reverseArray(int[] arr)
    {
    //decrement the array size
    int size=arr.length-1;
    /**As long as current first index is less than array size swap current index with current last index
    */
    for(int i=0;i<size;i++)
    {
    swap(arr,i,size--);

    }

    return arr;
    }

    public static void main(String []args)
    {

    //create array
    int [] arr = {3,5,7,9,11,13,15,17,19};
    //print original array
    System.out.println("Original Array:" + " " + Arrays.toString(arr));
    //call reverse method
    reverseArray(arr);
    //print reversed array
    System.out.println("Reversed Array:" + " " + Arrays.toString(arr));
    }
    }

    OUTPUT:
    $javac reverseArrayInPlace.java
    $java -Xmx128M -Xms16M reverseArrayInPlace
    Original Array: [3, 5, 7, 9, 11, 13, 15, 17, 19]
    Reversed Array: [19, 17, 15, 13, 11, 9, 7, 5, 3]

    ReplyDelete
  11. TapanMarch 15, 2020 at 10:25 AM

    public class ReverseArray {

    public static void main(String[] args) {
    int arr[] = { 1, 2, 3, 4, 5, 6 };
    reverse(arr);
    }

    static void reverse(int[] arr) {
    int size = (arr.length) / 2;
    int count = 0;

    for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
    count++;
    int tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
    if (count == size)
    break;
    }
    System.out.println(Arrays.toString(arr));
    }
    }

    ReplyDelete
  12. javin paulMarch 18, 2020 at 8:01 AM

    Good job Tapan, nice

    ReplyDelete
  13. UnknownApril 22, 2020 at 10:37 PM

    easy way to reverse array

    public class Reversearr {

    public static void main(String[] args) {

    Integer ar[]= {1, 1, 2, 3, 5, 5, 7, 9, 9, 9};

    for(int i=ar.length-1; i>=0;i--) {
    System.out.println(ar[i]);
    }
    }

    }

    ReplyDelete
  14. javin paulApril 30, 2020 at 9:46 PM

    @Unknown, this is not reversing the array but printing the array in reverse order. You need to return array itself where elements are in reverse order of given array.

    ReplyDelete
  15. Nathaniel CobbinahOctober 2, 2021 at 1:14 PM


    public class bubbleSort {

    public static void main(String[] args) {

    int[] values = {32, 1,5, 9 ,90,16};
    int[] values2 = {32, 1,5, 9 ,90,16};

    int[] result = bubbleSortAsc(values);
    int[] result2 = bubbleSortDes(values2);

    System.out.println("ascending");
    for(int value : result) {
    System.out.println(value);
    }

    System.out.println("descending");
    for(int value : result2) {
    System.out.println(value);
    }

    }

    public static int[] bubbleSortAsc(int[] list) {
    int temp = 0;
    int n = list.length;

    for(int i=0; i < n; i++) {
    for(int j=1; j < n; j++) {
    if(list[j - 1] > list[j]) {
    //swap
    temp = list[j - 1];
    list[j - 1] = list[j];
    list[j] = temp;
    }
    }
    }
    return list;
    }


    //descending sort
    public static int[] bubbleSortDes(int[] list) {
    int temp = 0;
    for(int i=0; i < list.length; i++) {
    for(int j=0; j < list.length; j++) {
    if(list[i] > list[j]) {
    //swap
    temp = list[i];
    list[i] = list[j];
    list[j] = temp;
    }
    }
    }
    return list;
    }

    }

    ReplyDelete
  16. AnonymousAugust 4, 2024 at 9:52 PM

    I will use temp method use this function to reverse array which takes tow parameters int say (arr[]) and size of the array say (n).

    this function reverse array and store it in another array.

    inside function new array (arr, the size of the array ) is initilized.

    the array is iterated from the first element and each element arr [] is placed in the new array from the back.


    Public static int reverse (int arr[] , int n) {
    int b = new int [n] ;
    int j = n;
    for(int i = 0; i < n; i ++) {
    b[j - 1] = arr[i];
    j = j -1;
    }
    System. out. println("the reverse array: /n" ) ;
    for(Int K =0; k < n; k++) {
    System. Out. println(b[k] );
    }
    return j;
    }
    public static void main(String args[] ) {
    int odd = {10, 20, 30, 40, 50} ;
    odd. reverse (odd , odd. length) ;
    }

    ReplyDelete
  17. AhmedAugust 6, 2024 at 5:07 AM

    sir we want use the manual method but you have used is actually counted in library
    I had solved using swaps array like this : 1- this function to reverse the array in less number of swaps

    2- this function to reverse array it self the first element swaps with the last element

    3- and the second element swap with the last second element and so on

    4- for instance consider array [ 1, 2, 3, n , n - 1 , n - 2] we swap 1 with n and 2 with n - 1 and 3 with n - 2 and so on

    public static int swap (int a[], int n) {
    int i , k , t;
    for(i = 0; i< n /2; i++) {
    t = a[i] ;
    a[i] = a[i - n - 1] ;
    a[i - n - 1] = t;

    }
    System. out. println ("the reverse array is : /n" ) ;
    for (K = 0; k< n; k++) {
    System.out.println (a[k]);
    }
    return k;
    }

    public static void main (String args []) {
    int odd [] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    odd. swap (odd, odd. length) ;
    }

    ReplyDelete