Reverse a String using Java Collections reverse() method

This post will discuss how to reverse a string using the Java Collections Framework reverse() method.

The following example demonstrates how to use Collections.reverse() to reverse Java’s string. Following are the complete steps:

Create an empty ArrayList of characters and initialize it with characters of the given string using String.toCharArray().
Reverse the list using java.util.Collections reverse() method.
Finally, convert the ArrayList into String using StringBuilder and return the formed string.

The following program demonstrates it:

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

import java.util.stream.Collectors;

class Main

{

// Method to reverse a string in Java using `Collections.reverse()`

public static String reverse(String str)

{

// base case: if the string is null or empty

if (str == null || str.equals("")) {

return str;

}

// create an empty list of characters

List<Character> list = new ArrayList<Character>();

// push every character of the given string into it

for (char c: str.toCharArray()) {

list.add(c);

}

// reverse list using `java.util.Collections` `reverse()`

Collections.reverse(list);

// convert `ArrayList` into string and return it

return list.stream().map(String::valueOf).collect(Collectors.joining());

}

public static void main(String[] args)

{

String str = "Reverse me!";

// String is immutable

str = reverse(str);

System.out.println("The reversed string is " + str);

}

Download Run Code

Output:

The reversed string is !em esreveR

The above solution first converts ArrayList into StringBuilder, then StringBuilder into a string. We can directly convert ArrayList into a string by removing square brackets, commas, and single space from it. The following code uses String.replaceAll() to achieve that. But this approach is highly inefficient and should be avoided at all costs.

import java.util.List;

import java.util.ArrayList;

import java.util.Collections;

class Main

{

// Method to reverse a string in Java using `Collections.reverse()`

public static String reverse(String str)

{

// return if the string is null or empty

if (str == null || str.equals("")) {

return str;

}

// create an empty list of characters and push every

// character of the given string into it

List<Character> list = new ArrayList<Character>();

for (char c: str.toCharArray()) {

list.add(c);

}

// reverse list using `java.util.Collections` `reverse()`

Collections.reverse(list);

// list.toString() is [t, h, g, i, l, e, D, , e, i, h, c, e, T]

// convert `list.toString()` to valid string

return list.toString()

.replaceAll("[,\\[\\]]", "") // t h g i l e D e i h c e T

.replaceAll(" ", "@") // t h g i l e D@ e i h c e T

.replaceAll(" ", "") // thgileD@eihceT

.replaceAll("@", " "); // !em esreveR

}