Reverse a String using character array in Java

This post will discuss how to reverse a string using a character array in Java.

We know that we cannot make any change in the string object as string is immutable in Java. But we can use a character array that can be modified easily:

Create an empty character array of the same size as that of the given string.
Fill the character array backward with characters of the given string.
Finally, convert the character array into string using String.copyValueOf(char[]) and return it.

The following program demonstrates it:

class Main

{

// Method to reverse a string in Java using a character array

public static String reverse(String str)

{

// return if the string is null or empty

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

return str;

}

// get string length

int n = str.length();

// create a character array of the same size as that of string

char[] temp = new char[n];

// fill character array backward with characters in the string

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

temp[n - i - 1] = str.charAt(i);

}

// convert character array to string and return it

return String.copyValueOf(temp);

}

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

Using swap():

Following is another efficient way to reverse a string in Java using character array:

Create a character array and initialize it with characters of the given string using String.toCharArray().
Start from the two endpoints l and h of the given string. Run the loop till two endpoints intersect (l <= h). In each iteration of the loop, swap values present at indexes l & h and increment l & decrement h.
Finally, convert the character array into string using String.copyValueOf(char[]) and return.

The following program demonstrates it:

class Main

{

// Method to reverse a string in Java using a character array

public static String reverse(String str)

{

// return if the string is null or empty

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

return str;

}

// create a character array and initialize it with the given string

char[] c = str.toCharArray();

for (int l = 0, h = str.length() - 1; l < h; l++, h--)

{

// swap values at `l` and `h`

char temp = c[l];

c[l] = c[h];

c[h] = temp;

}

// convert character array to string and return

return String.copyValueOf(c);

}