Reverse string

A Java string contains a sequence of characters. To create another version of that string, we can reverse it. This can be used to form a string key.

We use many parts from Java to reverse a string. We create a mutable char array buffer. We use charAt to get (in inverse order) chars from the original string.

Method example

We introduce the reverseString method. First we allocate a char array with a size equal to the string's length. Then we loop the array elements.

public class Program {

    public static String reverseString(String value) {
        // Create a char buffer.
        char[] array = new char[value.length()];
        for (int i = 0; i < array.length; i++) {
            // Assign to the array from the string in reverse order.
            // ... charAt uses an index from the last.
            array[i] = value.charAt(value.length() - i - 1);
        }
        return new String(array);
    }

    public static void main(String[] args) {

        // Test our reverse string method.
        String original = "abc";
        String reversed = reverseString(original);
        System.out.println(original);
        System.out.println(reversed);

        original = "qwerty";
        reversed = reverseString(original);
        System.out.println(original);
        System.out.println(reversed);
    }
}
abc
cba
qwerty
ytrewq

A discussion

The Arrays class has no reverse method. This means that we must reverse these characters manually. We find a reverse() on Collections.

StringBuilder

Here is an alternative method. StringBuilder has a reverse method. We can create a StringBuilder from a string and then reverse it.

String reversal is not a common requirement. But for certain problems (like key generation) or for homework assignments, it has use.