How to reverse character elements in array using java?

StringBuilder have a reverse method.

 System.out.print("REVERSE WORD: " + new StringBuilder(word).reverse().toString());

Or if you don't want to use inbuilt methods

String result= "";
for(int i=word.length(); i>0; i--) {
    result+= word.charAt(i-1);
}  
 System.out.print("REVERSE WORD: " + result);

public class test {
    public static void main(String[] args) {
        String word = "Stack Overflow";
        char[] wordArray = word.toCharArray();
        System.out.println("NORMAL WORD=" + Arrays.toString(wordArray));
        test.reverse(wordArray);
        System.out.println("REVERSE WORD=" + Arrays.toString(wordArray));
    }

    public static void reverse(char[] array) {
        if (array == null) {
            return;
        }
        int i = 0;
        int j = array.length - 1;
        char tmp;
        while (j > i) {
            tmp = array[j];
            array[j] = array[i];
            array[i] = tmp;
            j--;
            i++;
        }
    }
}

Output:

NORMAL WORD=[S, t, a, c, k,  , O, v, e, r, f, l, o, w]
REVERSE WORD=[w, o, l, f, r, e, v, O,  , k, c, a, t, S]

Do this:
1. Convert the string into a character array.
2. Swap the elements (first and last -> second and second last -> ... Till the middle).
3. You will get a linear complexity of O(n/2) = O(n).

public static String reverseWord(String str)
{
    if(str.length()>0)
    {
        char arr[] = str.toCharArray();
        int length = arr.length;
        for(int i=0; i<length/2; i++)
        {
            char temp = arr[i];
            arr[i] = arr[length-i-1];
            arr[length-i-1] = temp;
        }
        return new String(arr);
    }
    return str;
}

I use StringBuilder for constructed the reversed string. In your main method.

    String word = getWord();
    StringBuilder sb = new StringBuilder();
    for(char c : word.toCharArray()) {
        sb.insert(0, c);
    }
    String reversed = sb.toString();
    System.out.print("NORMAL WORD: " + word);
    System.out.print("REVERSE WORD: " + reversed);

Let's look at the code in main:

char[]wordArray = word.toCharArray();

The String is a group of chars with one after the other.

We need to add each char to the beginning of the result in a loop:

String result = "";
for(char wordChar : word.toCharArray()) {
    result = wordChar + result;
}

This would cause the first char to go last and the second to go second to last etc. This way we can reverse the order of the String.

public static void main(String args[]) {
    String a = "12345";
    char [] original= a.toCharArray();
    char[] reverse = new char[a.length()];
    int j =0;
    for(int i=reverse.length-1; i>=0; i--) {
        reverse[j] = original[i]; 
        j++;
    }

    System.out.println(new String(reverse));
}