Java program to print the output like- input will be "a4k3b2" and output should be after 'a' 4th character, after 'k' 3rd character and the final output : "aeknbd"

public class StringTest {

    public static void main(String[] args) {

        String str = "a4k3b2";// output aeknbd


        System.out.println(replaceChar(str));;
    }

    private static StringBuilder replaceChar(String str) {

        StringBuilder stB = new StringBuilder();

        char[] c = str.toCharArray();

        for (int i = 0; i < c.length; i++) {
            char cha = c[i];
            
            if(Character.isAlphabetic(c[i])) {
                stB.append(cha);
            }else{
                
                int v = c[i-1];
                char value = c[i];
                char digitChar = (char) (v + (value-48));
                stB.append(digitChar);
            }
        }
        return stB;

    }
}