Adding two Strings simultaneously

Assuming both strings are the same length:

StringBuilder sb = new StringBuilder();
for(int i = 0; i < str1.length(); i++) {
    sb.append(str1.charAt(i)).append(str2.charAt(i));
}
String result = sb.toString();

If one string is longer, you’ll need something like this:

StringBuilder sb = new StringBuilder();
int i = 0;
for(int len = Math.min(str1.length(), str2.length()); i < len; i++) {
    sb.append(str1.charAt(i)).append(str2.charAt(i));
}
if(str1.length() != str2.length()) {
    String longerStr = str1.length() > str2.length() ? str1 : str2;
    sb.append(longerStr.substring(i));
}
String result = sb.toString();