Java Programming

Hi, for an "encryption" project, i need your help creating a method that will be incrementing by 5 every char in a string that occupies an odd position in that same string.

For example : 'adam' should return 'aiar'

-Method can't use arrays.
-Method can't use regular expressions like 'split' etc..in order words the teacher wants us to use the simplest stuff inside the String class.
-Only letters must be modified.

so far i have this : (names of values are in french, sorry)

public class Main {

public static String incrementerDeCinqImpaires (String chaine) {


char j = chaine.charAt(1);
char x = chaine.charAt(3);
char o = chaine.charAt(5);


j+=5;
x+=5;
o+=5;


System.out.println(x);
System.out.println(j);
System.out.println(o);
String   tmpString = chaine.replace( chaine.charAt(1), j );
String   tmpString2 = tmpString.replace( tmpString.charAt(3), x );
String   tmpString3 = tmpString2.replace( tmpString2.charAt(5), o );
System.out.println( "Original = " + chaine );
System.out.println( "Result   = " + tmpString3 );



return tmpString3;

}
public static void main (String args [] ) {
        
        
       System.out.println (incrementerDeCinqImpaires("adam"));
       
        
  
    }
}

The thing is i need a "for" loop (which has been a nightmare for me trying to figure out how to make it) in order to increment every odd positioned char in the string.

So all I need really is to sum up what i posted above inside a for loop (or more). Then just add ifs and elses so i can increment only letters.