java program to insert an character in an array

public class Answer implements QuestionInterface {
@Override
public String func(String str, int index, char ch)
{
   if(str==null)
   {
      
       return null;
   }
                if(index>str.length()+1)
                {
                    return str;
                }
              
                else
                {
StringBuilder st= new StringBuilder(str);
                st.insert(index-1, ch);
                String e=st.toString();
return e;
                }
}
}