Styjun

I am creating a method my compsci class that need to fulfill this task:
"Write a recursive method called String buildWeirdString(String s) While building the ‘forward’ part of the original string, it should not keep any digits that were part of the initial string - and while building the ‘backward’ part of the original string, it should not keep any letters of the alphabet that were part of the initial input. (you should find the reverseLine() method we did in class helpful) Tiny hint for ignoring case – there is a helpful method in the Character class that will tell you if a character is a letter or not. for example,

userinput:abCD123e!$f%
would produce the output:abCDe!$f%$!321

I don't exactly know how to use the Stringbuilder class because we haven't gone over it in class yet, but it seems like it would be useful for this application, so it is very likely my error is it its usage but it very well could be anywhere else.

This task begins at Static String buildWeirdString, the rest is other already completed tasks.

 package recursionPrograms;

 import java.util.*;

 public class recursion 

 public static void main(String[] args) 
 Scanner s = new Scanner(System.in);
 Scanner st = new Scanner(System.in);
 int b = 0;
 int e = 0;
 String str;
 String string;
 // exp
 System.out.println("Base: ");
 b = s.nextInt();
 System.out.println("Exponent: ");
 e = s.nextInt();
 System.out.println(recPow(b, e));
 // palidrome
 System.out.println("String: ");
 str = st.nextLine();
 System.out.println(isPalindrome(str));
 //string
 System.out.println("String: ");
 string = st.nextLine();
 System.out.println(buildWeirdString(string));
 

 public static double recPow(int base, int exp) 
 if (exp == 0)
 return 1;
 else if (exp < 0)
 return 1 / (base * recPow(base, Math.abs(exp) - 1));
 else
 return base * recPow(base, exp - 1);

 

 static boolean isPalindrome(String str) 
 if (str.length() < 2) 
 return true;
 
 if (str.charAt(0) == str.charAt(str.length() - 1)) 
 return isPalindrome(str.substring(1, (str.length() - 1)));
 
 return false;
 

 static String buildWeirdString(String string) 
 StringBuilder stb = new StringBuilder();
 StringBuilder stb2 = new StringBuilder();

 if (string.length() == 0) 
 String rtrn = stb.toString() + stb2.reverse().toString();
 return rtrn;
 
 if (Character.isLetter(string.charAt(0))) 

 buildWeirdString(string.substring(1));
 stb.insert(string.length() - 1, 
 Character.toString(string.charAt(0)));
 
 if (!Character.isLetter(string.charAt(0))) 
 buildWeirdString(string.substring(1));
 stb2.insert(string.length() - 1, 
 Character.toString(string.charAt(0)));
 
 return null;
 
 

Desired results:
in: abc1d234
out: abcd4321

What happens:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 1
 at java.lang.AbstractStringBuilder.insert(Unknown Source)
 at java.lang.StringBuilder.insert(Unknown Source)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:62)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:61)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:61)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:57)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:57)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:57)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:57)
 at recursionPrograms.recursion.buildWeirdString(recursion.java:57)
 at recursionPrograms.recursion.main(recursion.java:25)