How to recursively take an input string and return a string with each character repeated?

Question

Basically, I have a task that requires me to write a method called stutter. It is assumed that the stuttering method takes an input string s and returns a string with each character repeated. For example, if the input string was "help", then the result of running this method should be "hheellpp". I tried a bunch of different things and can't make it work. Here is my code:

import java.util.*;

public class Stutter {  
    static String stutterString = "";

    public static String stutter ( String s ) {
        char ch = s.charAt (0);
        String tempString = String.valueOf ( ch );
        if ( s.length() == 0 ) {
            return stutterString;
        } else {
            stutterString += tempString + tempString;
            return stutter ( s.substring (1) );
        }
    }

    public static void main ( String [] args ) {
        Scanner inputScanner = new Scanner ( System.in );
        System.out.println ( "What word would you like to stutter?" );
        String userInput = inputScanner.next();     
        inputScanner.close();       
        System.out.println ( stutter ( userInput ) );
    }
}

I get an error that I'm not sure what to do. This is mistake:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
    at java.lang.String.charAt(Unknown Source)
    at Stutter.stutter(Stutter.java:12)
    at Stutter.stutter(Stutter.java:23)
    at Stutter.main(Stutter.java:41)

. . , Stutter, . , , , .

Answer 1

char ch = s.charAt (0);

char ch = s.length() > 0 ? s.charAt(0) : ' ';

, .

:

if (s.length() == 0) {
    return stutterString;
} else {
    char ch = s.charAt(0);
    String tempString = String.valueOf(ch);
    stutterString += tempString + tempString;
    return stutter(s.substring (1));
}

?
→ abcdefg
→ aabbccddeeffgg

:

, s.charAt(0), s ? , s , s.length() > 0 - , .

. , , , . , , .

Answer 2

:

- 1, "".

, .

, , . , , .

import java.util.*;

public class Stutter {  

    public static String stutter ( String s ) {
        if(s.length() < 1) return "";
        else{
            return "" + s.charAt(0) + s.charAt(0) + stutter(s.substring(1,s.length()));
        }
    }

    public static void main ( String [] args ) {
        Scanner s = new Scanner ( System.in );
        System.out.println ( "What word would you like to stutter?" );
        String userInput = s.nextLine();
        System.out.println(stutter(userInput));
    }
}

Answer 3

: s.length() , s.charAt(0) ... .

Answer 4

s s.charAt (0). ,

char ch = s.charAt (0);
String tempString = String.valueOf ( ch );

to else block

Answer 5

:

public class Stutter {
    private static String head(String str) {
        return str.substring(0,1);
    }

    private static String tail(String str) {
        return str.substring(1);
    }

    private static String stutter(String str) {
        if (str.length() > 0)
            return head(str)+head(str)+stutter(tail(str));
        else
            return "";
    }

    public static void main(String args[]) throws Exception  {
        if (args.length > 0) {
            System.out.println(stutter(args[0]));
        }
    }
}

Answer 6

This can be done very simply using String.replaceAll () :

public class Stutter {
    public static void main (String args[]) {
        String TEST_STRING = "abcdefg";
        System.out.println(stutter(TEST_STRING));
    }

    private static String stutter(String s) {
        return s.replaceAll("(.)", "$1$1");
    }
}

Answer 7

Editted.

import java.util.*;

public class Stutter {

    static String stutterString = "";

    public static String stutter ( String s ) {
        if(s.length() > 0)
        {
            return stutterString.concat(s.substring(0,1)).concat(s.substring(0,1)).concat(stutter(s.substring(1)));
        }
        else
        {
            return stutterString;
        }
    }

    public static void main ( String [] args ) {


        Scanner inputScanner = new Scanner ( System.in );

        System.out.println ( "What word would you like to stutter?" );
        String userInput = inputScanner.next();

        inputScanner.close();

        System.out.println ( stutter ( userInput ) );

    }

}