Question

I am practicing over the summer to try and get better and I am a little stuck on the following:

http://www.javabat.com/prob/p123384

Given a string, return a new string where the first and last chars have been exchanged.

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Examples:

frontBack("code") → "eodc"
frontBack("a") → "a"
frontBack("ab") → "ba"

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Code:

public String frontBack(String str) 
{
    String aString = "";
    
    if (str.length() == 0){
        return "";
    }
    
    char beginning = str.charAt(0);
    char end = str.charAt(str.length() - 1);
    
    str.replace(beginning, end);
    str.replace(end, beginning);
    
    return str;
}

Answer 1

Rather than using the String.replace method, I’d suggest using the String.substring method to get the characters excluding the first and last letter, then concatenating the beginning and end characters.

Furthermore, the String.replace method will replace all occurrences of the specified character, and returns a new String with the said replacements. Since the return is not picked up in the code above, the String.replace calls really don’t do much here.

This is because String in Java is immutable, therefore, the replace method cannot make any changes to the original String, which is the str variable in this case.

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Also to add, this approach won’t work well with Strings that have a length of 1. Using the approach above, a call to String.substring with the source String having a length of 1 will cause a StringIndexOutOfBoundsException, so that will also have to be taken care of as a special case, if the above approach is taken.

Frankly, the approach presented in indyK1ng’s answer, where the char[] is obtained from the String and performing a simple swap of the beginning and end characters, then making a String from the modified char[] is starting to sound much more pleasant.