I got an error while using StringBuilder

Question

The int input represents a number k, and the method should replace the kth occurrence of the first char input with the second char input. If the first char does not occur at least k times, then nothing is replaced.

public class HW2{
    public static String replaceKth(char a, char b, int k, String c){
        StringBuilder builder = new StringBuilder();
        int occ = 0;
        for(int i = 0; i < c.length(); i = i + 1){
            if (c.charAt(i) == a){
                occ = occ + 1;
                if(occ == k){
                    builder.setCharAt(i, b);
                }
            }
        }
        return builder.toString();
    }
}

When I tried to test:

HW2.replaceKth(a, b, 3, "aabaa")
Static Error: Undefined name 'a'

yep I define a and another error appears

HW2.replaceKth('a', 'a', 3, "aabaa")
java.lang.StringIndexOutOfBoundsException: String index out of range: 3
    at java.lang.AbstractStringBuilder.setCharAt(AbstractStringBuilder.java:347)
    at java.lang.StringBuilder.setCharAt(StringBuilder.java:55)
    at HW2.replaceKth(HW2.java:15)
> 

Answer 1

You never set the builder instance equal to your String object that you wanted to modify.

public static String replaceKth(char a, char b, int k, String c){
    StringBuilder builder = new StringBuilder(c);
    int occ = 0;
    for(int i = 0; i < c.length(); i++){
        if (c.charAt(i) == a){
            occ = occ + 1;
            if(occ == k){
                builder.setCharAt(i, b);
            }
        }
    }
    return builder.toString();
}

Answer 2

Without single-quotes, a is just a variable name, which appears not to be defined here.

When you call replaceKth, either define a and b to be a chars with specific values, or pass char literals, each enclosed in single-quotes.

char a = 'a';
char b = 'b';

OR

HW2.replaceKth('a', 'b', 3, "aabaa");

Answer 3

It looks like your error is because you are sending a variable a and not a letter a.

a is a variable you didn't declare. Either make one, or use quotes around the letters when calling the method.

Answer 4

You are not converting your String to StringBuilder

Change the code line as

 StringBuilder builder = new StringBuilder(c);

you can do like this to replace the character at an index

public class StringTest {
    public static void main(String[] args) {
        System.out.println(StringTest.replaceKth('a', 'b', 3, "aabaa"));
    }

    public static String replaceKth(char a, char b, int k, String c){
        StringBuilder builder = new StringBuilder(c);
        if(builder.charAt(k)==a){
            builder.setCharAt(k, b);
        }
        return builder.toString();
    }
}