Replace String

Problem

From a given string, replace all instances of 'a' with 'one' and 'A' with 'ONE'.
Example Input: " A boy is playing in a garden"
Example Output: " ONE boy is playing in onegarden"
-- Not that 'A' and 'a' are to be replaced only when theyare single characters, not as part of another word.

Solution

public static String replaceString(String s) {
    // s = s.replace("a ", "one ");
    // s = s.replace("A ", "ONE ");

//        s = s.replaceAll("\b(a)", "one");
//        s = s.replaceAll("\b(A)", "ONE");

    StringBuilder sb = new StringBuilder();
    for(String str : s.split(" ")) {
        if(str.equals("a"))    sb.append("one ");
        else if(str.equals("A")) sb.append("ONE ");
        else sb.append(str + " ");
    }
    
    return sb.toString();
}