例如：输入字符串：aaaaaaaaaa bbbbbbbbbbbbcccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
什么字符应该转换成大写？b
输出应该是什么：aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbcccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc

public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);
    System.out.println("Enter a String");
    StringBuilder sb = new StringBuilder(keyboard.nextLine());
    System.out.println("Which character should be converted to uppercase?");
    char c = keyboard.nextLine().charAt(0);

    ParsingUtils.changeLetter(sb, c);

    System.out.println(sb);
}

public static void changeLetter(StringBuilder sb, char letter)
{
    String change = Character.toString(letter); //changes the char in the parameter to string
    String change2 = change.toUpperCase(); //make an uppercase copy of the string
    char upper = change2.charAt(0); //change the upper case copy to a char

    for (int counter = 0; counter < sb.length(); counter++) //Is supposed to replace every instance of the given character with uppercase
    {
        if (sb.toString().contains(change))
        {
            sb.setCharAt(counter, upper);
        }
    }
}

我的代码输出什么：bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbcccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
有没有一个更好的，不太复杂的方法来做这件事，我太沮丧看到？
我还是一个java新手，所以可能有一个简单的答案完全出乎我的意料。