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class StrBuilder {
    /**
     * Sets the character at the specified index.
     *
     * @see #charAt(int)
     * @see #deleteCharAt(int)
     * @param index  the index to set
     * @param ch  the new character
     * @return this, to enable chaining
     * @throws IndexOutOfBoundsException if the index is invalid
     */
    public StrBuilder setCharAt(int index, char ch) {
        if (index < 0 || index >= length()) {
            throw new StringIndexOutOfBoundsException(index);
        }
        buffer[index] = ch;
        return this;
    }

}