问在替换短语中的字符串时遇到问题

EN

private String line; //The line to format

public TextFormatter(String lineToFormat) {
    line = lineToFormat;
}

/**
 * Finds the first single instance of str in line, starting at the postion start
 * <p>
 * @param str   the string of length 1 to find. Guaranteed to be length 1.
 * @param start the position to start searching. Guaranteed to be in the string line.
 * <p>
 * @return the index of first instance of str if string found or -1 otherwise.
 */
private int findString(String str, int start) {
    String phrase = line;
    int psn = phrase.indexOf(str, start);
    return psn;
}

/**
 * Count the number of times single instances of str appear in the line.
 * <p>
 * @param str the string to find. Guaranteed to be length 1.
 * <p>
 * @return the number of times the string appears in the line.
 */
private int countStrings(String str) {
    int counter = 0;

    for (int i = 0; i < line.length(); i++) {
        if (line.charAt(i) == '_') {
            counter++;
        }
    }
    return counter;
}

/**
 * Replaces all single instances of underscores in the line given by line with italics tags. There must be an even
 * number of underscores in the line, and they will be replaced by <I>, </I>, alternating.
 * <p>
 * @return the line with single instances of underscores replaced with
 * <I> tags or the original line if there are not an even number of underscores.
 * <p>
 */
public String convertItalics() {
    String toReturn = " ";

    if (countStrings("_") % 2 == 0) { //makes sure there is even number of "_"
        for (int i = 0; i < line.length(); i++) {
            if (line.indexOf("_") + i == line.indexOf("_")) {
                toReturn += line.replace("_", "<I>");
            }
        }
    } else if (countStrings("_") % 2 != 0) {
        toReturn += line.replace("_", " ");
    }
    return toReturn;
}