public void applyFont(int startIndex, int endIndex, WriteFont writeFont) {
        if (intervalFontList == null) {
            intervalFontList = ListUtils.newArrayList();
        }
        intervalFontList.add(new IntervalFont(startIndex, endIndex, writeFont));
    }
Applies a font to the specified characters of a string.

@param startIndex The start index to apply the font to (inclusive)
@param endIndex   The end index to apply to font to (exclusive)
@param writeFont  The font to use.
fastexcel-core/src/main/java/cn/idev/excel/metadata/data/RichTextStringData.java