@Override
    public String get(Integer key) {
        if (key == null || key < 0) {
            return null;
        }
        return cache.get(key);
    }
Putting temporary data directly into a map is a little more efficient but very memory intensive

@author Jiaju Zhuang
fastexcel-core/src/main/java/cn/idev/excel/cache/MapCache.java