@SuppressWarnings("unchecked")
    protected <K, V2> Map<K, V2> newMap() {
        try {
            return (Map<K, V2>) protoMapClass.newInstance();
        } catch (InstantiationException e) {
            throw new IllegalArgumentException("wrong proto type map "
                    + protoMapClass);
        } catch (IllegalAccessException e) {
            throw new IllegalArgumentException("wrong proto type map "
                    + protoMapClass);
        }
    }
multi-key map, some basic objects *

@author yihua.huang
webmagic-extension/src/main/java/us/codecraft/webmagic/utils/MultiKeyMapBase.java