In class org.apache.commons.collections(4).MapUtils there's a method putAll(final Map<K, V> map, final Object array) which starts with
Actually map.size() is not that harmless call for any map. For example, consider java.util.concurrent.ConcurrentHashMap size() implementation: depending on the circumstances it may take even more time than the rest of the putAll method (at least prior to JDK 8). Things are even worse for ConcurrentSkipListMap: its size() method iterates over all the map elements. Thus I'd suggest to replace this call with more conventional check like:
If you still want to save bytes, you may use map.getClass(). It's final, so it will never be overridden to do something strange and it's even used by JavaC for the same purpose (to trigger NPE). For example, if you compile and disassemble this code:
You will see a getClass() call which is used to trigger NPE.