public static <K, V> Map<K, V> singletonMap(final K key, final V value) {
		return Collections.singletonMap(key, value);
	}
public static < K , V > Map < K , V > singletonMap ( final K key , final V value ) { return Collections . singletonMap ( key , value ) ; }
返回一个只含一个元素但结构特殊的Map，节约空间.

注意返回的Map不可写, 写入会抛出UnsupportedOperationException.

@see java.util.Collections#singletonMap(Object, Object)