Java Stream

Collections and streams, while bearing some superficial similarities, have different goals. Collections are primarily concerned with the efficient management of, and access to, their elements. By contrast, streams do not provide a means to directly access or manipulate their elements, and are instead concerned with declaratively describing their source and the computational operations which will be performed in aggregate on that source. However, if the provided stream operations do not offer the desired functionality, the BaseStream.iterator() and BaseStream.spliterator() operations can be used to perform a controlled traversal. - Oracle Java 8

Stream에 대한 정리를 정말 잘한 링크

Stream 총정리
Stream 고급

Java Stream API는 왜 for-loop보다 느릴까?