참고자료
https://futurecreator.github.io/2018/08/26/java-8-streams/ http://iloveulhj.github.io/posts/java/java-stream-api.html https://homoefficio.github.io/2016/06/26/for-loop-를-Stream-forEach-로-바꾸지-말아야-할-3가지-이유/
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