関連ワード:  java stream transform list to map    java stream convert list to map    java stream convert list to map duplicate key    java 8 convert list to map    java 8 convert list to map with object as value    java 8 convert list to map group by    java 8 convert list to map with composite key    java 8 convert list to map with custom key    java stream convert list to hashmap    java 8 stream convert list to map