map() and flatMap()
概要
コピーして自身のテキストエディターで見たほうが見やすいです。
e.g. map() and flatMap() view @Java
# ========================================================================================
# List map()
# ==========
List<Member> memberList = memberBhv.selectList(...);
List<Integer> memberIdList = memberList.stream()
.map(mb -> mb.getMemberId()) // returns Integer
.collect(Collectors.toList());
+- List<Member> ---+ |- List<Integer> -----+
| | | |
| {1, "sea"} | ---- map() ----> | 1 |
| {2, "land"} | mb.getMemberId() | 2 |
| {3, "piary"} | | 3 |
| | | |
+------------------+ +---------------------+
empty list:
+- List<Member> ---+ |- List<Integer> -----+
| | | |
| *empty | ---- map() ----> | *empty |
| | mb.getMemberId() | |
| | | |
+------------------+ +---------------------+
# ========================================================================================
# List flatMap()
# ==============
List<Member> memberList = memberBhv.selectList(...);
memberBhv.loadPurchase(memberList, ...);
List<Purchase> purchaseList
= memberList.stream()
.flatMap(mb -> mb.getPurchaseList().stream()) // returns Stream
.collect(Collectors.toList());
+- List<Member> ---------------+ +- List<Purchase> ----+
| | | |
| {1, "sea"} | | {101, 1, ...} |
| |-{101, 1, ...} | | {102, 1, ...} |
| |-{102, 1, ...} | ---- flatMap() ----> | {103, 2, ...} |
| {2, "land"} | mb.getPurchaseList() | {104, 2, ...} |
| |-{103, 2, ...} | .stream() | {105, 3, ...} |
| |-{104, 2, ...} | | |
| {3, "piary"} | +---------------------+
| |-{105, 3, ...} |
| |
+------------------------------+
without flat:
+- List<Member> ---------------+ +- List<List<Purchase>> --+
| | | |
| {1, "sea"} | | +-----------------+ |
| |-{101, 1, ...} | | | {101, 1, ...} | |
| |-{102, 1, ...} | | | {102, 1, ...} | |
| {2, "land"} | ------ map() ------> | +-----------------+ |
| |-{103, 2, ...} | mb.getPurchaseList() | | {103, 2, ...} | |
| |-{104, 2, ...} | | | {104, 2, ...} | |
| {3, "piary"} | | +-----------------+ |
| |-{105, 3, ...} | | | {105, 3, ...} | |
| | | +-----------------+ |
+------------------------------+ +-------------------------+
empty list:
+- List<Member> ---------------+ +- List<Purchase> ----+
| | | |
| *empty | ---- flatMap() ----> | *empty |
| | mb.getPurchaseList() | |
| | .stream() | |
+------------------------------+ +---------------------+
# ========================================================================================
# Optional map()
# ==============
Optional<Member> optMember = memberBhv.selectEntity(...);
Optional<Integer> optMemberId = optMember.map(mb -> mb.getMemberId()); // returns Integer
+- Optional<Member> --+ +- Optional<Integer> ---+
| | | |
| {1, "sea"} | ---- map() -----> | (1) |
| | mb.getMemberId() | |
+---------------------+ +-----------------------+
+- Optional<Member> --+ +- Optional<Integer> ---+
| | | |
| *empty | ---- map() -----> | *empty |
| | mb.getMemberId() | |
+---------------------+ +-----------------------+
return null:
+- Optional<Member> --+ +- Optional<Integer> -+
| | | |
| {null, "sea"} | ---- map() -----> | *empty |
| | mb.getMemberId() | |
+---------------------+ +---------------------+
detail:
+- Optional<Member> --+ +- Optional<Integer> ---+
| | | |
| | ------------ map() -------------------> | |
| | | |
| {1, "sea"} | if present: | |
| or | mb.getMemberId() => (1) ----(wrap)---/`\---> (1) |
| {null, "sea"} | => null | |
| or | \------+ | or |
| *empty | else | | |
| | *empty-------(cast only)------+----->>| *empty |
| | | |
+---------------------+ +-----------------------+
# ========================================================================================
# Optional flatMap()
# ==================
Optional<Member> optMember = memberBhv.selectEntity(...);
Optional<MemberWithdrawal> optWdl = optMember.flatMap(mb -> mb.getMemberWithdrawalAsOne()) // returns Optional
+- Optional<Member> ---------------------+ +- Optional<MemberWithdrawal> ----+
| | | |
| mb:{1, "sea"} | -------- flatMap() ---------> | wdl:{1, 2016/7/20, ...} |
| | | mb.getMemberWithdrawalAsOne() | |
| | | +---------------------------------+
| | +- Optional<MemberWithdrawal> -+ | ^
| +-| wdl:{1, 2016/7/20, ...} |--/`\-->--->--->--->--->--->--->--->---^
| +------------------------------+ |
+----------------------------------------+
+- Optional<Member> ---------------------+ +- Optional<MemberWithdrawal> ----+
| | | |
| mb:{1, "sea"} | -------- flatMap() ---------> | *empty |
| | | mb.getMemberWithdrawalAsOne() | |
| | | +---------------------------------+
| | +- Optional<MemberWithdrawal> -+ | ^
| +-| *empty |--/`\-->--->--->--->--->--->--->--->---^
| +------------------------------+ |
+----------------------------------------+
+- Optional<Member> ---------------------+ +- Optional<MemberWithdrawal> ----+
| | | |
| *empty | -------- flatMap() ---------> | *empty |
| | mb.getMemberWithdrawalAsOne() | |
+----------------------------------------+ +---------------------------------+
without flat:
+- Optional<Member> --------------------+ +- Optional<Optional<MemberWithdrawal>> -+
| | | |
| mb:{1, "sea"} | ----------- map() ----------> | +- Optional<MemberWithdrawal> -+ |
| | | mb.getMemberWithdrawalAsOne() | | {101, 1, ...} | |
| | | | +------------------------------+ |
| | +- Optional<MemberWithdrawal> -+ | | ^ |
| +-| wdl:{1, 2016/7/20, ...} |-/`\-->--->--->--->--->--->--->--/`\-->---^ |
| +------------------------------+ | | |
+---------------------------------------+ +----------------------------------------+
detail:
+- Optional<Member> -------------+ +- Optional<MemberWithdrawal> -+
| | | |
| | -------------- flatMap() -----------------> | |
| | | |
| mb:{1, "sea"} - wdl:{101, ...} | if present: | |
| | mb.getMemberWithdrawalAsOne() | |
| or | => Optional({101, ...}) | |
| | \--(non wrap)---(null check)--->>| {101, ...} |
| mb:{2, "land"} - *empty | => *empty | |
| or | \------------------+ | or |
| *empty | else | | |
| | *empty -------(cast only)---+----------->>| *empty |
| | | |
+--------------------------------+ +------------------------------+
