# Lambda

## list -> map

#### account list -> account map.
map<id,account>

public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
}

public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, Function.identity()));
}

因為list有順序，但是map沒有，若要有順序，則可轉成 LinkedHashMap

Map<Long, RestorationScheme> restorationSchemeMap = queryRestorationScheme()
                .stream()
                .sorted(Comparator.comparing(RestorationScheme::getId, Comparator.naturalOrder()))
                .collect(Collectors.toMap(RestorationScheme::getId, Function.identity(), (e1, e2) -> e1, LinkedHashMap::new));

計算list中重複的string次數

Map<String, Long> countMap = strings.stream()
				.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

Pair<K,V> 雙重key值 找value

Map<Pair<Integer, Integer>, ActTemp> actTempMapByCompanyAndProductCode = actTempService.selectByEntity(searchDTO)
                .stream()
                .collect(Collectors.toMap(actTemp -> new Pair(actTemp.getCompanyCode(), actTemp.getProductCode()), Function.identity()));

reference: https://www.cnblogs.com/xujanus/p/6133865.html