鉴于以下List
类的Transaction
,使用Java 8 lambdas,我希望获得List
ResultantDTO
,每个帐户类型一个。
public class Transaction {
private final BigDecimal amount;
private final String accountType;
private final String accountNumber;
}
public class ResultantDTO {
private final List<Transaction> transactionsForAccount;
public ResultantDTO(List<Transaction> transactionsForAccount){
this.transactionsForAccount = transactionsForAccount;
}
}
到目前为止,我使用以下代码将List<Transaction>
分组为accountType
。
Map<String, List<Transaction>> transactionsGroupedByAccountType = transactions
.stream()
.collect(groupingBy(Transaction::getAccountType));
如何返回List<ResultantDTO>
,将List从每个地图键传递到构造函数中,每个ResultantDTO
包含一个accountType
?
答案 0 :(得分:5)
您可以在单流操作中执行此操作:
public List<ResultantDTO> convert(List<Transaction> transactions) {
return transactions.stream().collect(
collectingAndThen(
groupingBy(
Transaction::getAccountType,
collectingAndThen(toList(), ResultantDTO::new)),
map -> new ArrayList<>(map.values())));
}
此处collectingAndThen
使用了两次:一次用于下游Collector
将列表转换为ResultantDTO
个对象,一次将结果映射转换为其值列表。
答案 1 :(得分:2)
假设您有:
static ResultantDTO from(List<Transaction> transactions) {...}
你可以写:
Map<String, List<Transaction>> transactionsGroupedByAccountType = transactions
.stream()
.collect(groupingBy(Transaction::getAccountType));
Map<String, ResultantDTO> result = transactionsGroupedByAccountType.entrySet().stream()
.collect(toMap(Entry::getKey, e -> from(e.getValue)));
您可以在一个流中执行此操作,但这可能更简洁。
答案 2 :(得分:1)
您可以使用toMap
收藏家:
Map<String, ResultantDTO> transactionsGroupedByAccountType = transactions
.stream()
.collect(toMap(Transaction::getAccountType,
t -> new ResultantDTO(t.getAmount(),
Stream.of(new SimpleEntry<>(t.getAccountNumber(), t.getAmount()))
.collect(toMap(SimpleEntry::getKey, SimpleEntry::getValue))),
(dt1, dt2) -> new ResultantDTO(dt1.getSumOfAmountForAccountType().add(dt2.getSumOfAmountForAccountType()),
Stream.of(dt1.getAccountNumberToSumOfAmountForAccountNumberMap(), dt2.getAccountNumberToSumOfAmountForAccountNumberMap())
.flatMap(m -> m.entrySet().stream())
.collect(toMap(Map.Entry::getKey, Map.Entry::getValue)))));
虽然它不太可读。也许您可以添加一个构造函数,该构造函数在Transaction
类中使用单个ResultantDTO
作为参数和合并:
public ResultantDTO(Transaction t) {
///
}
static ResultantDTO merger(ResultantDTO r1, ResultantDTO r2) {
///
}
并且更具可读性:
Map<String, ResultantDTO> transactionsGroupedByAccountType = transactions
.stream()
.collect(toMap(Transaction::getAccountType,
ResultantDTO::new,
ResultantDTO::merger));