Java 8 lambdas分组和映射
给定以下Java 8 lambdas分组和映射,java,java-8,java-stream,Java,Java 8,Java Stream,给定以下事务类的列表,使用Java 8 lambdas,我想获得ResultantTo的列表,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
事务类的列表
,使用Java 8 lambdas,我想获得ResultantTo的列表
,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;
}
}
如何返回一个列表
,将列表从每个映射键传递到构造函数中,每个accountType都包含一个ResultantTo
static ResultantDTO from(List<Transaction> transactions) {...}
static resultdto from(列表事务){…}
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)));
映射事务GroupedByAccountType=事务
.stream()
.collect(分组依据(事务::getAccountType));
映射结果=TransactionGroupedByAccountType.entrySet().stream()
.collect(toMap(Entry::getKey,e->from(e.getValue));
您可以在一个流中执行此操作,但这可能更干净、更简单。您可以使用toMap
收集器:
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)))));
在那里它更具可读性:
Map<String, ResultantDTO> transactionsGroupedByAccountType = transactions
.stream()
.collect(toMap(Transaction::getAccountType,
ResultantDTO::new,
ResultantDTO::merger));
映射事务GroupedByAccountType=事务
.stream()
.collect(toMap)(事务::getAccountType,
ResultantDTO::新建,
ResultantDTO::合并);
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())));
}
公共列表转换(列表事务){
return transactions.stream().collect(
收集然后(
分组依据(
事务::getAccountType,
收集然后(toList(),ResultantTo::new)),
map->newarraylist(map.values());
}
这里使用了两次collecting和:一次用于下游Collector
将列表转换为ResultantDTO
对象,一次用于将结果映射转换为其值列表。您能展示一下,如果不使用流
操作,您将如何做您想要做的事情吗?
Map<String, ResultantDTO> transactionsGroupedByAccountType = transactions
.stream()
.collect(toMap(Transaction::getAccountType,
ResultantDTO::new,
ResultantDTO::merger));
public List<ResultantDTO> convert(List<Transaction> transactions) {
return transactions.stream().collect(
collectingAndThen(
groupingBy(
Transaction::getAccountType,
collectingAndThen(toList(), ResultantDTO::new)),
map -> new ArrayList<>(map.values())));
}