Java8收集两个列表以按条件映射
我有一个目标:Java8收集两个列表以按条件映射,java,collections,java-8,java-stream,Java,Collections,Java 8,Java Stream,我有一个目标: public class CurrencyItem { private CurrencyName name; private BigDecimal buy; private BigDecimal sale; private Date date; //... } 其中,CurrencyName是以下其中之一:欧元、美元、卢布等 和两份清单 List<CurrencyItem> currenciesByCommercialBank
public class CurrencyItem {
private CurrencyName name;
private BigDecimal buy;
private BigDecimal sale;
private Date date;
//...
}
其中,CurrencyName
是以下其中之一:欧元、美元、卢布等
和两份清单
List<CurrencyItem> currenciesByCommercialBank = ...
List<CurrencyItem> currenciesByCentralBank = ...
下面的代码是O(n2),但对于小集合(您的列表可能是这样的)应该可以:
在这种情况下,映射将包含所有匹配项,并且不会抱怨缺少条目。这应该是最佳选择。您首先构建一张从货币到商业银行的地图。然后,通过中心线构建一个从商业到中心的地图(在第一个地图中查找)
List currencesbycommercialbank=new ArrayList();
List CurrencesByCentralBank=新建ArrayList();
//从CurrencyName到CommercialBank构建我的查找。
Map commercials=商业银行的货币
.stream()
.收集(
汤姆(
//从货币名称映射。
ci->ci.getName(),
//给商业银行本身。
ci->ci);
Map commercialToCentral=中央银行的货币
.stream()
.收集(
汤姆(
//等效商业地图
ci->commercials.get(ci.getName()),
//到这个中心。
ci->ci
));
currenciesByCommercialBank.CurrencyName == currenciesByCentralBank.CurrencyName
return currenciesByCommercialBank
.stream()
.map(c ->
new AbstractMap.SimpleImmutableEntry<>(
c, currenciesByCentralBank.stream()
.filter(c2 -> c.currencyName == c2.currencyName)
.findFirst()
.get()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
}
currenciesByCommercialBank
.stream()
.flatMap(c ->
currenciesByCentralBank.stream()
.filter(c2 -> c.currencyName == c2.currencyName)
.map(c2 -> new AbstractMap.SimpleImmutableEntry<>(c, c2)))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
List<CurrencyItem> currenciesByCommercialBank = new ArrayList<>();
List<CurrencyItem> currenciesByCentralBank = new ArrayList<>();
// Build my lookup from CurrencyName to CommercialBank.
Map<CurrencyName, CurrencyItem> commercials = currenciesByCommercialBank
.stream()
.collect(
Collectors.toMap(
// Map from currency name.
ci -> ci.getName(),
// To the commercial bank itself.
ci -> ci));
Map<CurrencyItem, CurrencyItem> commercialToCentral = currenciesByCentralBank
.stream()
.collect(
Collectors.toMap(
// Map from the equivalent commercial
ci -> commercials.get(ci.getName()),
// To this central.
ci -> ci
));