[英]Stream of a nested POJOs List to a another POJO LIST
希望可以有人帮帮我。 我有一个POJO,结构如下:
public class Invoice{
private String docNum;
private String customer;
private ArrayList<InvoiceDetails> invoiceDetails;
/* Getters and setters*/
}
还有另外一个POJO如下
public class InvoiceDetails{
private String vatGroup;
private Double vatAmount;
private Double amount;
/* Getters and setters*/
}
除此之外我还有第三个以下内容
public class VatType{
private String vatGroup;
private Double vatAmount;
private Double amount;
/* Getters and setters*/
}
我要做的是将Invoice
List
减少为按VatType
分组的VatType List
。 就像SQL
de DISTINCT
子句一样。 假设我有以下列表:
InvoiceDetails idA1 = new InvoiceDetails("S1", 100.0, 40.0);
InvoiceDetails idA2 = new InvoiceDetails("S2", 140.0, 40.0);
InvoiceDetails idA3 = new InvoiceDetails("A1", 50.0, 10.0);
ArrayList<InvoiceDetails> listA = new ArrayList<>();
listA.add(idA1);
listA.add(idA2);
listA.add(idA3);
Invoice invoiceA = new Invoice();
invoiceA.setDetailList(listA);
InvoiceDetails idB1 = new InvoiceDetails("S1", 200.0, 50.0);
InvoiceDetails idB2 = new InvoiceDetails("S2", 240.0, 50.0);
InvoiceDetails idB2 = new InvoiceDetails("A1", 100.0, 20.0);
ArrayList<InvoiceDetails> listB = new ArrayList<>();
listB.add(idB1);
listB.add(idB2);
listB.add(idB3);
Invoice invoiceB = new Invoice();
invoiceB.setDetailList(listB);
List<Invoice> invoiceList = new ArrayList<>();
invoiceList.add(invoiceA);
invoiceList.add(invoiceB);
预期结果将是具有以下项目的VatType
List
:
("S1",300.0,90.0)
("S2",380.0,90.0)
("A1",150.0,30.0)
如何一次性使用stream
获取此列表。 避免迭代Lists
? 提前致谢
首先需要一个flatMap来创建列表中所有Invoice
中所有InvoiceDetails
的流。 之后,您可以使用获取合并方法的toMap变体进行缩减 。 最后, Map
values()
方法将获得VatType
的Collection
:
Collection<VatType> values = invoiceList.stream()
.flatMap(invoices -> invoices.getInvoiceDetails().stream())
.collect(toMap(InvoiceDetails::getVatGroup, VatType::new, (i1, i2) -> {
i1.setAmount(i1.getAmount() + i2.getAmount());
i1.setVatAmount(i1.getVatAmount() + i2.getVatAmount());
return i1;
}))
.values();
使用此VatType
构造函数的位置:
VatType(InvoiceDetails invoiceDetails) {
vatGroup = invoiceDetails.getVatGroup();
vatAmount = invoiceDetails.getVatAmount();
amount = invoiceDetails.getAmount();
}
如果需要,您可以轻松地从Collection
创建List
:
List<VatType> vatTypes = new ArrayList<>(values);
您可以使用Java流功能:
Function<InvoiceDetails, VatType> myF = t -> new VatType(t.getVatGroup(), t.getVatAmount(), t.getAmount());
然后流比使用:
List<VatType> myLocations =
invoiceList.stream()
.map(Invoice::getInvoiceDetails)
.flatMap(Collection::stream)
.map(myF)
.collect(Collectors.<VatType>toList());
这是使用Java 8的List
和Map
的新功能来实现它的方法,没有流:
Map<String, VatType> map = new HashMap<>();
invoiceList.forEach(i -> i.getDetailList().forEach(d ->
map.merge(
d.getVatGroup(),
new VatType(d.getVatGroup(), d.getVatAmount(), d.getAmount()),
(left, right) -> {
left.setAmount(left.getAmount() + right.getAmount());
left.setVatAmount(left.getVatAmount() + right.getVatAmount());
return left;
})));
List<VatType> result = new ArrayList<>(map.values());
如果您可以将构造函数添加到接受InvoiceDetail
实例的VatType
和组合两个VatType
实例的merge
方法:
public VatType(String vatGroup, Double vatAmount, Double amount) {
this.vatGroup = vatGroup;
this.vatAmount = vatAmount;
this.amount = amount;
}
public VatType(InvoiceDetails details) {
this(details.getVatGroup(), details.getVatAmount(), details.getAmount());
}
public VatType merge(VatType another) {
this.vatAmount += another.vatAmount;
this.amount += another.amount;
return this;
}
然后,您可以将第一个代码段简化为:
Map<String, VatType> map = new HashMap<>();
invoiceList.forEach(i -> i.getDetailList().forEach(d ->
map.merge(d.getVatGroup(), new VatType(d), VatType::merge)));
List<VatType> result = new ArrayList<>(map.values());
如果这是项目中的常见任务,您还可以使用dozer库 。
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