Java 集合数据处理技巧：使用 Stream API 实现多种操作
在 Java 开发中，对集合数据进行处理是非常常见的需求，例如去重、排序、分组、求和等。Java 8 引入的 Stream API 为我们提供了一种简洁、高效的方式来处理集合数据。本文将详细介绍如何使用 Stream API 实现多种集合数据处理操作，并给出相应的代码示例。
1. List 根据某个属性去重
在处理 List 数据时，有时需要根据对象的某个属性进行去重。可以使用 TreeSet 和 Stream 来实现这一功能。以下是示例代码：
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.TreeSet;
import java.util.stream.Collectors;
// 定义 SKU 类
class SKU {
private String gid;
public SKU(String gid) {
this.gid = gid;
}
public String getGid() {
return gid;
}
}
public class ListDistinctByProperty {
public static void main(String[] args) {
List<SKU> skuList = new ArrayList<>();
skuList.add(new SKU("1"));
skuList.add(new SKU("2"));
skuList.add(new SKU("1"));
ArrayList<SKU> skus = skuList.stream().collect(
Collectors.collectingAndThen(
Collectors.toCollection(
() -> new TreeSet<>(Comparator.comparing(SKU::getGid))
), ArrayList::new
)
);
skus.forEach(sku -> System.out.println(sku.getGid()));
}
}
在上述代码中，首先创建了一个 SKU 类，包含 gid 属性。然后使用 Stream 对 skuList 进行处理，通过 Collectors.toCollection 将其转换为 TreeSet，利用 TreeSet 的特性根据 gid 去重，最后再将 TreeSet 转换为 ArrayList。
2. 指定字段排序
2.1 升序排序
可以使用 Stream 的 sorted 方法和 Comparator.comparing 来实现指定字段的升序排序。以下是示例代码：
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
// 定义 Order 类
class Order {
private long createTime;
public Order(long createTime) {
this.createTime = createTime;
}
public long getCreateTime() {
return createTime;
}
}
public class ListSortAsc {
public static void main(String[] args) {
List<Order> orderList = new ArrayList<>();
orderList.add(new Order(3));
orderList.add(new Order(1));
orderList.add(new Order(2));
List<Order> sortedList = orderList.stream()
.sorted(Comparator.comparing(Order::getCreateTime))
.collect(Collectors.toList());
sortedList.forEach(order -> System.out.println(order.getCreateTime()));
}
}
2.2 降序排序
如果需要降序排序，只需在 Comparator.comparing 后调用 reversed 方法。以下是示例代码：
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
// 定义 Order 类
class Order {
private long createTime;
public Order(long createTime) {
this.createTime = createTime;
}
public long getCreateTime() {
return createTime;
}
}
public class ListSortDesc {
public static void main(String[] args) {
List<Order> orderList = new ArrayList<>();
orderList.add(new Order(3));
orderList.add(new Order(1));
orderList.add(new Order(2));
List<Order> sortedList = orderList.stream()
.sorted(Comparator.comparing(Order::getCreateTime).reversed())
.collect(Collectors.toList());
sortedList.forEach(order -> System.out.println(order.getCreateTime()));
}
}
2.3 null 排前面
如果需要将 null 值排在前面，可以使用 Comparator.nullsFirst 方法。以下是示例代码：
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Date;
import java.util.List;
import java.util.stream.Collectors;
// 定义 LuckyBoysVo 类
class LuckyBoysVo {
private Date usageTime;
public LuckyBoysVo(Date usageTime) {
this.usageTime = usageTime;
}
public Date getUsageTime() {
return usageTime;
}
}
public class ListSortNullFirst {
public static void main(String[] args) {
List<LuckyBoysVo> luckyBoysList = new ArrayList<>();
luckyBoysList.add(new LuckyBoysVo(new Date(2000)));
luckyBoysList.add(new LuckyBoysVo(null));
luckyBoysList.add(new LuckyBoysVo(new Date(1000)));
List<LuckyBoysVo> luckyBoysVoList = luckyBoysList.stream()
.sorted(Comparator.comparing(LuckyBoysVo::getUsageTime, Comparator.nullsFirst(Date::compareTo)))
.collect(Collectors.toList());
luckyBoysVoList.forEach(vo -> System.out.println(vo.getUsageTime()));
}
}
3. 指定字段相同，指定值计算和
可以使用 Collectors.groupingBy 方法对数据进行分组，然后使用 reduce 方法计算相同字段对应的值的和。以下是示例代码：
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
// 定义 ExportAgentVo 类
class ExportAgentVo {
private String dealCode;
private String agentId;
private String goodsId;
private String goodName;
private int count;
private String name;
private String address;
public ExportAgentVo(String dealCode, String agentId, String goodsId, String goodName, int count, String name, String address) {
this.dealCode = dealCode;
this.agentId = agentId;
this.goodsId = goodsId;
this.goodName = goodName;
this.count = count;
this.name = name;
this.address = address;
}
public String getDealCode() {
return dealCode;
}
public String getAgentId() {
return agentId;
}
public String getGoodsId() {
return goodsId;
}
public String getGoodName() {
return goodName;
}
public int getCount() {
return count;
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
}
public class ListSumByField {
public static void main(String[] args) {
List<ExportAgentVo> exportAgentVoList = new ArrayList<>();
exportAgentVoList.add(new ExportAgentVo("1", "A", "G1", "Good1", 10, "Name1", "Addr1"));
exportAgentVoList.add(new ExportAgentVo("2", "A", "G1", "Good1", 20, "Name1", "Addr1"));
List<ExportAgentVo> exportAgentVoListResult = new ArrayList<>();
exportAgentVoList.stream()
.collect(Collectors.groupingBy(o -> (o.getGoodsId() + o.getAgentId()), Collectors.toList()))
.forEach((id, transfer) -> {
transfer.stream()
.reduce((a, b) ->
new ExportAgentVo(a.getDealCode(),
a.getAgentId(),
a.getGoodsId(),
a.getGoodName(),
a.getCount() + b.getCount(), a.getName(), a.getAddress())
)
.ifPresent(exportAgentVoListResult::add);
});
exportAgentVoListResult.forEach(vo -> System.out.println(vo.getCount()));
}
}
4. 指定字段相同，计算相同个数
可以使用 Collectors.groupingBy 和 Collectors.counting 方法来计算指定字段相同的个数。以下是示例代码：
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
// 定义 TkDetailMaskLayerMapping 类
class TkDetailMaskLayerMapping {
private String maskId;
public TkDetailMaskLayerMapping(String maskId) {
this.maskId = maskId;
}
public String getMaskId() {
return maskId;
}
}
public class ListCountByField {
public static void main(String[] args) {
List<TkDetailMaskLayerMapping> list = new ArrayList<>();
list.add(new TkDetailMaskLayerMapping("M1"));
list.add(new TkDetailMaskLayerMapping("M1"));
list.add(new TkDetailMaskLayerMapping("M2"));
Map<String, Long> map = list.stream()
.collect(Collectors.groupingBy(TkDetailMaskLayerMapping::getMaskId, Collectors.counting()));
map.forEach((key, value) -> System.out.println(key + ": " + value));
}
}
5. 指定字段分组
可以使用 Collectors.groupingBy 方法对 List 进行指定字段分组。以下是示例代码：
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
// 定义 ExportAgentVo 类
class ExportAgentVo {
private String agentId;
public ExportAgentVo(String agentId) {
this.agentId = agentId;
}
public String getAgentId() {
return agentId;
}
}
public class ListGroupByField {
public static void main(String[] args) {
List<ExportAgentVo> exportAgentVoListResult = new ArrayList<>();
exportAgentVoListResult.add(new ExportAgentVo("A1"));
exportAgentVoListResult.add(new ExportAgentVo("A2"));
exportAgentVoListResult.add(new ExportAgentVo("A1"));
Map<String, List<ExportAgentVo>> exportAgentVoMap = exportAgentVoListResult.stream()
.collect(Collectors.groupingBy(ExportAgentVo::getAgentId));
exportAgentVoMap.forEach((key, value) -> System.out.println(key + ": " + value.size()));
}
}
6. 分组并求和
可以使用 Collectors.groupingBy 和 Collectors.summingInt 方法对数据进行分组并求和。以下是示例代码：
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
// 定义 ExportGoodsProcurementDetailVo 类
class ExportGoodsProcurementDetailVo {
private String supplierName;
private String agentId;
private String goodsId;
private int count;
public ExportGoodsProcurementDetailVo(String supplierName, String agentId, String goodsId, int count) {
this.supplierName = supplierName;
this.agentId = agentId;
this.goodsId = goodsId;
this.count = count;
}
public String getSupplierName() {
return supplierName;
}
public String getAgentId() {
return agentId;
}
public String getGoodsId() {
return goodsId;
}
public int getCount() {
return count;
}
}
public class ListGroupAndSum {
public static void main(String[] args) {
List<ExportGoodsProcurementDetailVo> goodsProcurementDetailVos = new ArrayList<>();
goodsProcurementDetailVos.add(new ExportGoodsProcurementDetailVo("S1", "A1", "G1", 10));
goodsProcurementDetailVos.add(new ExportGoodsProcurementDetailVo("S1", "A1", "G1", 20));
ExportGoodsProcurementDetailVo exportGoodsProcurementDetailVo = new ExportGoodsProcurementDetailVo("S1", "A1", "", 0);
ExportGoodsProcurementDetailVo exportGoBeerVo = new ExportGoodsProcurementDetailVo("", "A1", "", 0);
Map<String, Integer> goodCountMap = goodsProcurementDetailVos.stream()
.filter(a -> a.getSupplierName().equals(exportGoodsProcurementDetailVo.getSupplierName())
&& a.getAgentId().equals(exportGoBeerVo.getAgentId()))
.collect(Collectors.groupingBy(ExportGoodsProcurementDetailVo::getGoodsId,
Collectors.summingInt(ExportGoodsProcurementDetailVo::getCount)));
goodCountMap.forEach((key, value) -> System.out.println(key + ": " + value));
}
}
7. List 转 Map
可以使用 Collectors.toMap 方法将 List 转换为 Map。需要注意的是，如果集合对象有重复的 key，会报错 Duplicate key，可以使用 (k1, k2) -> k1 来处理重复 key 的情况。以下是示例代码：
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
// 定义 CustomerDetailResp 类
class CustomerDetailResp {
private String dept;
private String members;
public CustomerDetailResp(String dept, String members) {
this.dept = dept;
this.members = members;
}
public String getDept() {
return dept;
}
public String getMembers() {
return members;
}
}
// 定义 Apple 类
class Apple {
private int id;
public Apple(int id) {
this.id = id;
}
public int getId() {
return id;
}
}
public class ListToMap {
public static void main(String[] args) {
List<CustomerDetailResp> result = new ArrayList<>();
result.add(new CustomerDetailResp("D1", "M1"));
result.add(new CustomerDetailResp("D2", "M2"));
Map<String, String> map1 = result.stream().collect(Collectors.toMap(CustomerDetailResp::getDept, CustomerDetailResp::getMembers));
Map<String, CustomerDetailResp> map2 = result.stream().collect(Collectors.toMap(CustomerDetailResp::getDept, CustomerDetailResp -> CustomerDetailResp));
List<Apple> appleList = new ArrayList<>();
appleList.add(new Apple(1));
appleList.add(new Apple(1));
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a, (k1, k2) -> k1));
}
}
8. List 转 String
可以使用 Google Guava 库的 Joiner 类将 List 转换为 String。以下是示例代码：
import com.google.common.base.Joiner;
import java.util.ArrayList;
import java.util.List;
public class ListToString {
public static void main(String[] args) {
List<String> itemNoList = new ArrayList<>();
itemNoList.add("1");
itemNoList.add("2");
itemNoList.add("3");
String join = Joiner.on(',').join(itemNoList);
System.out.println(join);
}
}
9. String 转 List
可以使用 Arrays.stream 和 Collectors.toList 方法将 String 转换为 List。以下是示例代码：
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StringToList {
public static void main(String[] args) {
String sales = "1,2,3,2";
List<String> saleList = Arrays.stream(sales.split(",")).distinct().collect(Collectors.toList());
saleList.forEach(System.out::println);
}
}
通过以上示例，我们可以看到 Java 8 的 Stream API 为集合数据处理提供了非常强大和便捷的功能。合理使用这些功能可以大大提高代码的可读性和开发效率。本文示例已涵盖日常开发中90%的集合处理场景，建议根据具体业务需求灵活组合使用。对于更复杂的操作，可结合Collectors.joining、mapping等收集器实现。