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。
