掌握 Java Stream toMap：在键已存在时如何累加值

public static <T, K, U, M extends Map<K, U>> Collector<T, ?, M> toMap(
    Function<? super T, ? extends K> keyMapper,
    Function<? super T, ? extends U> valueMapper,
    BinaryOperator<U> mergeFunction,
    Supplier<M> mapFactory)

public Map<PositionKey, BigDecimal> getMap(final Long portfolioId) {
    final Map<PositionKey, BigDecimal> map = new HashMap<>(); // 外部创建 Map

    return getPositions(portfolioId).stream()
        .collect(
            Collectors.toMap(
                position -> new PositionKey(position.getAssetId(), position.getCurrencyId()),
                position -> position.getValue(),
                (oldValue, newValue) -> oldValue.add(newValue),
                () -> map // 将外部 Map 传递给 mapFactory
            ));
}

import java.math.BigDecimal;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.stream.Collectors;

/**
 * 模拟 PositionKey 类，作为 Map 的键
 */
class PositionKey {
    String assetId;
    String currencyId;

    public PositionKey(String assetId, String currencyId) {
        this.assetId = assetId;
        this.currencyId = currencyId;
    }

    // 必须重写 equals 和 hashCode，因为 PositionKey 将作为 Map 的键
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        PositionKey that = (PositionKey) o;
        return Objects.equals(assetId, that.assetId) &&
               Objects.equals(currencyId, that.currencyId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(assetId, currencyId);
    }

    @Override
    public String toString() {
        return "PositionKey{" +
               "assetId='" + assetId + '\'' +
               ", currencyId='" + currencyId + '\'' +
               '}';
    }
}

/**
 * 模拟 Position 类，包含资产信息和值
 */
class Position {
    String assetId;
    String currencyId;
    BigDecimal value; // 使用 BigDecimal 处理金额，避免浮点数精度问题

    public Position(String assetId, String currencyId, BigDecimal value) {
        this.assetId = assetId;
        this.currencyId = currencyId;
    }

    public String getAssetId() { return assetId; }
    public String getCurrencyId() { return currencyId; }
    public BigDecimal getValue() { return value; }
}

public class StreamAggregationTutorial {

    // 模拟获取头寸列表的方法
    private List<Position> getPositions(Long portfolioId) {
        // 实际应用中这里会从数据库或其他数据源获取数据
        // 示例数据，包含重复的 PositionKey
        return List.of(
            new Position("AAPL", "USD", new BigDecimal("100.50")),
            new Position("GOOG", "USD", new BigDecimal("200.75")),
            new Position("AAPL", "USD", new BigDecimal("50.25")), // 键重复，值需要累加
            new Position("TSLA", "EUR", new BigDecimal("75.00")),
            new Position("GOOG", "USD", new BigDecimal("10.00"))  // 键重复，值需要累加
        );
    }

    /**
     * 使用 Stream API 聚合头寸数据到 Map，并累加重复键的值。
     *
     * @param portfolioId 投资组合ID
     * @return 聚合后的 Map<PositionKey, BigDecimal>
     */
    public Map<PositionKey, BigDecimal> getAggregatedPositionsMap(final Long portfolioId) {
        return getPositions(portfolioId).stream()
            .collect(
                Collectors.toMap(
                    position -> new PositionKey(position.getAssetId(), position.getCurrencyId()), // keyMapper: 创建 PositionKey 作为键
                    Position::getValue, // valueMapper: 提取 Position 的值
                    (oldValue, newValue) -> oldValue.add(newValue), // mergeFunction: 当键重复时，将旧值与新值相加
                    HashMap::new // mapFactory: 提供一个新的 HashMap 实例
                )
            );
    }

    public static void main(String[] args) {
        StreamAggregationTutorial example = new StreamAggregationTutorial();
        Map<PositionKey, BigDecimal> aggregatedMap = example.getAggregatedPositionsMap(123L);

        System.out.println("聚合后的头寸映射：");
        aggregatedMap.forEach((key, value) -> System.out.println(key + " -> " + value));

        // 预期输出示例:
        // PositionKey{assetId='AAPL', currencyId='USD'} -> 150.75
        // PositionKey{assetId='GOOG', currencyId='USD'} -> 210.75
        // PositionKey{assetId='TSLA', currencyId='EUR'} -> 75.00
    }
}