前言

今天想用下jdk1.8的新特性，对记账流水进行按照账户id进行统计汇总金额，查找jdk1.8的新特性发现没有实现对BigDecimal的求和方法。一开始用下面的方式搞定的，比较土鳖。另外还有个需求，更新账户余额的时候，为了避免死锁，要按照账户号顺序进行更新，这就涉及到汇总后按账户号排序的问题，这里用的是TreeMap。

第一版实现

TreeMap<Long, List<AccountingDetailVo>> resultList222 = voList.stream().collect(
        Collectors.groupingBy(AccountingDetailVo::getAcctNo,TreeMap::new,Collectors.toList()));

    TreeMap<Long, BigDecimal> acctMap = new TreeMap<>();
    for (Entry<Long, List<AccountingDetailVo>> entry:resultList222.entrySet()) {
      acctMap.put(entry.getKey(),
          entry.getValue().stream().map(AccountingDetailVo::getAmtByBalDir).reduce(BigDecimal.ZERO, BigDecimal::add));
    }

后来从网上查了查，可以自定义实现，参考文章我给关了，这里就不引用了，具体实现如下：

自定义个Function

@FunctionalInterface
public interface ToBigDecimalFunction <T> {

  BigDecimal applyAsBigDecimal(T value);

}

自定义个CollectorsUtils

public class CollectorsUtils {

  static final Set<Characteristics> CH_NOID = Collections.emptySet();

  private CollectorsUtils() {
  }

  @SuppressWarnings("unchecked")
  private static <I, R> Function<I, R> castingIdentity() {
    return i -> (R) i;
  }

  //自定义个Collector实现类
  static class CollectorImpl<T, A, R> implements Collector<T, A, R> {
    private final Supplier<A> supplier;
    private final BiConsumer<A, T> accumulator;
    private final BinaryOperator<A> combiner;
    private final Function<A, R> finisher;
    private final Set<Characteristics> characteristics;

    CollectorImpl(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner,
        Function<A, R> finisher, Set<Characteristics> characteristics) {
      this.supplier = supplier;
      this.accumulator = accumulator;
      this.combiner = combiner;
      this.finisher = finisher;
      this.characteristics = characteristics;
    }

    CollectorImpl(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner,
        Set<Characteristics> characteristics) {
      this(supplier, accumulator, combiner, castingIdentity(), characteristics);
    }

    @Override
    public BiConsumer<A, T> accumulator() {
      return accumulator;
    }

    @Override
    public Supplier<A> supplier() {
      return supplier;
    }

    @Override
    public BinaryOperator<A> combiner() {
      return combiner;
    }

    @Override
    public Function<A, R> finisher() {
      return finisher;
    }

    @Override
    public Set<Characteristics> characteristics() {
      return characteristics;
    }
  }

  // 创建个汇总的方法
  public static <T> Collector<T, ?, BigDecimal> summingBigDecimal(ToBigDecimalFunction<? super T> mapper) {
    return new CollectorImpl<>(() -> new BigDecimal[1], (a, t) -> {
      if (a[0] == null) {
        a[0] = BigDecimal.ZERO;
      }
      a[0] = a[0].add(mapper.applyAsBigDecimal(t));
    }, (a, b) -> {
      a[0] = a[0].add(b[0]);
      return a;
    }, a -> a[0], CH_NOID);
  }

}

要计算的bean

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class AccountingDetailVo implements Serializable{


  private static final long serialVersionUID = 6226733035919673733L;

  @ApiModelProperty(value = "资金类型")
  private Long amtType;

  @ApiModelProperty(value = "账户")
  private Long acctNo;

  @ApiModelProperty(value = "账户类型")
  private Long acctType;

  @ApiModelProperty(value = "客户")
  private Long custNo;

  @ApiModelProperty(value = "发生方向")
  private String balDir;

  @ApiModelProperty(value = "金额")
  private BigDecimal amt;

  /**
   * 根据资金方向取得金额
   */
  public double getAmtByBalDirToDouble() {
    return this.getBalDir().equals(BalDirEnum.ADD.getValue()) ? this.getAmt().doubleValue() : this.getAmt().negate().doubleValue();
  }

  public BigDecimal getAmtByBalDir() {
    return this.getBalDir().equals(BalDirEnum.ADD.getValue()) ? this.getAmt() : this.getAmt().negate();
  }

}

测试用例

public class ListGroupTest2 {
  public static void main(String[] args) {

    List<AccountingDetailVo> voList = new ArrayList<>();
    AccountingDetailVo vo1 = AccountingDetailVo.builder().acctNo(1L).balDir("-").amt(BigDecimal.valueOf(500.22)).build();
    AccountingDetailVo vo2 = AccountingDetailVo.builder().acctNo(2L).balDir("+").amt(BigDecimal.valueOf(500.33)).build();
    AccountingDetailVo vo3 = AccountingDetailVo.builder().acctNo(1L).balDir("-").amt(BigDecimal.valueOf(500.44)).build();
    AccountingDetailVo vo4 = AccountingDetailVo.builder().acctNo(2L).balDir("+").amt(BigDecimal.valueOf(500.55)).build();
    AccountingDetailVo vo5 = AccountingDetailVo.builder().acctNo(3L).balDir("+").amt(BigDecimal.valueOf(500.66)).build();
    voList.add(vo1);
    voList.add(vo2);
    voList.add(vo3);
    voList.add(vo4);
    voList.add(vo5);
    TreeMap<Long, BigDecimal> resultList333 = voList.stream().collect(
        Collectors.groupingBy(AccountingDetailVo::getAcctNo,TreeMap::new, CollectorsUtils.summingBigDecimal(AccountingDetailVo::getAmtByBalDir)));

    System.out.println(JSON.toJSONString(resultList333, SerializerFeature.PrettyFormat));

  }
}

如果是多个字段要进行group by呢？

可以看下groupingBy的参数信息，第一个传入的是Function，可以在要计算的bean里面增加个get方法就好，把要group by的字段拼串吧。

Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream)