Stream流

1.Stream流

1.1 分组

                Map<String, List<PaymentVarietyStatisticsVo>> mapList =         
                paymentVarietyStatisticsVoList.stream()
                .collect(Collectors.groupingBy(item -> {
                    if (Strings.isNullOrEmpty(item.getProductName())) {
                        return "";
                    } else return item.getProductName();
                }));

将List集合中的数据按照某个字段分组，返回map

1.2 求和

        DecimalFormat doubleFormatter = new DecimalFormat("#.00");
        doubleFormatter.setRoundingMode(RoundingMode.HALF_UP);
        for (String s : result.keySet()) {
            List<Map<String, Object>> list = result.get(s);
            Double weightSum = list.stream().mapToDouble(item ->                 
            MapUtils.getDoubleValue(item, "value")).sum();
            String format = doubleFormatter.format(weightSum);
            System.err.println(format);
            list.add(new HashMap<String, Object>() {{
                this.put("timeColumn", "合计");
                this.put("value", new 
            BigDecimal(format).stripTrailingZeros().toPlainString());
            }});
        }

创建一个DecimalFormat对象，用来格式化结果

list.stream().mapToDouble(User::getName).sum();求和
list.stream().mapToDouble(User::getName).max();最大
list.stream().mapToDouble(User::getName).min();最小
list.stream().mapToDouble(User::getName).average();平均值

1.3 List转Map

        List<Map<String, Object>> pinzhongNow = baseMapper.pinzhong()
        HashMap<String, Map<String, Object>> name =         
        pinzhongNow.stream().collect(Collectors.toMap(item ->
                        MapUtils.getString(item, "name"),
                item ->
                        item, (oldVal, currVal) -> oldVal, HashMap::new)
        );

toMap()第一个参数为key，第二个参数为value,第三个参数为 如果oldVal与currVal的key值相同，选择oldVal作为那个key所对应的value值

1.4 遍历

productList1.stream().forEach(e -> {
  String productNameByCode = setProductNameService.getProductNameByCode(e);
  productList.add(productNameByCode);
});

1.5 BigDecimal类型的求和

BigDecimal weightReduce = weightList.stream().reduce(BigDecimal.ZERO, BigDecimal::add);

BigDecimal reduce = vegetablesList.stream().map(PaymentRecordVo::getWeight).reduce(BigDecimal.ZERO, BigDecimal::add);

1.6 Integer类型求和，设置默认值

Integer integer = valueList.stream().reduce(Integer::sum).orElse(0);

1.7 去重

List<String> productNameList1 = productNameList.stream().distinct().collect(Collectors.toList());

1.8 过滤

List<PaymentRecordVo> fruitList = fruitTopFiveList.stream().filter(s -> s.getWeight1() != null && !s.getWeight1().toString().equals("0.00")).collect(Collectors.toList());

取到集合中 weight字段不等于空或者0.00的

最后

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