知识记录

使用Stream流获取唯一一个或首个的操作

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if (CollectionUtils.isNotEmpty(supplierList)) {
    supplierList.stream().filter(s -> s.getSupplierCode().equals(resultData.getSupplierCode())).findFirst().ifPresent(s -> resultData.setSupplierName(s.getSupplierName()));
}

分组

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Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));

List转Map

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Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));

total

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BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);

groupBy 实现的 list 转 map

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public static <T, K> Map<K, T> list2Map(@NonNull Collection<T> list, @NonNull Function<? super T, K> keyFunc) {
        return list.stream().collect(Collectors.toMap(keyFunc, Function.identity(),
                (u, v) -> {
                    throw new IllegalStateException(String.format("Multiple entries with same key,%s=%s,%s=%s",
                            keyFunc.apply(u), u,
                            keyFunc.apply(v), v));
                },
                HashMap::new));
}

去重

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List<Player> newList = playerList.stream().collect(Collectors .collectingAndThen( Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Player::getName))), ArrayList::new)); newList.forEach(System.out::println);

创建空集合

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Collections.emptyList()

获取集合最小值(为空添加默认值)

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evaluateBaseEntry.getValue().stream()
        .map(SupplierEvaluateBaseinfo::getSecretLevel)
        .filter(Objects::nonNull)
        .min(Comparator.comparing(Integer::valueOf)).orElse("5");

list对象两个属性相乘在相加

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item.stream().map(p -> p.getQuantity().multiply(p.getUnitPrice())).reduce(BigDecimal::add).orElse(BigDecimal.ZERO)

Java Stream递归

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/**
* Java Stream递归
* 当找不到子级权限的时候map操作不会再递归调用covert
*/
private UmsPermissionNode covert(UmsPermission permission, List<UmsPermission> permissionList) {
    UmsPermissionNode node = new UmsPermissionNode();
    BeanUtils.copyProperties(permission, node);
    List<UmsPermissionNode> children = permissionList.stream()
           .filter(subPermission -> subPermission.getPid().equals(permission.getId()))
           .map(subPermission -> covert(subPermission, permissionList)).collect(Collectors.toList());
    node.setChildren(children);
    return node;
}

java stream 多属性过滤

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List<SupplierEvaluationYear> filter = supEvaluationYearList.stream()
        .distinct()
        .filter(e -> !(supplierEvaluationYears.stream()
                .map(y -> y.getSupplierId() + ":" + y.getPurchaseId() + ":" + y.getProductType() + ":" + y.getYear())
                .collect(toList())
                .contains(e.getSupplierId() + ":" + e.getPurchaseId() + ":" + e.getProductType() + ":" + e.getYear())))
        .collect(toList());