使用Java 8的Stream API可以很容易地实现按订单ID分组并按项最小日期排序的功能。以下是示例代码：

1.假设我们有一个Order类，它有一个orderId和一个itemLeastDate属性：

public class Order { private int orderId; private LocalDate itemLeastDate;

public Order(int orderId, LocalDate itemLeastDate) {
    this.orderId = orderId;
    this.itemLeastDate = itemLeastDate;
}

public int getOrderId() {
    return orderId;
}

public LocalDate getItemLeastDate() {
    return itemLeastDate;
}

}

2.现在我们可以使用Stream API来对订单集合进行分组，并按项最小日期对每个组进行排序：

List orders = Arrays.asList( new Order(1, LocalDate.parse("2022-01-01")), new Order(2, LocalDate.parse("2022-01-02")), new Order(1, LocalDate.parse("2022-01-03")), new Order(2, LocalDate.parse("2022-01-04")));

Map> groupedOrders = orders.stream() .collect(Collectors.groupingBy(Order::getOrderId));

Map> sortedGroupedOrders = new HashMap<>();

groupedOrders.forEach((k, v) -> { List sortedOrders = v.stream() .sorted(Comparator.comparing(Order::getItemLeastDate)) .collect(Collectors.toList()); sortedGroupedOrders.put(k, sortedOrders); });

3.现在，我们可以以每个订单ID为键，按项最小日期排列的订单列表为值，将其打印出来：

sortedGroupedOrders.forEach((k, v) -> { System.out.println("Order ID: " + k); v.forEach(order -> System.out.println("\tItem least date: " + order.getItemLeastDate())); });

这将输出以下内容：

Order ID: 1 Item least date: 2022-01-01 Item least date: 2022-01-03 Order ID: 2 Item least date: 2022-01-02 Item least date: