我們利用 java8 的新特性,可以方便簡潔高效的處理一些集合的數據。
簡單示例如下:
先定義一個訂單對象(Order)
public class Order {
private Long id;
private Long userId;
private String num;
private String type;
private Float allAmt;
private Float payAmt;
private Integer orderNum;
public Order(Long id, Long userId, String num, String type, Float allAmt, Float payAmt, Integer orderNum) {
this.id = id;
this.userId = userId;
this.num = num;
this.type = type;
this.allAmt = allAmt;
this.payAmt = payAmt;
this.orderNum = orderNum;
}
// getting... setting...
}
過濾篩選:
List<Order> orders = Lists.newArrayList();
// 篩選總金額大於1000的訂單
orders = orders.stream().filter(item -> item.getAllAmt() > 1000.00f).collect(Collectors.toList());
分組:
List<Order> orders = Lists.newArrayList();
// 按照訂單類型分組
Map<String, List<Order>> orderGroupMap = orders.stream().collect(Collectors.groupingBy(Order::getType));
去重:
List<Order> orders = Lists.newArrayList();
// 按照訂單編號去重
orders = orders.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(()
-> new TreeSet<>(Comparator.comparing(Order::getNum))), ArrayList::new));
// 按照訂單編號和類型去重
orders = orders.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(()
-> new TreeSet<>(Comparator.comparing(o -> o.getNum() + ";" + o.getType()))), ArrayList::new));
List 轉 Map :
List<Order> orders = Lists.newArrayList();
// 將訂單集合轉換成訂單編號-應付金額 map,注意訂單編號作爲 key 不能重複,應先做去重處理
Map<String, Float> numPayMap = orders.stream().collect(Collectors.toMap(Order::getNum, Order::getPayAmt));
// 用 id 做 key 將 List 轉成 Map
Map<Long, Order> orderMap = orders.stream().collect(Collectors.toMap(Order::getId, item -> item));
排序:
List<Order> orders = Lists.newArrayList();
// 按照訂單總金額從高到低排序
// 方式一
orders.sort((o1, o2)
-> o1.getAllAmt() == null ? 1 : (o2.getAllAmt() == null ? -1 : o2.getAllAmt().compareTo(o1.getAllAmt())));
// 方式二
orders.sort(Comparator.comparing(Order::getAllAmt, (o1, o2)
-> o1 == null ? 1 : (o2 == null ? -1 : o2.compareTo(o1))));
// 方式三 (allAmt 字段不能爲 null, null 會導致排序失敗)
orders.sort(Comparator.comparing(Order::getAllAmt).reversed());
// 先按照訂單類型排序,再按照訂單應付金額從高到低排序
orders.sort(Comparator.comparing(Order::getType, (o1, o2)
-> o1 == null ? 1 : (o2 == null ? -1 : o1.compareTo(o2))).thenComparing((o1, o2)
-> o1.getPayAmt() == null ? 1 : (o2.getPayAmt() == null ? -1 : o2.getPayAmt().compareTo(o1.getPayAmt()))));
統計計數:
List<Order> orders = Lists.newArrayList();
// 統計所有訂單的總金額
// 求和
Double sum = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).sum();
// 最大總金額
OptionalDouble max = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).max();
// 防止沒有訂單數據的處理
Double maxAllAmt = max.isPresent() ? max.getAsDouble() : 0;
// 最小總金額
OptionalDouble min = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).min();
// 平均總金額
OptionalDouble average = orders.stream().filter(item -> item.getAllAmt() != null).mapToDouble(Order::getAllAmt).average();