项目中常用的JDK8之Stream流，长期补充

1、JDK8 List转Map

public class ListToMapExample {
    public static void main(String[] args) {
        List<User> users = Arrays.asList(
            new User(1, "Alice"),
            new User(2, "Bob"),
            new User(3, "Charlie")
        );
        Map<Integer, User> userMap = users.stream().collect(Collectors.toMap(User::getId, user -> user));
        userMap.forEach((id, user) -> System.out.println(id + " -> " + user.getName()));
    }
}

2、JDK8从集合中根据条件查找元素

假如集合groupOpts是List<HashMap<String,String>>类型，现在要查找groupOpts中HashMap键为id，值为123456的元素，找不到就返回null。

1.返回一个元素

HashMap<String, String> groupOpt = groupOpts.stream().filter(group -> "123456".equals(group.get("id"))).findAny().orElse(null);

2.返回符合条件多个元素

List<HashMap<String, String>> collect = groupOpts.stream().filter(tag -> "123456".equals(group.get("id"))).collect(Collectors.toList());

3、JDK8 对集合中某类进行合计

BigDecimal rkChaoShiWeight = inboundOvertimeAlarmOrderList.stream().map(order -> order.getOrderWeight()).reduce(BigDecimal.ZERO, BigDecimal::add);