串行和并行

串行

stream()

并行

parallelStream()

stream().parallel()

  • 对象中取字段
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List<String> names = userEntities.stream().map(UserEntity::getName).collect(Collectors.toList());
  • reduce求和
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List<Integer> list = Arrays.asList(1,2,3,4,5,6);
//串行
int all = list.stream().reduce(0,((tempTatal, tempParam) -> tempTatal + tempParam));
System.out.println(all);
all = list.stream().reduce(0,Integer::sum);
System.out.println(all);
//并行
all = list.stream().parallel().reduce(0,Integer::sum,Integer::sum);
System.out.println(all);

//串行
BigDecimal all = list.stream().reduce(BigDecimal.ZERO, BigDecimal::add);
//并行
BigDecimal all = list.stream().parallel().reduce(BigDecimal.ZERO, BigDecimal::add, BigDecimal::add);

Optional判空

  • 举例

    • 实体类User
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    @Data
    public class User {
        //用户名
        private String userName;
        //信息
        private String message;
        //爱好
        private Hobby hobby;
    }

    • 实体类Hobby
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    @Data
    public class Hobby {
        //爱好名
        private String hobbyName;
        //描述
        private String hobbydesc;
    }
    • 模拟使用
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    public static void main(String[] args) throws MyException {

        //简单的一层结构(效果不明显，用ifPresent 和 ==null区别不大)
        User user = null;
        Optional.ofNullable(user).ifPresent( //判断是否为空
            user1 -> {
                System.out.println(user1.getUserName());
                System.out.println(user.getMessage());
            }
        );
        //抛出异常
        Optional.ofNullable(user).map(User::getMessage).orElseThrow(() -> new MyException("这是抛出我的自定义异常"));
        //不抛出异常直接给默认值
        String message = Optional.ofNullable(user).map(User::getMessage).orElse("");
        System.out.println("这是我给的值"+message);

        //多层结构(多层的时候可以多个map.然后通过ifPresent可以直接判断空,比不断地判断 ==null 简洁方便)
        Optional.ofNullable(user).map(User::getHobby).ifPresent(
            hobby -> {
                System.out.println("这里执行获取到hobby的事");
            }
        );
        //抛出异常
        Optional.ofNullable(user).map(User::getHobby).map(Hobby::getHobbyName).orElseThrow(() -> new MyException("这是抛出我的自定义异常"));
        //直接给默认值
        String hobbyName = Optional.ofNullable(user).map(User::getHobby).map(Hobby::getHobbyName).orElse("");
        System.out.println("获取Hobby中默认给的值为"+hobbyName);
        //给对象中的对象默认对象
        Hobby hobby = Optional.ofNullable(user).map(User::getHobby).orElse(new Hobby());
        System.out.println("获取Hobby中的值不报异常为"+hobby.getHobbyName());

    }