java-8

问题 I have a list of activities(Activity) and I want to determine a data structure of the form Map(String, DateTime) (not Duration or Period; DateTime it's a must) that maps. For each activity the total duration computed over the monitoring period. The class Activity has: activityLabel(String) , startTime(DateTime) , endTime(DateTime) . I use joda time. This is what I have done: Map<String, DateTime> durations = activities.stream().collect(Collectors.toMap( it -> it.activityLabel, it ->new

问题 I started to learn Lambda expressions of Java 8, and wrote below program to get sum of all numbers in the list: import java.util.Arrays; import java.util.List; public class MainClass { public static void main(String[] args) { List<Integer> number = Arrays.asList(1, 2, 3, 4, 5); System.out.println(number.stream().reduce((c,e) -> { return c + e; })); } } I was expecting the output to be: 15 but I got: Optional[15] Java version: 1.8.0_45 Please explain what does Optional[] means in the output?

问题 Swing applications show a blank window when launched on a VNC desktop with Java 8. I don't have this problem with JRE 7 and non-swing applications (eg. Eclipse) runs properly with the same settings. Are there some sort of default settings like 3D acceleration or something that have been enabled by default on JRE 8 ? How does i disable it ? 回答1: I had blank Java windows in VNC too, but with Java 7 and Java 8, so it wasn't much of a surprise that disabling xrender didn't help me. That probably

问题 Some people override the CrudRepository's method findAll to return an Stream (java 8), but I saw they finally transform the Stream to a List in order to send it through a rest controller. Why are they using a Stream ? What is the advantage to use a Stream here? If they want to filter the records I think would be better filter on DataBase. 回答1: This is already supported in Spring Data JPA, look here; so there's not real advantage to override those to return Stream . If you really want a Stream

问题 Some people override the CrudRepository's method findAll to return an Stream (java 8), but I saw they finally transform the Stream to a List in order to send it through a rest controller. Why are they using a Stream ? What is the advantage to use a Stream here? If they want to filter the records I think would be better filter on DataBase. 回答1: This is already supported in Spring Data JPA, look here; so there's not real advantage to override those to return Stream . If you really want a Stream

问题 When saving a LocalDate field (e.g. '2017-09-27') to a mySQL Date column using JPA CriteriaBuilder API, the result is different (e.g. '2017-09-26'). I have validated that my database's timezone is set to UTC using SELECT TIMEDIFF(NOW(), UTC_TIMESTAMP) as the result is '00:00:00'. I am testing this locally, and I have a timezone of GMT + 2, so my suspicion is that when the conversion occurs from LocalDate to Date , 2 hours are being deducted and producing a date 1 day before the requested date

问题 When saving a LocalDate field (e.g. '2017-09-27') to a mySQL Date column using JPA CriteriaBuilder API, the result is different (e.g. '2017-09-26'). I have validated that my database's timezone is set to UTC using SELECT TIMEDIFF(NOW(), UTC_TIMESTAMP) as the result is '00:00:00'. I am testing this locally, and I have a timezone of GMT + 2, so my suspicion is that when the conversion occurs from LocalDate to Date , 2 hours are being deducted and producing a date 1 day before the requested date

问题 When saving a LocalDate field (e.g. '2017-09-27') to a mySQL Date column using JPA CriteriaBuilder API, the result is different (e.g. '2017-09-26'). I have validated that my database's timezone is set to UTC using SELECT TIMEDIFF(NOW(), UTC_TIMESTAMP) as the result is '00:00:00'. I am testing this locally, and I have a timezone of GMT + 2, so my suspicion is that when the conversion occurs from LocalDate to Date , 2 hours are being deducted and producing a date 1 day before the requested date

问题 I was writing some testcode for java-8 conversion between java.util.Date and java.time.LocalDateTime , and discovered an anomaly seems to occur in the hour after the transition from normaltime-to-summertime, when the year is 2038 or higher. I just wanted to know if this is a bug in jdk8, or if I am doing something wrong? Note: I am on Windows-7, 64-bit jdk, so should not be affected by the 2038-unix bug, which would have a much worse effect. Here my demo-code: package conversiontest; import

问题 I've just started exploring some concurrency features of Java 8. One thing confused me a bit is these two static methods: CompletableFuture<Void> runAsync(Runnable runnable) CompletableFuture<U> supplyAsync(Supplier<U> supplier) Do anyone know why they choose to use interface Supplier? Isn't it more natural to use Callable, which is the analogy of Runnable that returns a value? Is that because Supplier doesn't throw an Exception that couldn't be handled? 回答1: Short answer No, it's not more