更新时间:2023-01-07 08:00:39
ArrayListlist = new ArrayList();list.add( new Course(350, "5/20/2020"));list.add( new Course(350, "4/20/2019"));list.add( new Course(350, "3/20/2018"));list.add( new Course(360, "6/20/2020"));list.add( new Course(360, "5/20/2019"));list.add( new Course(370, "5/20/2020"));list.add( new Course(370, "5/19/2018"));list.add( new Course(360, "4/10/2016"));//查找每个课程编号的最新日期映射latestDates = new HashMap();对于(课程当前课程:列表){LocalDate latestHitherto = latestDates.get(currentCourse.getCourseNumber());if (latestHitherto == null || currentCourse.getDate().isAfter(latestHitherto)) {latestDates.put(currentCourse.getCourseNumber(), currentCourse.getDate());}}//删除没有最新日期的课程迭代器courseIterator = list.iterator();而 (courseIterator.hasNext()) {课程 currentCourse = courseIterator.next();如果 (currentCourse.getDate().isBefore(latestDates.get(currentCourse.getCourseNumber()))) {courseIterator.remove();}}//打印结果对于(课程当前课程:列表){System.out.format(%3d %s%n",currentCourse.getCourseNumber(),currentCourse.getDate().format(Course.dateFormatter));}
这个片段的输出是:
350 5/20/2020360 6/20/2020370 5/20/2020
我在 jdk1.7.0_67 上运行.我添加了 ThreeTen Backport 1.3.6.请参阅下面的链接.
这是我使用的 Course
类:
公开课课程{public static final DateTimeFormatter dateFormatter= DateTimeFormatter.ofPattern(M/d/u");private int courseNumber;私人本地日期;公共课程(int courseNumber,字符串日期字符串){this.courseNumber = courseNumber;this.date = LocalDate.parse(dateString, dateFormatter);}公共 int getCourseNumber() {返回课程编号;}公共本地日期 getDate() {归期;}}
如果您坚持使用 java.util.Date
— 一个设计糟糕且过时的类 — (1) 我不明白您为什么要这样做,(2) 您可以将我的代码修改为改用 Date
.
您可能需要考虑升级您的 Java 版本.Java 15 已经发布,并且是 Java 16 的早期访问版本. Java 8 中已经出现了流.它们将允许您以更少的代码行获得相同的结果:
合集过滤课程 = list.stream().collect(Collectors.groupingBy(Course::getCourseNumber,Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparing(Course::getDate)),可选::或ElseThrow))).values();列表<课程>过滤列表=新的数组列表(过滤课程);FilterList.forEach(c -> System.out.format(%3d %s%n", c.getCourseNumber(), c.getDate().format(Course.dateFormatter)));
370 5/20/2020360 6/20/2020350 5/20/2020
就代码而言,它不会在结果列表中给出相同的课程顺序,并且它需要 Java 10(在那里引入了过度的无参数 orElseThrow
方法).即使您需要相同的顺序,流仍然会非常有用.
java.time 在较旧和较新的 Android 设备上都能很好地工作.它只需要至少 Java 6.
org.threeten.bp
导入日期和时间类.java.time
首先被描述.java.time
的 backport 到Java 6 和 7(JSR-310 为 ThreeTen).This is my arraylist
ArrayList<Courses> list = new ArrayList<Courses>();
list.add( new Courses(350,5/20/2020) );
list.add( new Courses(350,4/20/2019 );
list.add( new Courses(350,3/20/2018) );
list.add( new Courses(360,6/20/2020) );
list.add( new Courses(360,5/20/2019) );
list.add( new Courses(370,5/20/2020) );
list.add( new Courses(370,5/19/2018) );
list.add( new Courses(360,4/10/2016) );
public class Courses{
int coursenum;
Date date;
}
How can I remove elements so that the arraylist contains only those elements with the latest date?
Should like this;
350, 5/20/2020}
360,6/20/2020}
370, 5/20/2020
ArrayList<Course> list = new ArrayList<>();
list.add( new Course(350, "5/20/2020") );
list.add( new Course(350, "4/20/2019") );
list.add( new Course(350, "3/20/2018") );
list.add( new Course(360, "6/20/2020") );
list.add( new Course(360, "5/20/2019") );
list.add( new Course(370, "5/20/2020") );
list.add( new Course(370, "5/19/2018") );
list.add( new Course(360, "4/10/2016") );
// Find latest date for each course number
Map<Integer, LocalDate> latestDates = new HashMap<>();
for (Course currentCourse : list) {
LocalDate latestHitherto = latestDates.get(currentCourse.getCourseNumber());
if (latestHitherto == null || currentCourse.getDate().isAfter(latestHitherto)) {
latestDates.put(currentCourse.getCourseNumber(), currentCourse.getDate());
}
}
// Remove courses that haven’t got the latest date
Iterator<Course> courseIterator = list.iterator();
while (courseIterator.hasNext()) {
Course currentCourse = courseIterator.next();
if (currentCourse.getDate().isBefore(latestDates.get(currentCourse.getCourseNumber()))) {
courseIterator.remove();
}
}
// Print result
for (Course currentCourse : list) {
System.out.format("%3d %s%n",
currentCourse.getCourseNumber(),
currentCourse.getDate().format(Course.dateFormatter));
}
Output from this snippet was:
350 5/20/2020 360 6/20/2020 370 5/20/2020
I ran on jdk1.7.0_67. I had added ThreeTen Backport 1.3.6. See the link below.
Here is the Course
class I used:
public class Course {
public static final DateTimeFormatter dateFormatter
= DateTimeFormatter.ofPattern("M/d/u");
private int courseNumber;
private LocalDate date;
public Course(int courseNumber, String dateString) {
this.courseNumber = courseNumber;
this.date = LocalDate.parse(dateString, dateFormatter);
}
public int getCourseNumber() {
return courseNumber;
}
public LocalDate getDate() {
return date;
}
}
If you insist on using java.util.Date
— a poorly designed and long outdated class — (1) I would not understand why you would, (2) you can probably modify my code to use Date
instead.
You may want consider upgrading your Java version. Java 15 is out, and an early access edition of Java 16. Already in Java 8 comes streams. They will allow you to obtain the same in much fewer lines of code:
Collection<Course> filteredCourses = list.stream()
.collect(Collectors.groupingBy(Course::getCourseNumber,
Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparing(Course::getDate)),
Optional::orElseThrow)))
.values();
List<Course> filteredList = new ArrayList<>(filteredCourses);
filteredList.forEach(c -> System.out.format(
"%3d %s%n", c.getCourseNumber(), c.getDate().format(Course.dateFormatter)));
370 5/20/2020 360 6/20/2020 350 5/20/2020
As the code stands it doesn’t give the same order of courses in the resulting list, and it requires Java 10 (the overlaoded no-arg orElseThrow
method was introduced there). Even if you require the same order, streams will still be extremely helpful.
java.time works nicely on both older and newer Android devices. It just requires at least Java 6.
org.threeten.bp
with subpackages.java.time
was first described.java.time
to Java 6 and 7 (ThreeTen for JSR-310).