问题
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
回答1:
Java 1.7 and java.time through ThreeTen Backport
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.
Stream version
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.
Question: Can that work on Java 1.7?
java.time works nicely on both older and newer Android devices. It just requires at least Java 6.
- In Java 8 and later and on newer Android devices (from API level 26) the modern API comes built-in.
- In non-Android Java 6 and 7 get the ThreeTen Backport, the backport of the modern classes (ThreeTen for JSR 310; see the links at the bottom).
- On older Android either use desugaring or the Android edition of ThreeTen Backport. It’s called ThreeTenABP. In the latter case make sure you import the date and time classes from
org.threeten.bpwith subpackages.
Links
- Oracle tutorial: Date Time explaining how to use java.time.
- Java Specification Request (JSR) 310, where
java.timewas first described. - ThreeTen Backport project, the backport of
java.timeto Java 6 and 7 (ThreeTen for JSR-310). - Java 8+ APIs available through desugaring
- ThreeTenABP, Android edition of ThreeTen Backport
- Question: How to use ThreeTenABP in Android Project, with a very thorough explanation.
回答2:
I will recommend to use LocalDate from java-8, since Date is older version
public class Courses{
int coursenum;
LocalDate date;
}
And then you can use removeIf
list.removeIf(d->d.isBefore(LocalDate.now());
回答3:
Define a key date and remove all courses whose date is before your key date. Provided you have the appropriate getter:
ArrayList<Courses> list = new ArrayList<Courses>();
//populate list
Date someDate = new SimpleDateFormat("MM/dd/yyyy").parse("01/01/2020");
list.removeIf(c -> c.getDate().before(someDate));
Kindly @Randy Casburn has written a minimal yet complete example which I do not want to withhold from the interested reader:
/**
*
* @author Randy Casburn
*/
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
public class Main {
public static void main(String[] args) {
Date latest = new Date("1/1/2020");
List<Courses> list = new ArrayList<>();
list.add(new Courses(350, new Date("5/20/2020")));
list.add(new Courses(350, new Date("4/20/2019")));
list.add(new Courses(350, new Date("3/20/2018")));
list.add(new Courses(360, new Date("6/20/2020")));
list.add(new Courses(360, new Date("5/20/2019")));
list.add(new Courses(370, new Date("5/20/2020")));
list.add(new Courses(370, new Date("5/19/2018")));
list.add(new Courses(360, new Date("4/10/2016")));
list.removeIf(course -> !course.date.after(latest));
list.forEach(l-> System.out.println(l.date));
}
}
class Courses {
int coursenum;
Date date;
Courses(int coursenum, Date date){
this.coursenum = coursenum;
this.date = date;
}
}
回答4:
I am attaching my code as an idea for you request. You can play around with it.
public class Courses {
int courseNum;
LocalDate date;
public Courses(int courseNum, LocalDate date) {
this.courseNum = courseNum;
this.date = date;
}
public int getCourseNum() {
return courseNum;
}
public void setCourseNum(int courseNum) {
this.courseNum = courseNum;
}
public LocalDate getDate() {
return date;
}
public void setDate(LocalDate date) {
this.date = date;
}
@Override public String toString() {
return new StringJoiner(", ", Courses.class.getSimpleName() + "[", "]")
.add("courseNum=" + courseNum)
.add("date=" + date)
.toString();
}
public class Main {
public static void main(String[] args) {
List<Courses> list = new ArrayList<>();
list.add(new Courses(350, LocalDate.of(2020, 5, 20)));
list.add(new Courses(350, LocalDate.of(2019, 4, 20)));
list.add(new Courses(350, LocalDate.of(2018, 3, 20)));
list.add(new Courses(360, LocalDate.of(2020, 6, 20)));
list.add(new Courses(360, LocalDate.of(2019, 5, 20)));
list.add(new Courses(370, LocalDate.of(2020, 5, 20)));
list.add(new Courses(370, LocalDate.of(2018, 5, 19)));
list.add(new Courses(360, LocalDate.of(2016, 4, 10)));
List<Courses> res = list.stream().filter(c -> c.getDate().getYear() == 2020).collect(Collectors.toList());
res.forEach(System.out::println);
}
来源：https://stackoverflow.com/questions/65327012/java-arraylist-of-object-remove-elements-from-an-arraylist-by-date