java按日期和时间对ArrayList的对象进行排序

Question

我在ArrayList中有10个对象，其中患者ID和预约日期作为实例变量。如何按预约日期对患者进行分类

由于某些原因，我无法使用Comparator对ArrayList进行排序；编译器给出了一个错误：

java.util.Comparator is abstract; cannot be instantiated

当不使用Comparator时Collections.sort()对Object不起作用

ArrayList<Appointment> testApp = new ArrayList<>();

while (!App.isEmpty()) {
    tempA = (Appointment) App.dequeue();

    if (tempA.getPatID().equals(search)) {
        testApp.add(tempA);
    }

    tempApp.enqueue(tempA);
}

while (!tempApp.isEmpty()) {
    tempA = (Appointment) tempApp.dequeue();
    App.enqueue(tempA);
}

Collections.sort(testApp, new Comparator<Appointment>());

Answer 1

您不能直接实例化Comparator，您需要自己实现逻辑，请列出

    Comparator<Appointment> comp = new Comparator<>() {
        public int compare(Appointment a1, Appointment a2) {
            // Logic to compare here, see javadoc of that method
            return 0;
        }
    }

    testApp.sort(comp);

使用lambda表达式，可以编写如下代码：

    testApp.sort((a1, a2) -> {
        // Logic to compare here, see javadoc of that method
        return 0;
    });

如果Appointment是您自己编写的类，则可以使其实现Comparable<Appointment>：

class Appointment implements Comparable<Appointment> {
        public int compareTo(Appointment a2) {
            // Logic to compare here, see javadoc of that method
            return 0;
        }
}

这意味着您可以在不指定比较器的情况下对列表进行排序：

    testApp.sort(null);

Answer 2

我假设您的Appointment类如下所示：

class Appointment {
    private int patientId;
    private LocalDateTime appointmentDate;

    // Getters & setters
}

如果是这样，您应该创建一个Comparator来使用^{}方法进行排序：

ArrayList<Appointment> appointments = new ArrayList<>();

Collections.sort(appointments, Comparator.comparing(appointment -> {
    return appointment.getAppointmentDate();
}));

lambda函数告诉比较器如何获取对象（一个Appointment）并提取用于排序的元素（键；appointmentDate）。如果您愿意，可以将其浓缩为方法引用：

Collections.sort(appointments, Comparator.comparing(Appointment::getAppointmentDate));

您也可以直接在ArrayList上调用^{}：

appointments.sort(Comparator.comparing(Appointment::getAppointmentDate));

---

如果您总是（或通常）按日期排序您的^ {< CD1>}对象，则可以考虑使^ {CD1>}实现^{}，这样您就可以调用^ {< CD11>}而不传递^ {< CD2>}：

class Appointment implements Comparable<Appointment> {
    private int patientId;
    private LocalDateTime appointmentDate;

    // Getters & setters

    @Override
    public int compareTo(final Appointment other) {
        return appointmentDate.compareTo(other.appointmentDate);
    }
}

Answer 3

private static final Comparator<Appointment> SORT_BY_DATE_DESC = Comparator.comparing(Appointment::getDate).reversed();

public static void main(String... args) {
    List<Appointment> appointments = Collections.emptyList();
    appointments.sort(SORT_BY_DATE_DESC);
}

public static class Appointment {
    private Date date;

    public Date getDate() {
        return date;
    }
}

Answer 4

为了使您的类实例彼此具有可比性，您的类大多数实现“可比”接口。例如，如果您有一个名为C1的类，并且您希望使C1实例具有可比性，则大多数情况下会以以下方式声明C1：

public class C1 implements Comparable<C1>{
//some codes

@Override
public int compareTo(C1 other){
// here you can compare this instance of C1 (references with this keyword) with other

}

}

现在，如果您使用Collections.sort()，您的arrayList将被排序