TreeSet实现？

Question

我尝试实现TreeSet而不是ArrayList，因为我需要对该列表进行排序。原始代码是：

private final List<Report> reports = new ArrayList<Report>();
public void receiveReport(final Report report) {
        this.reports.add(report);
    }

对于我的版本：

private final TreeSet<Report> reports = new TreeSet<>();
public void receiveReport(final Report report) {
        reports.add(report);
    }

它只向我显示一个报告和这个错误：

Exception in thread "main" java.lang.ClassCastException: class dsa.speedcamera.Report cannot be cast to class java.lang.Comparable (dsa.speedcamera.Report is in unnamed module of loader 'app'; 
java.lang.Comparable is in module java.base of loader 'bootstrap')
    at java.base/java.util.TreeMap.compare(TreeMap.java:1563)
    at java.base/java.util.TreeMap.addEntryToEmptyMap(TreeMap.java:768)
    at java.base/java.util.TreeMap.put(TreeMap.java:777)
    at java.base/java.util.TreeMap.put(TreeMap.java:534)
    at java.base/java.util.TreeSet.add(TreeSet.java:255)
    at dsa.speedcamera.ProvidedImplementation.receiveReport(ProvidedImplementation.java:19)
    at dsa.speedcamera.ProvidedImplementation.demonstration(ProvidedImplementation.java:73)
    at dsa.speedcamera.ProvidedImplementation.main(ProvidedImplementation.java:92)

Process finished with exit code 1

这是报表文件: package dsa.speedcamera；

public class Report {
    public TimeID timeID;
    public int speedLimit;
    public String carRegistration;
    public int speedRecorded;
    /** 
     * Returns a String representing the values in this object so that is can
     * be directly printed with System.out.println();
     */
    public String toString() {
        return timeID.toString() +  
        "\nSpeed limit: " + speedLimit + " mph" +
        "\nCar registration: " + carRegistration +
        "\nSpeed recorded: " + speedRecorded + " mph";
    }
}

我该怎么办？

Answer 1

对于用户定义的对象，不能直接添加到TreeSet。您需要为TreeSet提供一个排序应该适用的属性。例如，您是基于speedLimit还是基于speedRecorded进行排序？我怎么知道？TreeSet是怎么知道的？

要向TreeSet提供该信息，您需要提供我们所称的自定义比较器。

请阅读下面的文章，它们会对你有所帮助。

https://www.geeksforgeeks.org/treeset-comparator-method-in-java/

https://www.callicoder.com/java-treeset/