Question

所以我一直在试图弄清楚Collections如何在Java中工作（通过查看我们在uni中做的一个旧项目），但是我一直遇到一些我无法解决的错误，它与Comparable函数有关

这是我的第一堂课：

import java.io.*; 
import java.util.*;

public class Client  {

private int hour;
private int minute;
private int min_per_stay;
private double price;

public Client(int h, int m, int mps, double p)
{
    this.hour = h;
    this.minute = m;
    this.min_per_stay = mps;
    this.price = p;
}

public void SetHour(int h)
{
    this.hour = h;
}

public void SetMin(int m)
{
    this.minute = m;
}

public void SetMinPStay(int mps)
{
    this.min_per_stay = mps;
}

public void SetPrice(double p)
{
    this.price = p;
}

public int GetHour()
{
    return hour;
}

public int GetMin()
{
    return minute;
}

public int GetMinPStay()
{
    return min_per_stay;
}

public double GetPrice()
{
    return price;
}

public String toString()
{
    return "Hours: " + this.hour + 
            " Minutes: " + this.minute +
            " Minutes per stay:" + this.min_per_stay +
            " Price: " + this.price + "\n";

}

public boolean equals(Client c) {
    return (this.hour == ((Client)c).GetHour() && this.minute == ((Client)c).GetMin());
}

public int CompareTo(Client c)
{
    if (this.hour<((Client)c).hour) return -1;
    if(this.minute<((Client)c).minute) return -1;
    if (this.hour>((Client)c).hour) return 1; 
    if(this.minute>((Client)c).minute) return 1;
    return 0;
}
}

我的收藏：

import java.io.*;
import java.util.*;

public class ClientCollection{

private SortedSet<Client> oClient = new TreeSet<Client>();

public ClientCollection()
{

}

public ClientCollection(String FileName)
{
    try
    {
    Scanner sc = new Scanner(new File(FileName));
    while(sc.hasNextLine())
    {
        oClient.add(new Client(sc.nextInt(),sc.nextInt(),
                sc.nextInt(),sc.nextDouble()));
    }
    sc.close();
    }
    catch (FileNotFoundException e)
    {
        System.out.println("File not found!");
    }
}

public void addClient(Client c)
{
    oClient.add(c);
}

public  List<Client> reverseList()
{
    List<Client> oList1 = new ArrayList<Client>(oClient);
    List<Client> oList2 = new ArrayList<Client>();

    for(ListIterator<Client> it = (oList1.listIterator(oClient.size())); it.hasPrevious(); )
    {
        Client res = it.previous();
        oList2.add(res);
    }
    return oList2;
}

public void printColl()
{
    System.out.println(oClient.toString());
}

public static void main(String[] args)
{
    ClientCollection oc = new ClientCollection("test.txt");
    oc.printColl();

    oc.reverseList();
    oc.printColl();
}

}

我得到的错误是：

Exception in thread "main" java.lang.ClassCastException: Client cannot be cast to java.base/java.lang.Comparable
    at java.base/java.util.TreeMap.compare(Unknown Source)
    at java.base/java.util.TreeMap.put(Unknown Source)
    at java.base/java.util.TreeSet.add(Unknown Source)
    at ClientCollection.<init>(ClientCollection.java:20)
    at ClientCollection.main(ClientCollection.java:56)

如果有人向我解释错误，我会非常高兴，我仍然有点新鲜:(

Answer 1

您的inds <- do.call(intersect, unname(by(num, num$x5, function(x) x$x4))) num[num$x4 %in% inds, ] x1 x2 x3 x4 x5 2 74 96 36 2 2017 3 61 83 7 3 2017 4 63 35 33 2 2018 5 44 95 67 3 2018课程没有实现Client界面（即Comparable<Client>没有自然排序）。

因此，为了将其用作Client的元素，您必须将TreeSet构造函数传递给TreeSet<Client>，它指定Comparator<Client>的顺序} elements。

如果不这样做会导致Client，因为ClassCastException类（或者更确切地说是在幕后使用的TreeSet类）假定如果你没有提供{ {1}}在构造函数中，这意味着您的TreeMap实现了Comparator。

编辑：由于Client课程中有Comparable方法，因此您似乎打算实施CompareTo。

更改：

Client

为：

Comparable

并改变：

public class Client

为：

public class Client implements Comparable<Client>

您可能希望修改public int CompareTo(Client c)方法的逻辑。

Answer 2

除了@Eran写的内容之外，您还需要编写以小写字母开头的方法名称。您可以看到StackOverflow的格式化程序被它弄糊涂了。它也会让任何人不得不混淆你的代码。

您的compareTo方法应如下所示：

public int compareTo(final Client c)
{
    final int diffHours = this.hour - c.hour;
    if (diffHours == 0) {
        return this.minute - c.minute;
    } else {
        return diffHours;
    }
}

转换是不必要的，因为c参数已经是Client类型。

关于compareTo＆amp; amp的错误TreeSet（Java）

2 个答案: