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我正在尝试实现一个通用的 Bag 类，它可以计算包中大于对象 x 的对象数量，为此我使用 Date 类来测试它。

包码：

class Bag<T extends Comparable<T>> 
{// generic class with T standing for a class

private T[] bag = (T[])(new Object[100]);                 
private int numElements = 0; // junk in bag[numElements..] 

void add(T x) 
{ // put x in bag
        bag[numElements] = x; numElements++;
}

int freq(T x) 
{ // how many x’s in bag?
        int count = 0;
        for (int i=0; i<numElements; i++)
            if (bag[i].equals(x)) // .equals because T a class 
                count++;
        return count;
}

int numGreater(T x)
{//number of objects greater than x 
    int greaterThan = 0;
    for(int i = 0; i<= bag.length; i++)
    {
        if(bag[i].compareTo(x) == 1)
        {
            greaterThan++;
        }
    }
    return greaterThan;
}
}

日期代码：

import java.util.*;
import java.lang.*;

class Date implements Comparable<Date>
  { // dates

private int day, month, year;

Date(int day0, int month0, int year0) 
{
    day = day0; month = month0; year = year0;
}

public String toString() 
{
    return "" + day + "/" + month + "/" + year;
}

private int elapsedDays() 
{
    // Number of days elapsed from 1/1/1900 to this date
    int days = (year-1900)*365+(year-1901)/4; 
    int k = 1;
    while (k<month) 
    {
        int daysInMonth;
    if (k==9||k==4||k==6||k==11)daysInMonth = 30;
        else if (k==2) 
        {
            if (year%4==0 && year!=1900) daysInMonth = 29;
            else daysInMonth = 28;
        }
        else daysInMonth = 31;
        days = days + daysInMonth; 
        k++; 
    }
    return days + day;
    }

    public int compareTo(Date d)
    {
        if(d.elapsedDays() > elapsedDays())
        {
            return -1;
        }
        if(d.elapsedDays() == elapsedDays())
        {
            return 0;
        }
        return 1;
    }

  }

最后是我正在使用的测试程序：

import java.util.*;
import java.lang.*;

class BagTest
{
public static void main(String[] args) 
{//finds dates greater than today
    Bag<Date> ds = new Bag<>();
    Date today = new Date(07,11,2013);
    Date d1 = new Date(1,11,2013);
    Date d2 = new Date(8,11,2013);
    Date d3 = new Date(9,11,2013);

    ds.add(today);
    ds.add(d1);
    ds.add(d2);
    ds.add(d3);

    System.out.print(ds.numGreater(today));
}
}

我收到的错误消息如下：

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; can
not be cast to [Ljava.lang.Comparable;
        at Bag.<init>(Bag.java:6)
        at BagTest.main(BagTest.java:8)

提前感谢您的帮助！

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