Question

import java.util.HashMap;
import java.io.*;
import java.util.*;
public class Fegan implements Comparable{
HashMap<String, Integer> cart = new HashMap<String, Integer>();
List list = new ArrayList<FoodItems>();
int x =0;
public void addToCart(FoodItems f)
{
    cart.put(f.name, f.valueOfFood);
}
public String display(FoodItems f)
{
    return(f.name + " costs " + f.valueOfFood);
}
public void addToList(FoodItems f)
{
    //FoodItems temp = (FoodItems) f;
    list.add(f);
}
public int compareTo(Object o)
{
    //FoodItems temp = (FoodItems) o;
    if(this.x == ((FoodItems)o).valueOfFood)
        return 0;
    else if (this.x <((FoodItems)o).valueOfFood)
        return 1;
    else 
        return -1;
}
public void sortMap(List list)
{
    for(int i =0; i< list.size(); i++)
    {
        FoodItems temp = (FoodItems) list.get(i);
        cart.put(temp.name, temp.valueOfItem);

    }
}
}

import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.io.*;
import java.util.*;
public class Test {
public static void main(String [] args)
{
    HashMap<String, Integer> cart = new HashMap<String, Integer>();
    FoodItems firts = new FoodItems("Chocolate" , 50);
    FoodItems second = new FoodItems("Juice", 79);
    FoodItems third = new FoodItems("Apple", 200);
    FoodItems forth = new FoodItems("Orange", 300);
    FoodItems fifth = new FoodItems("Milk" , 400);
    ArrayList items = new ArrayList();
    items.add(firts);
    items.add(second);
    items.add(third);
    items.add(forth);
    items.add(fifth);
    Collections.sort(items);
    Iterator itr = items.iterator();
    Fegan myFegan = new Fegan();
    myFegan.sortMap(items);

    while(itr.hasNext()){
        Object element = itr.next();
        System.out.println(element + "\n");
    }
}
}

为什么写作

Exception in thread "main" java.lang.ClassCastException: FoodItems cannot be cast to java.lang.Comparable
at java.util.ComparableTimSort.countRunAndMakeAscending(Unknown Source)
at java.util.ComparableTimSort.sort(Unknown Source)
at java.util.ComparableTimSort.sort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at java.util.Collections.sort(Unknown Source)
at Test.main(Test.java:21)

提前谢谢！

Answer 1

实现Comparable的对象是Fegan。

您要覆盖的方法compareTo应该有一个Fegan对象作为参数，而您将其转换为FoodItems。您的compareTo实施应说明Fegan与另一个Fegan的比较。

要实际进行排序，您可能需要制作FoodItems工具Comparable并复制粘贴您的实际compareTo逻辑。

Answer 2

在使用自定义对象作为Treemap中的键时，我遇到了类似的问题。每当将自定义对象用作hashmap中的键时，都将覆盖两个函数equals和hashcode，但是，如果在此对象上使用Treemap的ContainsKey方法，则还需要覆盖CompareTo方法，否则将收到此错误在Kotlin中，使用自定义对象作为hashmap中的键的人应该喜欢

 data class CustomObjectKey(var key1:String = "" , var 
 key2:String = ""):Comparable<CustomObjectKey?>
 {
override fun compareTo(other: CustomObjectKey?): Int {
    if(other == null)
        return -1
   // suppose you want to do comparison based on key 1 
    return this.key1.compareTo((other)key1)
}

override fun equals(other: Any?): Boolean {
    if(other == null)
        return false
    return this.key1 == (other as CustomObjectKey).key1
}

override fun hashCode(): Int {
    return this.key1.hashCode()
} 
}

无法强制转换为java.lang.Comparable

2 个答案: