无法强制转换为java.lang.Comparable

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)

先感谢您!

Answer 1

• 实现的对象Comparable是Fegan.

compareTo你在其中覆盖的方法应该有一个Fegan对象作为参数,而你将它转换为FoodItems.您的compareTo实现应描述如何Fegan与另一个进行比较Fegan.

• 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树状图中的键时，我遇到了类似的问题。每当您使用自定义对象作为 hashmap 中的键时，您都会重写两个函数 equals 和 hashcode，但是，如果您在此对象上使用 Treemap 的 ContainsKey 方法，那么您还需要重写 CompareTo 方法，否则您将收到此错误在 kotlin 中使用自定义对象作为 hashmap 中的键应该如下所示

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()
    } 
}