Class cast Exception while using Comparable interface

Question

When I am trying to execute the below code,the compiler is throwing error at line 13 as "java.lang.ClassCastException". Can someone let me know what's wrong with below code?

package chapter11;

import java.util.*;

public class ComparableExample {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Item[] items = new Item[3];
        items[0] = new Item(102, "Duct Tape");
        items[1] = new Item(103, "Bailing Wire");
        items[2] = new Item(104, "Chewing Gum");

        Arrays.sort(items);
        for (Item i : items) {
            System.out.println(i.getNumber() + ":" + i.getDescription());
        }
    }
}

interface Comparable {
    int compareTo(Object o);
}

class Item implements Comparable {
    private int number;
    private String description;

    public Item(int number, String description) {
        this.number = number;
        this.description = description;
    }

    public int getNumber() {
        return number;
    }

    public String getDescription() {
        return description;
    }

    public int compareTo(Object o) {
        Item i = (Item) o;
        if (this.getNumber() < i.getNumber())
            return -1;
        if (this.getNumber() < i.getNumber())
            return 1;
        return 0;
    }
}

Any help is appreciated, Thanks!!

Your compareTo method in the Item class can be simplified to: return this.number - i.number; compareTo does not dictate that you return -1,1,or 0. You only need to return a pos/neg number or 0. — Solace
– Solace, Commented Mar 3, 2014 at 18:50
You're checking for < i.getNumber() twice. 1 will never be returned. — Menno
– Menno, Commented Mar 3, 2014 at 18:51
@Solace Frankly, no. That is not a good simplification. There is a chance of overflow when the result goes out of bounds. Best is to use Integer.compare() method. — Rohit Jain
– Rohit Jain, Commented Mar 3, 2014 at 18:57

rpax · Accepted Answer · 2014-03-03 18:59:40Z

2

Remove your Comparable interface, and use the Comparable interface from the Java api. And also, maybe you can change

public int compareTo(Object o) {
        Item i = (Item) o;
        if (this.getNumber() < i.getNumber())
            return -1;
        if (this.getNumber() < i.getNumber())
            return 1;
        return 0;
    }

into :

public int compareTo(Object o) {
            Item i = (Item) o;
            if (this.getNumber() < i.getNumber())
                return -1;
            if (this.getNumber() > i.getNumber())
                return 1;
            return 0;
        }

edited Mar 3, 2014 at 18:59

answered Mar 3, 2014 at 18:52

rpax

4,4947 gold badges35 silver badges60 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Sotirios Delimanolis Over a year ago

In this case, fix the raw type.

Rohit Jain Over a year ago

Comparable is an interface. Both in OP code, and in standard Java API.

Sandy Over a year ago

Removing Comparable interface doesn't change a thing. It still gives the same error(Exception in thread "main" java.lang.ClassCastException: chapter11.Item cannot be cast to java.lang.Comparable) at Line 14..Also,changed the compareTo class

rpax Over a year ago

@RohitJain That's true, I wrote the answer without re-reading it

Collectives™ on Stack Overflow

Class cast Exception while using Comparable interface

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related