Adding enums, implementing same interface, to a collection throws ClassCastException

Question

I've made an interface Attribute to be implemented by different Enums (as I know a Java-enum is a class actually) representing an attribute:

Interface Attribute:

public interface Attribute {
    void print();
}

Then I made two Enums: Size and Type with the Attribute behaviour:

Enum Size:

public enum Size implements Attribute {
    SMALL, MEDIUM, LARGE;

    @Override
    public void print() {
        System.out.println("I am a size.");
    }
}

Enum Type:

public enum Type implements Attribute {
    SQUARE, CIRCLE, TRIANGLE;

    @Override
    public void print() {
        System.out.println("I am a type.");
    }
}

Then I test the structure within my main class:

public class AttributesTestDrive {
    public static void main(String[] args) {
        Set<Attribute> attributes = new TreeSet<>();
        attributes.add(Size.MEDIUM);
        attributes.add(Type.CIRCLE);
        for (Attribute attribute : attributes) {
            attribute.print();
        }
    }
}

Can anyone explain why I get a java.lang.ClassCastException and how to get rid of this:

Exception details:

at java.lang.Enum.compareTo(Enum.java:180)
at java.lang.Enum.compareTo(Enum.java:55)
at java.util.TreeMap.put(TreeMap.java:568)
at java.util.TreeSet.add(TreeSet.java:255)

score 2 · Accepted Answer · answered Dec 30 '15 at 14:03

2

Try using HashSet instead of TreeSet

answered Dec 30 '15 at 14:03

user1211

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Thank you. I prefer a wrong answer; it's at least an answer. – LowLevel Dec 30 '15 at 14:35
@LowLevel why do you think this answer is wrong? – Louis Wasserman Dec 30 '15 at 16:49
@LouisWasserman, it just didn't work. – LowLevel Dec 30 '15 at 19:04
What happened when you tried it? – Louis Wasserman Dec 30 '15 at 19:06
I tried again and now it works. However, thank you once again. – LowLevel Dec 30 '15 at 19:14
I probably did something wrong. Sorry ;) – LowLevel Dec 30 '15 at 19:14
And your answer helped me actually to solve the whole problem: http://stackoverflow.com/questions/34530898/how-to-add-different-enum-values-to-a-set-as-unique-entities – LowLevel Dec 30 '15 at 19:42
There I used `CopyOnWriteArraySet` instead – LowLevel Dec 30 '15 at 19:48

Adding enums, implementing same interface, to a collection throws ClassCastException

1 Answers1