getClass() and enums

[Reghuram Variyam]

1.The result of

List <String> l1 = new ArrayList<String>();
List<Integer> l2 = new ArrayList<Integer>();
System.out.println(l1.getClass() == l2.getClass());

is "true".

2. the result of

enum Operation {PLUS,MINUS,TIMES,DIVIDE }

public static void main(String args[]){
System.out.print(Operation.PLUS.getClass() == Operation.MINUS.getClass());
System.out.println(Operation.PLUS.getClass().equals(Operation.MINUS.getClass()));
}

is "falsefalse"

Can some one explain how we are getting the results 'true' in first case and 'falsefalse' in second case ?

[Wes Misenheimer]

Try again, I'm getting "truetrue"

import java.util.*;

class Test {
enum Operation {PLUS,MINUS,TIMES,DIVIDE }
public static void main(String[] args) {
System.out.print(Operation.PLUS.getClass() == Operation.MINUS.getClass());
System.out.println(Operation.PLUS.getClass().equals
Operation.MINUS.getClass()));
}
}

[Jim Yingst]

Well, (like Wes I see) I get truetrue in the second case. Are you sure you aren't running code a little different than what you showed? In particular, have you perhaps omitted some extra details in the Operation definition?

I recommend you try running this:

    enum Operation {
        PLUS, MINUS, TIMES, DIVIDE
    }
 
    public static void main(String args[]) {
        Class cl1 = Operation.PLUS.getClass();
        Class cl2 = Operation.MINUS.getClass();
        System.out.println("cl1: " + cl1);
        System.out.println("cl2: " + cl2);
        System.out.print(cl1 == cl2);
        System.out.println(cl1.equals(cl2));
    }

And after that, replace the Operation definition with

    enum Operation {
        PLUS {}, MINUS {}, TIMES {}, DIVIDE {}
    }

to see what difference it makes.
[ November 26, 2006: Message edited by: Jim Yingst ]

[Greg L Tonn]

When the enum Operation { PLUS, MINUS, TIMES, DIVIDE } is set like this I get the below output:

PLUS class: class scjp.GetClass$Operation
MINUS class: class scjp.GetClass$Operation
true
true

When the enum Operation { PLUS {}, MINUS {}, TIMES {}, DIVIDE {} } is set like this I get the below output:

PLUS class: class scjp.GetClass$Operation$1
MINUS class: class scjp.GetClass$Operation$2
false
false

I see that the difference is the class. In the first case the class is Operation but in the second case the class is Operation.1 and Operation.2. Why is that?

Thanks

[Reghuram Variyam]

You are correct. I tried it out and I got truetrue as the answer. I got this question from a mock exam. It looks like the answer they provided is not correct. Thanks for the reply..

[Jim Yingst]

Who are "they"? Please always mention the source of any mock exam question you post.
[ November 27, 2006: Message edited by: Jim Yingst ]

[Reghuram Variyam]

http://home.wanadoo.nl/mrzljak/en/biz/exam.html

Questio 12 on scjp 5
[ November 27, 2006: Message edited by: Reghuram Variyam ]

[Greg L Tonn]

Can anyone tell me what the difference is between these two enums?

enum Operation { PLUS, MINUS, TIMES, DIVIDE }

enum Operation { PLUS {}, MINUS {}, TIMES {}, DIVIDE {} }

I know that they provide different output when you print getClass() for a specific element (see my post above) but I'm not sure why?

Thanks

[Jim Yingst]

In the second case, each enum instance is actually an anonymous subclass instance - all different. It's a bit silly to do this here, since none of those anonymous subclasses have any useful functionality. But it's possible to imagine a more useful, complex version, such as this:

enum Operation {
    PLUS {
        public int perform(int x, int y) { return x + y; }
    },
    MINUS {
        public int perform(int x, int y) { return x - y; }
    },
    TIMES {
        public int perform(int x, int y) { return x * y; }
    },
    DIVIDE {
        public int perform(int x, int y) { return x / y; }
    };
    
    public abstract int perform(int x, int y);
}

Here, again, each enum instance is a different anonymous subclass. with a different implementation of perform().
[ November 28, 2006: Message edited by: Jim Yingst ]

[Greg L Tonn]

Thanks Jim. I was thinking they were classes but it wasn't clear to me how it was all working since the output to the getClass() was Operation.1 and so on.

Thanks again and do you or anyone else know if that sort of thing will be on the exam?

[Jim Yingst]

Well, you probably need to be able to recognize the syntax for defining methods for an enum, which is essentially the same syntax as for anonymous classes. But note the use of commas and semicolon, and the fact that the abstract method declaration must come after the instance definitions, because the list of instance definitions must be the first thing inside the enum. I doubt you need to knowhow to figure the exact class names for the enum instances. (Class names of the form OuterClassName$1, OuterClassName$2 are typical for anonymous classes.)