the answer to a question!!

[Erico Doon]

Which of the following represent immutable classes?
a) String b) Double
c) StringBuffer d) Math
//can you rectify the answer for me? thank you!!

[Leandro Oliveira]

That's a good question! I guess all of them are true since all the classes mentioned (String,Double,StringBuffer,Math) are immutable, (I state this basing on the fact that these classes are declared final).

[Barry Gaunt]

Which ones do you think are immutable?
We cannot rectify your answers because you have not given us any

[Barry Gaunt]

One I can tell you (for leandro) StringBuffer is mutable. You can take a StringBuffer and change its contents. For example, if sb references a StringBuffer, and you do sb.reverse(), then sb refers to the same StringBuffer but the contents are reversed.

[Erico Doon]

thank you all for your reply!!
but i am still confused. Maybe i have not understood the meaning of the word "immutible".
it means this class cannot be derived or the content cannot be changed? please help me!
these all class are final!!

[Corey McGlone]

Originally posted by leandro oliveira:
That's a good question! I guess all of them are true since all the classes mentioned (String,Double,StringBuffer,Math) are immutable, (I state this basing on the fact that these classes are declared final).

Don't get final classes confused with immutable classes. A class being declared as final means that the class can not be extended. However, a class that is said to be immutable is a class which, once you create an instance of it, that instance can not be changed.
In this case, String and Double are immutable, while StringBuffer and Math are not.
I hope that helps,
Corey

[Erico Doon]

Thank you ,Corry !! the clods have disappeared.
but the answer is Math.
maybe the answer is wrong!

[Barry Gaunt]

Hold on Erico! Corey, java.lang.Math is final and consists only of zillions of static methods. You cannot even construct a Math object. So Math is safely considered to be an immutable class.
[ March 17, 2003: Message edited by: Barry Gaunt ]

[Leandro Oliveira]

what is the difference of immutable classes and immutable objects??? I thought immutable classes were classes that could not be extended, and it had something to do with polymorfism... Please clarify!!!

[Barry Gaunt]

My interpretation of an immutable class is one from which only immutable (unchangable) objects can be constructed. You can't construct any objects from Math so by a logical quirk it can be considered immutable. (See above for other characteristics of the Mathclass.)
Once you have constructed a String (or Double) object, you cannot change what's "inside" it. The methods of these classes only allow you to create new modified versions of the original object.
StringBuffer objects are not immutable because their contents can be changed. If you are given a StringBuffer object, then you can change its contents. You are still holding the same StringBuffer object but its contents may have been reversed or appended to.
[ March 17, 2003: Message edited by: Barry Gaunt ]

[Thomas Paul]

From the Sun web site:
Bloch defined an immutable class as a programming element whose instances cannot be modified. "All the information contained in each instance is provided when it is created and is fixed for the lifetime of the object," Bloch said.
By this defintion only String and Double are immutable classes. StringBuffer's instances can be modified and Math can't have any instances.

[Corey McGlone]

Originally posted by Thomas Paul:
From the Sun web site:
Bloch defined an immutable class as a programming element whose instances cannot be modified. "All the information contained in each instance is provided when it is created and is fixed for the lifetime of the object," Bloch said.
By this defintion only String and Double are immutable classes. StringBuffer's instances can be modified and Math can't have any instances.

This is exactly why I didn't include Math in the immutable class group - the contructor is private - you can never make an instance of it, but I suppose an argument could be made either way depending upon the definition you want to use for immutable. But, as immutability has to deal with the instances of a class and not really the class itself, I would have to exclude Math from the immutable classes list. However, I doubt such an ambiguous case would appear on the exam.
Thanks for the great reply, Tom.
Corey

[Barry Gaunt]

I'll throw Math out the window for this question.
Who's going to throw the author of the question (not Erico) off the roof?

[Rattan Mann]

If you think of "immutable" as "read-only" it will clarify a few things in the above discussion.Rattan

[Leandro Oliveira]

thank you guys! I understand it clearlly!!!
thx!