StringBuffer on overriding of equals

[sweta doshi]

Hello ,

I was just going through some previous topics and came across that StringBuffer does not override equals() . Any particualr reason as to why it does not ? Typically , it should override to check if the contents in it are the same just as String class does .

Thanks in advance
Sweta

[rex tony]

Because String is final class,but StringBuffer doesn't,Even as StringBuffer we can change the value

[sweta doshi]

Can you please elaborate on this ? I mean , how does that stop the StringBuffer from overriding equals ?

Thanks

[Deepak Jain]

Ok, i will try to answer this , I hope this is correct. But i will wait for what other ranchers have to say about this.
The equals() is overriden in String, Wrapper classes, because all these are immutable. So for Strings if a.equals(b) returns true that means contents of and b are same and also since a and b are strings its guaranteed that you cannot change the contents of strings, since strings are immutable.

String a = "String";
String b = "String";
a.equals(b) will return true for two reasons
a) Both a and b refer to same single "String" object in String pool.
b) Since strings are immutable its guarenteed that a.equals(b) will return true always, unless offcourse a is not reffereing to other string say "Text".

But for string buffer since StringBuffer objects are mutable that is the contents can be changed a.equals(b) will return false if a and b are referring to two different stringbuffer objects even though the contents are same, since its possible to change the contents of a with a call to append().

So i guess equals() is overridden to check the contents for immutable objects like String, Wrapper objects but not for mutable objects like StringBuffer.

[Henry Wong]

Originally posted by sweta doshi:
Can you please elaborate on this ? I mean , how does that stop the StringBuffer from overriding equals ?

Thanks

Admittedly, I don't understand the mutable argument either. While it is true that changing the equality and hash code will corrupt a hash map/set, if the mutable instance is used as a key, but so what?

IMHO, I think Sun should have overridden equals() and hashCode(), and let those, who choose to mutate the objects when they are used as keys, get burned.

Henry