Why String is immutable?

[dmjpro]

Can we have a discussion about it? ;)

[r035198x]

What really do you want to discuss about it?

[dmjpro]

@r035198x
Why String immutable ? i Just want to know the reason ?

[r035198x]

If they were mutable then there would be no String pool implementation and the performance gains of the String pool become lost.

[JosAH]

@dmjpro
Suppose they were mutable; would you've asked the opposite question?

kind regards,

Jos

[dmjpro]

No actually it seems to be odd. String simply character array. If want to change it then we need to have it another variable?

[r035198x]

It actually seems rather odd to me that you knowing that String is just a character array would expect changing it to be easy when you know all the restrictions of arrays ...

[dmjpro]

.........! I forgot that.
Then is the String is immutable for that reason?

[r035198x]

I did not say that is the reason. I was just using your words against you. I have already told you the performance reason in my first response.

[JosAH]

Note that this 'space saving' trick can turn its head against you; have a look at the following code snippet:

String humongous= ... ; // an extremely big String
String small= humongous.substring(0, 1): // a very small String
humongous= null; // free the big one?
 

The substring() method shares its char[] buffer with the String for which it is a substring and the entire String buffer can not be garbage collected. Line two from the above code snippet had better written as:

String small= new String(humongous.substring(0, 1));
 

The String constructor guarantees that a new char[] buffer will be used.

kind regards,

Jos

[dmjpro]

@r035198x
Actually sorry i didn't have a look at your first response.
@Josh
Thanks for your tricky code ;)

[askMeJFAQ]

I hope below comments make every one in commendable position to answer " Why String is immutable".

Look at this example: We have a file open method with login check. We pass a String to this method to process authentication which is necessary before the call will be passed to OS. If String was mutable it was possible somehow to modify its content after the authentication check before OS gets request from program then it is possible to request any file. So if you have a right to open text file in user directory but then on the fly when somehow you manage to change the file name you can request to open "passwd" file or any other. Then a file can be modified and it will be possible to login directly to OS.


Note: I do agree with "String pool implementation and the performance gains of the String pool" but it all w.r.t VM rather than OS.

Cheers
Bunty

[newbie325]

@JosAH

String humongous= ... ; // an extremely big String
String small= humongous.substring(0, 1): // a very small String
humongous= null; // free the big one?
 

The substring() method shares its char[] buffer with the String for which it is a substring and the entire String buffer can not be garbage collected. Line two from the above code snippet had better written as:

String small= new String(humongous.substring(0, 1));
 

This is not right. Call to substring is going to call the constructor String(originalString, startIndex, offset). So the substring is not sharing the char[] from original string