Immutable classes

Re: Immutable classes

Zalek Bloom wrote:
[color=blue]
> I am learning about difference between String and StringBuffer
> classes. In a book it sayes that a String class is immutable, that
> means that one an instance of String class is created, the string it
> contais cannot be changed, only reference will change to point to a
> different string. So results will be different in the following code
> depending is a, b are defined as String or StringBuffer:
>
> a = new String("test") (or new StringBuffer("t est")
> b = a
>
> a = a.any_method_th at_modifies_a[/color]

The thing with Strings is: when you call a.any_method_th at_modifies_a.. ., a
is not really modified, but instead a new String is created. So to say that
the references changes is incorrect: you only get a reference to a new
String object. The original String object still exists.
[color=blue]
>
> out.println(b)
>
> My question: how do I know if a class is Immutable? How to define an
> Immutable class?[/color]

As you said, a class is immutable if it's contents can not be changed, which
means that instances of the class will always be the same after they have
been constructed.

So if you have a class that contains a field which you can only set within
the constructor, you have an immutable class.

eg:

public class Immutable {
private Object something

public Immutable(Objec t something){
this.something = something;
}
}

Now instances of this class can not be changed anymore, it's something field
will always be the same.

Now consider this:

public class Immutable {
private Object something

public Immutable(Objec t something){
this.something = something;
}

public void setSomething(Ob ject something) {
this.something = something;
}
}

Now you can change the something field of the instance, so the class is no
longer immutable.
[color=blue]
>
> Thanks,
>
> Zalek[/color]

--
Regards,
Christophe Vanfleteren

Re: Immutable classes

Here is a discussion of immutables :


Zalek Bloom wrote:
[color=blue]
>I am learning about difference between String and StringBuffer
>classes. In a book it sayes that a String class is immutable, that
>means that one an instance of String class is created, the string it
>contais cannot be changed, only reference will change to point to a
>different string. So results will be different in the following code
>depending is a, b are defined as String or StringBuffer:
>
>a = new String("test") (or new StringBuffer("t est")
>b = a
>
>a = a.any_method_th at_modifies_a
>
>out.println( b)
>
>My question: how do I know if a class is Immutable? How to define an
>Immutable class?
>
>Thanks,
>
>Zalek
>
>[/color]