Why can final object be modified?
Solution 1
final
simply makes the object reference unchangeable. The object it points to is not immutable by doing this. INSTANCE
can never refer to another object, but the object it refers to may change state.
Solution 2
Being final is not the same as being immutable.
final != immutable
The final
keyword is used to make sure the reference is not changed ( that is, the reference it has can't be substituted with a new one )
But, if the attribute is self is modifiable it is ok to do what you have just described.
For instance
class SomeHighLevelClass {
public final MutableObject someFinalObject = new MutableObject();
}
If we instantiate this class, we won't be able to assign other value to the the attribute someFinalObject
because it is final.
So this is not possible:
....
SomeHighLevelClass someObject = new SomeHighLevelClass();
MutableObject impostor = new MutableObject();
someObject.someFinal = impostor; // not allowed because someFinal is .. well final
But if the object it self is mutable like this:
class MutableObject {
private int n = 0;
public void incrementNumber() {
n++;
}
public String toString(){
return ""+n;
}
}
Then, the value contained by that mutable object may be changed.
SomeHighLevelClass someObject = new SomeHighLevelClass();
someObject.someFinal.incrementNumber();
someObject.someFinal.incrementNumber();
someObject.someFinal.incrementNumber();
System.out.println( someObject.someFinal ); // prints 3
This has the same effect that your post:
public static void addProvider(ConfigurationProvider provider) {
INSTANCE.providers.add(provider);
}
Here you are not changing the value of INSTANCE, your are modifying its internal state ( via, providers.add method )
if you want to prevent that the class definition should be changed like this:
public final class ConfigurationService {
private static final ConfigurationService INSTANCE = new ConfigurationService();
private List providers;
private ConfigurationService() {
providers = new ArrayList();
}
// Avoid modifications
//public static void addProvider(ConfigurationProvider provider) {
// INSTANCE.providers.add(provider);
//}
// No mutators allowed anymore :)
....
But, it might not make much sense :)
By the way, you also have to synchronize access to it basically for the same reason.
Solution 3
The key to the misunderstanding is in your question's title. It's not the object which is final, it's the variable. The variable's value can't change, but the data within it can.
Always remember that when you declare a reference type variable, the value of that variable is a reference, not an object.
Solution 4
final just means the reference can't be changed. You can't reassign INSTANCE to another reference if it's declared as final. The internal state of the object is still mutable.
final ConfigurationService INSTANCE = new ConfigurationService();
ConfigurationService anotherInstance = new ConfigurationService();
INSTANCE = anotherInstance;
would throw a compilation error
Solution 5
Once a
final
variable has been assigned, it always contains the same value. If afinal
variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object. This applies also to arrays, because arrays are objects; if afinal
variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array.
Here's a guide on making an object immutable.
![Matt McCormick](https://i.stack.imgur.com/rZ6Es.jpg?s=256&g=1)
Matt McCormick
Updated on January 20, 2020Comments
-
Matt McCormick over 4 years
I came across the following code in a code base I am working on:
public final class ConfigurationService { private static final ConfigurationService INSTANCE = new ConfigurationService(); private List providers; private ConfigurationService() { providers = new ArrayList(); } public static void addProvider(ConfigurationProvider provider) { INSTANCE.providers.add(provider); } ...
INSTANCE
is declared asfinal
. Why can objects be added toINSTANCE
? Shouldn't that invalidate the use of final. (It doesn't).I'm assuming the answer has to do something with pointers and memory but would like to know for sure.
-
Abel Morelos over 14 years+1, For more details please check java.sun.com/docs/books/jls/second_edition/html/…, section 4.5.4.
-
diegoaguilar over 10 yearsSo let's say I deserialize an
ConfigurationService
object and I try to do a INSTANCE =deserializedConfigurationService
wouldn't be allowed? -
Sean Owen over 10 yearsYou could never assign
INSTANCE
to refer to another object. It doesn't matter where the other object came from. (NB there is oneINSTANCE
perClassLoader
that has loaded this class. You could in theory load the class several times in one JVM and each is separate. But this is a different, technical point.) -
Sean Owen about 7 years@AkhilGite your edit to my answer made it wrong; it actually reversed the sense of the sentence, which was correct. The reference is immutable. The object remains mutable. It does not "become immutable".
-
AkhilGite about 7 years@SeanOwen Sry for the edit I did, Your statement is completely right and Thanks.