In Java we use final
keyword with variables to specify its values are not to be changed.
But I see that you can change the value in the constructor / methods of the class. Again, if the variable is static
then it is a compilation error.
Here is the code:
import java.util.ArrayList;
import java.util.List;
class Test {
private final List foo;
public Test()
{
foo = new ArrayList();
foo.add("foo"); // Modification-1
}
public static void main(String[] args)
{
Test t = new Test();
t.foo.add("bar"); // Modification-2
System.out.println("print - " + t.foo);
}
}
Above code works fine and no errors.
Now change the variable as static
:
private static final List foo;
Now it is a compilation error. How does this final
really work?
You are always allowed to initialize a final
variable. The compiler makes sure that you can do it only once.
Note that calling methods on an object stored in a final
variable has nothing to do with the semantics of final
. In other words: final
is only about the reference itself, and not about the contents of the referenced object.
Java has no concept of object immutability; this is achieved by carefully designing the object, and is a far-from-trivial endeavor.