Re: Construct a superclass as itself
The87Boy wrote:
I am trying to construct a superclass [instance] as itself like this:
Your question applies to all classes, not just superclasses.
public Parent(Parent parent) {
// Set this class as itself
this = parent;
}
But it is not allowed by the compiler,
That is because 'this' always refers to the instance itself and cannot be
pointed to any other instance. Just think how confusing it'd be if 'this' was
not a pointer to "this" instance.
but do I have to set all variables like this:
public Parent(Parent parent) {
// Set this class as itself
this.id = parent.getID();
this.cclass = parent.getCClass();
This line is confusing. I assume you're referring to an instance member of
'Parent' called 'cclass' of type 'CClass', a singularly unfortunate choice of
type name. By convention (*not* by language requirement) you capitalize the
first letter of class names but not of variable names, which you've done, but
instances named after the class should retain the rest of the camel-case
capitalization where feasible: 'cClass'.
Given the existence of the confusingly similarly-named 'class' literal and the
instance method 'getClass()', and the usual uselessness of naming something
with 'Class' as part of the class name, you should pick a more meaningful,
less confusing name than 'cClass' or 'getCClass()'.
Theoretically the variable and corresponding 'get' method for "ID" should be
either 'id'/'getId()' or 'iD'/'getID()', but people actually do make the
exception 'ID'/'getID()' or better, as you've done, 'id'/'getID()'. The
convention is a bit looser for concepts normally expressed in all caps, like
"ID", "UPC", "URL", "ISBN" or "SSN".
}
Or are there any easier way?
There are other ways, not really easier ones. From your example it looks like
you want to copy member values from the "other" object, referenced by the
'parent' pointer, to the "this" object, referenced by the 'this' pointer. As
others have said, simply copying pointers as you showed is normal, but
overriding 'clone()' is something to do judiciously [Bloch]. There are a
number of things to watch out for, like the necessity to implement 'Cloneable'.
The possible need to copy deeply is shared by the solutions. Non-primitive
member variables are references, i.e., pointers to objects. Copying a
reference yields two pointers to the same object, so changes made through one
affect the other.
Read the Javadocs
<http://java.sun.com/javase/6/docs/api/java/lang/Object.html#clone()>
<http://java.sun.com/javase/6/docs/api/java/lang/Cloneable.html>
and the relevant chapter of Bloch's /Effective Java (2nd Edition)/, Sun, 2008.
(ISBN-13: 978-0-321-35668-0, ISBN-10: 0-321-35668-3)
<http://java.sun.com/docs/books/effective/>
"Item 11: Override clone judiciously"
--
Lew