Re: How to get a clone of the super class?
Lethal Possum wrote:
Let's say I have a class B that extends a class A. Now I have an
instance of B and I need to "clone" it but only as an instance of A,
not B. I can't just cast my instance of B into A, I need a new
instance of A, and only A. I need that the new object's getClass()
method to return A.class.
Is there an easy way to do this? I'd prefer not to copy each field one
by one as there is many of them and maintenance would be difficult
(i.e. how do I make sure someone adding a field to class A will update
my method accordingly).
I know I could probably achieve this by reflection, iterating on every
field of class A, but some fields of A need to be copied in a specific
way. This is already done properly by the clone() method of A so I
would really like to leverage that code.
I am not sure my problem is very clear so I wrote a short piece of
code to demonstrate it. I also included all the solution that I
already know not to work:
== Start of code ==
public class A implements Cloneable {
private boolean _copy = false;
The Java naming conventions call for no underscore in this variable name.
It initializes to 'false', then you set it to 'false'. This is redundant but
some consider it useful for internal documentation and don't mind the extra
assignment of 'false'.
public boolean isCopy() {
return _copy;
}
public Object clone() throws CloneNotSupportedException {
A a = (A)super.clone();
a._cloned = true;
return a;
}
}
I just noticed a typo in my code, "a._cloned = true;" should be
"a._copy = true;".
public class B extends A {
public A test1() throws Exception {
A a = (A)this;
Upcasts are superfluous. 'this' already /is-an/ 'A', and doesn't lose its
'B'-ness by having an 'A' pointer reference it.
return (A)a.clone();
}
public A test2() throws Exception {
A a = (A)clone();
Same remark - upcasts are superfluous.
return a;
}
public A test3() throws Exception {
A a = (A)super.clone();
Once again the cast is superfluous. It does nothing to upcast an instance to
a type that it already is.
This fails for the same reason that the call to 'super.clone()' in 'A'
succeeds. From the Javadocs for 'clone()' (which surely you have read while
researching this question - right?):
this method creates a new instance of the class of this object
Since the class of this object is 'B', the cloned instance is a 'B'.
You can't very well rely on this behavior in 'A' and simultaneously wish it
wouldn't work that way in 'B'.
return a;
}
public A test4() throws Exception {
A a = (A)clone();
return (A)a.clone();
Not only is the upcast superfluous, and not only does 'a' continue to point to
a 'B' instance (casts don't change the runtime type of an object), but now
you've created two instances of 'B' and thrown one away.
}
}
public class test {
public static void main (String[] args) throws Exception {
B b = new B();
A a1 = b.test1();
System.out.println(a1.getClass().toString() + " copy=" +
a1.isCopy());
A a2 = b.test2();
System.out.println(a2.getClass().toString() + " copy=" +
a2.isCopy());
A a3 = b.test3();
System.out.println(a3.getClass().toString() + " copy=" +
a3.isCopy());
A a4 = b.test4();
System.out.println(a4.getClass().toString() + " copy=" +
a4.isCopy());
}
}
== End of code ==
All that I get is:
class B copy=true
Not possible. You have four 'println()' calls. You must have gotten four
lines of output.
When I would like to get:
class A copy=true
The presence of the '_copy' variable (whose name violates the naming
conventions, btw) is against the spirit of 'clone()', but never mind. As the
Javadocs say, for the clone to be value-equal "is not an absolute requirement."
'clone()' is the wrong method for what you are asking. It is designed to
return an instance of the cloned object's class. You are trying to violate
that contract. Create your own method in 'A' to make a copy that instantiates
its own 'A' instance and copies over the desired state without using 'clone()'.
--
Lew