Calling constructors while cloning

Question

Here is the quote from Effective Java:

Here it is, copied from the specification for java.lang.Object [JavaSE6]:

Creates and returns a copy of this object. The precise meaning of “copy” may depend on the class of the object. The general intent is that, for any object x, the expression

x.clone() != x 

will be true, and the expression

x.clone().getClass() == x.getClass() 

will be true, but these are not absolute requirements. While it is typically the case that

x.clone().equals(x) 

will be true, this is not an absolute requirement. Copying an object will typically entail creating a new instance of its class, but it may require copying of internal data structures as well. No constructors are called.

Now let's look at the JavaSE6 javadocs

Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object. The general intent is that, for any object x, the expression:

x.clone() != x will be true,

and that the expression:

x.clone().getClass() == x.getClass() 

will be true, but these are not absolute requirements. While it is typically the case that:

x.clone().equals(x) 

will be true, this is not an absolute requirement.

Where did he find the emphasized text? What's the requirement?

Answer 1

A clone is usually regarded as a deep copy. If you only clone the "primary object", its member objects won't be deep copies. So in order to properly make a clone, you would have to go through the whole object hierarchy and clone all members that are clonable.

Using clone() is a bad idea usually, and very cumbersome. It's handier to serialize and deserialize an object graph instead. No constructors are called in that case either.

Answer 2

That's very badly worded. o != o.clone() because you have two objects, they are stored in different memory locations. They are not the same as in two references to the same thing.

Clone is just a method by convention you'd expect an object and it's clone to have the same class, and would probably want to hurt the person who wrote the method that didn't achieve that, but clone might have been the method name that meant something to them.

.equals is also a method. How was it implemented? There are also conventions around this as well, and an option for a developer to implement comparisons you might not agree with.

There is no requirement, as such just a convention and definitely not always followed, either by design or through incompetence.

All we are really talking about here is an extension of equality versus equivalence.