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In a book on Core Java, I found this excerpt :

Think about the way in which the Object class can implement clone. It knows nothing about the object at all, so it can make only a field-by-field copy. If all data fields in the object are numbers or other basic types, copying the fields is just fine. But if the object contains references to subobjects, then copying the field gives you another reference to the subobject, so the original and the cloned objects still share some information.

After reading this I was wondering, that How is the clone method originally implemented in Object Class?

What bothers me is that: how can a method in Object class make a field by field clone of a sub-class object, when it knows nothing about that class?

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Actually, clone() is implemented in native code, so I assume it just does a memory copy (copy all the bytes) without knowing the contents.

Besides that, there is the Reflection API to gain knowlegde about a class (which would be slower, however).

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Well, I dont' know about Reflection, I mean doing clone via Reflection would seem a pretty painstaking idea - I mean think about making all those Method, Field objects and calling all those getClass() ... eekkksss ... jUst sounds about very lengthy and tedious. However, I would buy your argument on clone() being a native method. Since, under the norms of JLS, it is just not possible to define clone() in Object to be able to access the fields of all non-existence classes . ---- AViD
In native code any reference to another object would be a pointer, so if you clone() an object using the default imlementation, what is copied is just the pointer. Thus the cloned object still references the objects the orignal points to. Primitive fields (like int etc.) are directly stored in the object (i.e. no pointer) so they would be copied.
Right. It does: VMMemoryManager.clone(this);
@O.D. depends on the implementation though, in Windows JDK 1.5 it's just protected native Object clone() throws CloneNotSupportedException;. But the result would be the same ;)
@VaidAbhishek: It doesn't need reflection, it just needs a bitwise copy. memcpy() for example. Your last sentence about non-existent classes doesn't make sense. You can't do anything with non-existent classes.
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Read this from the Javadoc:

protected Object clone() -

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. By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that

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

By convention, the object returned by this method should be independent of this object (which is being cloned).

To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.

Means when you have a subobject in your object you shouldnt just clone/copy its reference but the internal structure of this object (in order to create a new instance of it), if each object has its clean clone() methode you will be able to clone it like the parent object otherwise you will have to create a new instance of it and copy its internal premitive fields one by one.

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This is all okay, wisdom about perfect recipe of clone() method, but I want to know how exactly is clone() implemented in Object class ... you have given no clue on that ... :-(
take a look at this source code of the clone method: docjar.com/html/api/java/lang/Object.java.html

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