Path: csiph.com!aioe.org!.POSTED!not-for-mail From: =?UTF-8?Q?Arne_Vajh=c3=b8j?= Newsgroups: comp.lang.java.programmer Subject: Re: Are you sure, Mr. Gosling? Date: Tue, 7 Jun 2016 17:33:12 -0400 Organization: Aioe.org NNTP Server Lines: 73 Message-ID: References:

<1eb77568-ae31-4316-ac21-a33db79c5aa4@googlegroups.com> <5c864ac4-9634-4c88-9d6b-d14dbf3b3886@googlegroups.com> <746532a2-c6db-4d58-afc0-1cc57440eff5@googlegroups.com> NNTP-Posting-Host: 7SK6yTFfqwlIPNBLl3l7Qg.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 X-Notice: Filtered by postfilter v. 0.8.2 Xref: csiph.com comp.lang.java.programmer:34550 On 6/7/2016 1:22 PM, Eric Douglas wrote: > On Tuesday, June 7, 2016 at 1:10:36 PM UTC-4, Robert Klemme wrote: >> That sounds more like a use case for clone() to me as it seems to >> stress that the original type is preserved. It really depends on >> the usage. > > Maybe I'm getting confused then. I thought clone() sounded wrong > because it should return an instance of that class, so if you extend > the class and call super.clone it gets an instance of the base class? > If your extended class needs to return an instance of itself, which > needs to copy all the same values as the super class plus a few > more... You have already gotten several examples. I will just add a ref to the docs: https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#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. The method clone for class Object performs a specific cloning operation. First, if the class of this object does not implement the interface Cloneable, then a CloneNotSupportedException is thrown. Note that all arrays are considered to implement the interface Cloneable and that the return type of the clone method of an array type T[] is T[] where T is any reference or primitive type. Otherwise, this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation. The class Object does not itself implement the interface Cloneable, so calling the clone method on an object whose class is Object will result in throwing an exception at run time. Which pretty well explains why sub classing is actually a thing that clone handles very well. Arne