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Question about clone a Hashtable
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| George George via JavaKB.com 2005-05-15, 8:57 am |
| Hello everyone,
I want to clone the content of a Hashtable (its keys, its values and its
key-value relationship). But I have found that the clone method can not
completely clone a Hashtable, as is mentioned in the description of clone
method of Java API Specification,
Creates a shallow copy of this hashtable. All the structure of the
hashtable itself is copied, but the keys and values are not cloned. This is
a relatively expensive operation.
I am wondering what is the efficient and effective approach of cloning a
Hashtable, including the clone of its keys, its values and its key-value
relationship.
Thanks in advance,
George
--
Message posted via http://www.javakb.com
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| ByteCoder 2005-05-15, 8:57 am |
| "George George via JavaKB.com" <forum@nospam.JavaKB.com> wrote in
news:bb9ee96855a04663b0832e6975ba5d2f@Ja
vaKB.com:
> Hello everyone,
>
>
> I want to clone the content of a Hashtable (its keys, its values and
> its key-value relationship). But I have found that the clone method
> can not completely clone a Hashtable, as is mentioned in the
> description of clone method of Java API Specification,
>
> Creates a shallow copy of this hashtable. All the structure of the
> hashtable itself is copied, but the keys and values are not cloned.
> This is a relatively expensive operation.
>
> I am wondering what is the efficient and effective approach of cloning
> a Hashtable, including the clone of its keys, its values and its
> key-value relationship.
>
>
> Thanks in advance,
> George
>
Perhaps something like this would work (correct me if I'm wrong):
HashTable clone = originalHashTable;
--
-------------
- ByteCoder - ...I see stupid people
-------------
Curiosity *Skilled* the cat
| |
| Tony Morris 2005-05-15, 3:58 pm |
|
"ByteCoder" <me@none.com> wrote in message
news:Xns9657840B62E5FBCfromHolland@213.75.12.135...
> "George George via JavaKB.com" <forum@nospam.JavaKB.com> wrote in
> news:bb9ee96855a04663b0832e6975ba5d2f@Ja
vaKB.com:
>
>
> Perhaps something like this would work (correct me if I'm wrong):
>
> HashTable clone = originalHashTable;
>
> --
> -------------
> - ByteCoder - ...I see stupid people
> -------------
> Curiosity *Skilled* the cat
Yes, that is wrong. It will perform a reference assignment, and nothing
more.
Cloning a Hashtable is indicative of something seriously broken, and due to
the common misconception to the contrary, I won't bother justifying the
assertion for fear of boring, useless internet arguments.
--
Tony Morris
Software Engineer, IBM Australia.
BInfTech, SCJP 1.4, SCJP 5.0, SCJD
http://www.jtiger.org/ JTiger Unit Test Framework for Java
http://qa.jtiger.org/ Java Q&A (FAQ, Trivia)
http://xdweb.net/~dibblego/
| |
| ByteCoder 2005-05-15, 3:58 pm |
| "Tony Morris" <not@telling.you> wrote in news:428731d1@quokka.wn.com.au:
>
> "ByteCoder" <me@none.com> wrote in message
> news:Xns9657840B62E5FBCfromHolland@213.75.12.135...
>
> Yes, that is wrong. It will perform a reference assignment, and
> nothing more.
> Cloning a Hashtable is indicative of something seriously broken, and
> due to the common misconception to the contrary, I won't bother
> justifying the assertion for fear of boring, useless internet
> arguments.
>
Thank you for pointing that out to me. So you're actually saying there
is no really no proper way to clone a HashSet?
Actually, you could create a new HastSet and copy each item from the old
HashSet to the new one. Granted, it's not as fast as 'cloning' it, but
it should work.
--
-------------
- ByteCoder - ...I see stupid people
-------------
Curiosity *Skilled* the cat
| |
| George George via JavaKB.com 2005-05-16, 8:58 am |
| Thanks Tony,
I have written a simple sample below, but from the sample's output, I have
found out that it seems that the clone method of Hashtable copies the the
content of both keys and values, not as mentioned in the Java API
specification of clone method of class Hashtable "but the keys and values
are not cloned".
Do you know what points do the Java specification mean?
code:
Hashtable table1 = new Hashtable();
table1.put ("key1", "value1");
Hashtable table2 = (Hashtable) table1.clone();
if (table2.containsKey ("key1"))
{
System.out.println ("Contains!");
table2.put ("key1", "value2");
String newValue1 = (String) table1.get ("key1");
System.out.println ("New value1 of table1: " + newValue1);
newValue1 = (String) table2.get ("key1");
System.out.println ("New value1 of table2: " + newValue1);
}
else
{
System.out.println ("Not contains!");
}
Output is:
Contains!
New value1 of table1: value1
New value1 of table2: value2
regards,
George
--
Message posted via http://www.javakb.com
| |
| G Winstanley 2005-06-09, 3:58 am |
| On Mon, 16 May 2005 06:57:51 GMT, the cup of "George George via JavaKB.com"
<forum@JavaKB.com> overfloweth with the following:
> Thanks Tony,
>
>
> I have written a simple sample below, but from the sample's output, I have
> found out that it seems that the clone method of Hashtable copies the the
> content of both keys and values, not as mentioned in the Java API
> specification of clone method of class Hashtable "but the keys and values
> are not cloned".
>
> Do you know what points do the Java specification mean?
>
> code:
> Hashtable table1 = new Hashtable();
> table1.put ("key1", "value1");
> Hashtable table2 = (Hashtable) table1.clone();
> if (table2.containsKey ("key1"))
> {
> System.out.println ("Contains!");
> table2.put ("key1", "value2");
> String newValue1 = (String) table1.get ("key1");
> System.out.println ("New value1 of table1: " + newValue1);
> newValue1 = (String) table2.get ("key1");
> System.out.println ("New value1 of table2: " + newValue1);
> }
> else
> {
> System.out.println ("Not contains!");
> }
>
>
> Output is:
>
> Contains!
> New value1 of table1: value1
> New value1 of table2: value2
>
>
> regards,
> George
The clone() method creates new references to the same objects that are used
as keys/values for the original. All you are achieving is assigning a
different object to the same key.
Forgive my poor ASCII art, but this is roughly what I mean:
Original
--------
Hashtable1
Key1 -------------> "key1" <------
Value1 -----------> "value1" <- |
Hashtable2 | |
Key1 -------------------------)---
Value1 -----------------------
Modified
--------
Hashtable1
Key1 -------------> "key1" <------
Value1 -----------> "value1" |
Hashtable2 |
Key1 -----------------------------
Value1 -----------> "value2"
Stan
| |
| George George via JavaKB.com 2005-06-09, 8:58 am |
| Thanks Stan,
G Winstanley wrote:
>[quoted text clipped - 33 lines]
>
>The clone() method creates new references to the same objects that are used
>as keys/values for the original. All you are achieving is assigning a
>different object to the same key.
>
>Forgive my poor ASCII art, but this is roughly what I mean:
>
>Original
>--------
>Hashtable1
> Key1 -------------> "key1" <------
> Value1 -----------> "value1" <- |
>Hashtable2 | |
> Key1 -------------------------)---
> Value1 -----------------------
>
>Modified
>--------
>Hashtable1
> Key1 -------------> "key1" <------
> Value1 -----------> "value1" |
>Hashtable2 |
> Key1 -----------------------------
> Value1 -----------> "value2"
>
>Stan
What do you mean "All you are achieving is assigning a different object to
the same key"? Could you please provide a sample program to illustrate your
idea?
regards,
George
--
Message posted via JavaKB.com
http://www.javakb.com/Uwe/Forums.as...-setup/200506/1
| |
| G Winstanley 2005-06-10, 4:02 pm |
| On Thu, 09 Jun 2005 07:49:20 GMT, the cup of "George George via JavaKB.com"
<forum@JavaKB.com> overfloweth with the following:
> Thanks Stan,
>
>
> G Winstanley wrote:
>
> What do you mean "All you are achieving is assigning a different object to
> the same key"? Could you please provide a sample program to illustrate your
> idea?
>
>
> regards,
> George
Sure. This illustrates a bit of both...
import java.util.*;
public class HashCloneTest
{
public static void main(String[] args)
{
Hashtable table1 = new Hashtable();
Thing x = new Thing(1);
table1.put("key", x);
System.out.println("table1: " + table1.get("key"));
Hashtable table2 = (Hashtable)table1.clone();
System.out.println("table2: " + table2.get("key"));
((Thing)table1.get("key")).setNum(2);
System.out.println("table1: " + table1.get("key"));
System.out.println("table2: " + table2.get("key"));
table2.put("key", new Thing(3));
System.out.println("table2: " + table2.get("key"));
}
private static final class Thing
{
private int num;
private Thing(int i) { setNum(i); }
private void setNum(int i) { this.num = i; }
public String toString() { return "Thing[" + num + "]"; }
}
}
Stan
| |
| Robert Maas, see http://tinyurl.com/uh3t 2005-06-11, 3:58 am |
| > From: "George George via JavaKB.com" <forum@nospam.JavaKB.com>
> I want to clone the content of a Hashtable (its keys, its values and
> its key-value relationship). But I have found that the clone method
> can not completely clone a Hashtable, as is mentioned in the
> description of clone method of Java API Specification, ... Creates a
> shallow copy of this hashtable. All the structure of the hashtable
> itself is copied, but the keys and values are not cloned. ...
There are only three "canonical" copy operations possible:
- Copying the reference (pointer) but don't copy any of the structure.
(Java does this by simple assignment of pointer variables)
- Copy the structure of the container itself, but just copy references
to all components within the container.
(This is the shallow copy you are referring to.)
- Copy all the structure as deep as it goes. Unfortunately there can be
pointer loops, so if you do this by simple recursive algorithm you
get a recursive loop causing stack overflow. Java solves this in the
serialization mechanism by building a hash table of all items copied
and when it encounters a pointer to an item already in the table it
makes a back-reference instead of copying it a second time.
(If this is the behaviour you want, why don't you just serialize to a
string then deserialize back to a new structure which is an isomorphic
copy of the original?)
If you want anything more than a shallow copy but less than a full deep
serialization, how do you expect Java to guess what you want??
You should write your own class which sub-classes Hashtable, and define
whatever kind of very-deep-but-not-all-the-way cloning you want.
By the way, for enlightenment, you really should read this:
Linkname: P.S.: The Best of Intentions
URL: http://www.nhplace.com/kent/PS/EQUAL.html
It's from the point of view of lisp, but it applies equally well to java.
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