 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Java Forum / General / August 2007Call By Reference
class ABC { public static void swap(Integer x,Integer y) { x^=y^=x^=y; } public static void main(String args[]) { Integer a = 10; Integer b = 20; swap(a,b); System.out.println("a="+a+"\nb="+b); } } O/P a=10 b=20 Can anybody tell me why this program is not giving correct o/p i.e a=20 b=10 > public static void swap(Integer x,Integer y) > { > x^=y^=x^=y; > } That is (I think, I wouldn't write code like that!) public static void swap(Integer x, Integer y) { x = Integer.valueOf(x.intValue() ^ y.intValue()); y = Integer.valueOf(y.intValue() ^ x.intValue()); x = Integer.valueOf(x.intValue() ^ y.intValue()); } > Can anybody tell me why this program is not giving correct o/p i.e Anyway, Java is always call by value, even when those values are references (this makes code very much more easy to follow). Your code is just changing the references (which are copies), not the objects pointed to. You could write something similar that changes the object themselves. Integer is immutable, so you will need a different type. import java.util.concurrent.atomic.AtomicInteger; ... public static void swap( final AtomicInteger x, final AtomicInteger y ) { x.set(x.get() ^ y.get()); y.set(y.get() ^ x.get()); x.set(x.get() ^ y.get()); } Tom Hawtin ... > You could write something similar that changes the object themselves. > Integer is immutable, so you will need a different type. [quoted text clipped - 8 lines] > x.set(x.get() ^ y.get()); > } Also, many uses of swap really exchange elements of the same array: /* swap elements i and j of array.*/ public static void intArraySwap(int[] array, int i, int j){ int temp = array[i]; array[i] = array[j]; array[j] = temp; } Incidentally, in Java code, why do people prefer the xor-swap to using a temporary? To me, the temporary version seems much more direct and obvious. Patricia > Incidentally, in Java code, why do people prefer the xor-swap to using a > temporary? To me, the temporary version seems much more direct and obvious. Because it shows l33t h4x0r ski11z? It made sense in assembler on pre-RISC processors with few registers. It makes little sense on heavily optimised processors with large register files. It makes even less sense in a 3GL. I suggest to anyone who thinks xor swaps are k3wl, that they work out their frustrations with a copy of Hacker's Delight and not in their common or garden production or example code. http://www.amazon.co.uk/dp/0201914654 Tom Hawtin >> Incidentally, in Java code, why do people prefer the xor-swap to >> using a temporary? To me, the temporary version seems much more [quoted text clipped - 4 lines] > heavily optimised processors with large register files. It makes even > less sense in a 3GL. There is a "neat trick" for making a singly-linked list that can be traversed in both directions by having each "pointer" be the XOR of the addresses of he previous and next members. It doesn't work in Java, which is probably a good thing. And it breaks conservative garbage collectors in C++. > There is a "neat trick" for making a singly-linked list that can be > traversed in both directions by having each "pointer" be the XOR of the > addresses of he previous and next members. It doesn't work in Java, which > is probably a good thing. And it breaks conservative garbage collectors in > C++. Now that is a neat trick. You could write impure Java (with sun.misc.Unsafe for instance) to do it. However, you would still confuse the garbage collector. I guess you could have an array to store references for GC, but that would still hinder those GC algorithms that arrange objects close to referrers (as would a linked list algorithm that replaced nodes with arrays of references and (xor) indexes to next 'node' - but it makes me want to try it). Tom Hawtin >>> Incidentally, in Java code, why do people prefer the xor-swap to >>> using a temporary? To me, the temporary version seems much more [quoted text clipped - 9 lines] > is probably a good thing. And it breaks conservative garbage collectors in > C++. I'm aware of several situations in which the xor-swap trick is useful. For example, it can be used to implement swap as a C expression for use in a pre-processor macro. I'm trying to find out why people use it in ordinary Java code. I don't think any of the special cases apply, but maybe I'm missing something. The other possibility is that users of xor-swap consider it better style than temp-swap. If so, why? Patricia > The other possibility is that users of xor-swap consider it better style > than temp-swap. If so, why? At a guess, they simply find the use of a temporary, especially an explicit temporary, to be ugly. >The other possibility is that users of xor-swap consider it better style >than temp-swap. If so, why? It is magic. It is cool. It is a way of showing off.  SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > public static void swap(Integer x,Integer y) > { > x^=y^=x^=y; > } you can't write a swap routine in Java. See http://mindprod.com/jgloss/swap.html http://mindprod.com/jgloss/callbyreference.html http://mindprod.com/jgloss/callbyvalue.html Though it is theoretically allowed, chaining operators like than is just a way to tick people off. Don't do it. It's only function is writing unmaintainable code. See http://mindprod.com/jgloss/unmain.html SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
|
Reply to this Message
|
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.