AppletTalk.com Java discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  no pointer in Java => my problem Goto page 1, 2, 3, 4, 5  Next          AppletTalk.com Forum Index -> Java Language Programming View previous topic :: View next topic   Author Message rowla Guest Posted: Thu Apr 15, 2004 5:41 pm    Post subject: no pointer in Java => my problem I have a class: class Z { int i; public Z(int y) {this.i=y;} } and a function private void f(Z j) { Z s = new Z(65); j=s; } when I do: Z j = new Z(47); f(j); System.out.println(j.i); I get 47 from System.out.println(j.i); what should I do to get 65? -- best regards, jacek Back to top Joona I Palaste Guest Posted: Thu Apr 15, 2004 5:43 pm    Post subject: Re: no pointer in Java => my problem rowla <rowla.WYTNIJ.TO@i.to.o2.pl> scribbled the following: Quote: I have a class: class Z { int i; public Z(int y) {this.i=y;} } and a function private void f(Z j) { Z s = new Z(65); j=s; } when I do: Z j = new Z(47); f(j); System.out.println(j.i); I get 47 from System.out.println(j.i); what should I do to get 65? Change your design. Make the method f() return an int instead of trying to change its parameter. -- /-- Joona Palaste (palaste (AT) cc (DOT) helsinki.fi) ------------- Finland -------- -- http://www.helsinki.fi/~palaste --------------------- rules! --------/ "Life without ostriches is like coffee with milk." - Mika P. Nieminen Back to top rowla Guest Posted: Thu Apr 15, 2004 5:52 pm    Post subject: Re: no pointer in Java => my problem Quote: Change your design. Make the method f() return an int instead of trying to change its parameter. I forgot to add that I want to do it without returning anything... -- pozdrawiam, jacek Back to top Joona I Palaste Guest Posted: Thu Apr 15, 2004 5:57 pm    Post subject: Re: no pointer in Java => my problem rowla <rowla.WYTNIJ.TO@i.to.o2.pl> scribbled the following: Quote: Change your design. Make the method f() return an int instead of trying to change its parameter. I forgot to add that I want to do it without returning anything... Why? Does your religion forbid returning values? Do you have a childhood trauma about returning values? Did your boss tell you that if you return even one value, you're fired? Did someone dare you to see if you can do it without returning a value? -- /-- Joona Palaste (palaste (AT) cc (DOT) helsinki.fi) ------------- Finland -------- -- http://www.helsinki.fi/~palaste --------------------- rules! --------/ "That's no raisin - it's an ALIEN!" - Tourist in MTV's Oddities Back to top rowla Guest Posted: Thu Apr 15, 2004 6:15 pm    Post subject: Re: no pointer in Java => my problem 1) in the real program I change 3 different objects of 3 different types in my method and all I want to do is to return one of them and change in the method the 2 others so that the changes are seen externally. 2) what is more, I want to learn how to do a thing like that in Java. In C/C++ there's no problem with doing so, because you have pointers.. 3) if my ignorance is making you so nervous that you write so stupid questions, than just don't read my posts.. best wishes, jacek Back to top Joona I Palaste Guest Posted: Thu Apr 15, 2004 6:21 pm    Post subject: Re: no pointer in Java => my problem rowla <rowla.WYTNIJ.TO@i.to.o2.pl> scribbled the following: Quote: 1) in the real program I change 3 different objects of 3 different types in my method and all I want to do is to return one of them and change in the method the 2 others so that the changes are seen externally. 2) what is more, I want to learn how to do a thing like that in Java. In C/C++ there's no problem with doing so, because you have pointers.. You'll have to wrap the references up in some object and pass that along to your method. It's the only way. If you still don't like it, feel free to write your own language. Quote: 3) if my ignorance is making you so nervous that you write so stupid questions, than just don't read my posts.. No, I simply wanted a better justification for not returning a value than "I don't want to do it". -- /-- Joona Palaste (palaste (AT) cc (DOT) helsinki.fi) ------------- Finland -------- -- http://www.helsinki.fi/~palaste --------------------- rules! --------/ "The trouble with the French is they don't have a word for entrepreneur." - George Bush Back to top Bryce (Work) Guest Posted: Thu Apr 15, 2004 7:10 pm    Post subject: Re: no pointer in Java => my problem On Thu, 15 Apr 2004 19:41:50 +0200, "rowla" <rowla.WYTNIJ.TO@I.TO.o2.pl> wrote: Quote: I have a class: class Z { int i; public Z(int y) {this.i=y;} } and a function private void f(Z j) { Z s = new Z(65); j=s; } when I do: Z j = new Z(47); f(j); System.out.println(j.i); I get 47 from System.out.println(j.i); what should I do to get 65? Ok... Here's the problem. http://www.javaworld.com/javaworld/javaqa/2000-05/03-qa-0526-pass.html In short, java passes objects by value. A swap won't work because all you are doing is changing what the value passed in is pointing to. In effect, they both are 2 different pointers, pointing to the same object in memory (Java doesn't use pointers, but they are references. I'm not knowledgable enough to know the difference...). You are changing the reference that's passed into the function, but not the original reference. Since its just a copied value, when the procedure ends, its gone. You can work on the object itself though. For example, the following might work: private void f(Z j) { j.i = 65; ... } Now, when you return from your function, j.i will print 65. also, I know you probably were obfuscating your code so that company info was left out, etc... But on some newsreaders, i and j look very similar and can be confusing. Best to use descriptive words like int myInt; Z myZ etc... Makes it easier to read. -- now with more cowbell Back to top Christophe Vanfleteren Guest Posted: Thu Apr 15, 2004 7:16 pm    Post subject: Re: no pointer in Java => my problem Bryce (Work) wrote: Quote: In short, java passes objects by value. Just a nitpick: Java passes references to objects by value, not the objects themselves. -- Kind regards, Christophe Vanfleteren Back to top Dale King Guest Posted: Thu Apr 15, 2004 7:49 pm    Post subject: Re: no pointer in Java => my problem "rowla" <rowla.WYTNIJ.TO@I.TO.o2.pl> wrote Quote: 1) in the real program I change 3 different objects of 3 different types in my method and all I want to do is to return one of them and change in the method the 2 others so that the changes are seen externally. Then you have a very bad design. You need to fix the design, not try to figure out how to get Java to support your bad design. Usually it is the case that you just aren't thinking from an object-oriented perspective. The fact that you used the word function is also a good clue. Quote: 2) what is more, I want to learn how to do a thing like that in Java. In C/C++ there's no problem with doing so, because you have pointers.. And C/C++ has goto as well. There is no reason bad designs need to carry over to new languages. Quote: 3) if my ignorance is making you so nervous that you write so stupid questions, than just don't read my posts.. Joona asked a very good question. The question is "why?". The question you asked was "how?" You have to look deeper to what you are trying to accomplish and why, and not focus on how you've done it in other languages. To help you fix your design we have to understand what abstraction you are trying to represent. There are dozens of ways to implement it, but we don't know which is the right one without understanding the problem you are trying to represent. -- Dale King Blog: http://nobleware.homedns.org/Blog Back to top rowla Guest Posted: Thu Apr 15, 2004 8:19 pm    Post subject: Re: no pointer in Java => my problem Quote: Then you have a very bad design. You need to fix the design, not try to figure out how to get Java to support your bad design. Usually it is the case that you just aren't thinking from an object-oriented perspective. The fact that you used the word function is also a good clue. [...] To help you fix your design we have to understand what abstraction you are trying to represent. There are dozens of ways to implement it, but we don't know which is the right one without understanding the problem you are trying to represent. The thing was that after implementing some algorithm that was working OK I had to rewrite it to make it consistent with some coding standards. when of "the rules" is that a method shouls be no longer than approximately 20-25 lines, because otherwise it's hard to read. One of the methods was especially hard to divide into smaller ones and that's why I had problems with this pointer stuff.. But thanks to Bryce my roble has gone (with the wind;) Quote: And C/C++ has goto as well. There is no reason bad designs need to carry over to new languages. I'm not saying that C++ is better than Java, but I found pointers very useful (goto are not good of course:) -- pozdrawiam, jacek Back to top Bryce (Work) Guest Posted: Thu Apr 15, 2004 8:19 pm    Post subject: Re: no pointer in Java => my problem On Thu, 15 Apr 2004 19:16:34 GMT, Christophe Vanfleteren <c.v4nfl3t3r3n (AT) pandora (DOT) be> wrote: Quote: Bryce (Work) wrote: In short, java passes objects by value. Just a nitpick: Java passes references to objects by value, not the objects themselves. true. That's why I said "In short"... I generally think of objects as references anyways, so I get confused when trying to explain. -- now with more cowbell Back to top Chris Smith Guest Posted: Thu Apr 15, 2004 8:26 pm    Post subject: Re: no pointer in Java => my problem "Dale King" <kingd[at]tmicha[dot]net> wrote: Quote: To help you fix your design we have to understand what abstraction you are trying to represent. There are dozens of ways to implement it, but we don't know which is the right one without understanding the problem you are trying to represent. Dale's right, but http://jinx.swiki.net/66 might at least point in the right direction for finding a solution. -- www.designacourse.com The Easiest Way to Train Anyone... Anywhere. Chris Smith - Lead Software Developer/Technical Trainer MindIQ Corporation Back to top John C. Bollinger Guest Posted: Thu Apr 15, 2004 8:31 pm    Post subject: Re: no pointer in Java => my problem Joona I Palaste wrote: Quote: rowla <rowla.WYTNIJ.TO@i.to.o2.pl> scribbled the following: 1) in the real program I change 3 different objects of 3 different types in my method and all I want to do is to return one of them and change in the method the 2 others so that the changes are seen externally. 2) what is more, I want to learn how to do a thing like that in Java. In C/C++ there's no problem with doing so, because you have pointers.. You'll have to wrap the references up in some object and pass that along to your method. It's the only way. If you still don't like it, feel free to write your own language. Or change class Z so that you can modify a Z instead of creating a new instance. This is only possible if the code for Z is under your control, of course. Java simply does not have pass-by-reference, only pass-by-value. People can get confused about that when they start passing reference types around, probably more because of the clash of terminology than for any other reason. When you pass a reference, you are passing that reference _by_value_, so the dummy argument in the method (initially) contains a copy of the value of the reference. Assigning a new value to the dummy does not affect any other variable that contains the original value. As I understand it, in fact, C and C++ also have only pass-by-value. They just have a fairly facile way creating multiple levels of indirection. If you want to emulate C in this [NOT RECOMMENDED] then you can do this: [...] void fooPasser(Foo foo) { Foo[] fooPtr = new Foo[] { foo }; passByReference(fooPtr); // now fooPtr[0] != foo } void passByReference(Foo[] fooPtr) { Foo bar = new Foo(); fooPtr[0] = bar; } [...] The syntax is a bit messier than the C equivalent, but the semantics are as similar as you can get in this case. John Bollinger [email]jobollin (AT) indiana (DOT) edu[/email] Back to top andreia g. Guest Posted: Thu Apr 15, 2004 8:36 pm    Post subject: Re: no pointer in Java => my problem rowla wrote: Quote: I have a class: class Z { int i; public Z(int y) {this.i=y;} } and a function private void f(Z j) { Z s = new Z(65); j=s; } when I do: Z j = new Z(47); f(j); System.out.println(j.i); I get 47 from System.out.println(j.i); what should I do to get 65? When you create your local object "s" and assign it to "j", you are changing "j" only on a local scope. Seeing as "s" only exists inside the function "f", "j" returns to it's original value when you return from the function. Hence, you get 47 instead of 65. "j" is a reference, so you can change the "f" function to assign 65 to the variable "i" directly or through a method, instead of trying to change all of "j". The best way, imho, to go about it, is to have a method that takes an int and sets the variable "i" to that, just like when you construct the object. If you do that, you'll get the updated value when you return, instead of the original value. shana developer and otherwise computer nut Back to top Timo Kinnunen Guest Posted: Thu Apr 15, 2004 8:53 pm    Post subject: Re: no pointer in Java => my problem On Thu, 15 Apr 2004 19:41:50 +0200, rowla wrote: Quote: I have a class: class Z { int i; public Z(int y) {this.i=y;} } and a function private void f(Z j) { Z s = new Z(65); j=s; } when I do: Z j = new Z(47); f(j); System.out.println(j.i); I get 47 from System.out.println(j.i); what should I do to get 65? Here's an answer to the question. What you have to realize is that in C++ the compiler provides a default implementation for few member functions, unless you tell it otherwise. One of those functions is the assignment operator, .operator=(). You also have to realize that in C++, = in j=s is syntactic sugar to execute the assignment operator. The Java compiler neither provides such default methods nor is = syntactic sugar for them, so if you want to have the same effect, you'll have to do it manually. In your case, the code would look something like this: class Z { int i; public Z(int y) {this.i=y;} public Z assign(Z z) {this.i=z.i; return this;} } and private void f(Z j) { Z s = new Z(65); j.assign(s); } As you see, the "assignment operator" is just a normal method, which should you help you migrate the code towards a more Java-like design. Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AppletTalk.com Forum Index -> Java Language Programming All times are GMT Goto page 1, 2, 3, 4, 5  Next Page 1 of 5   Jump to: Select a forum Java Newsgroups----------------Java DiscussionsWeb-based java games comp.lang.java 3D Graphics API's for Java Java: Support and criticism Java System Announcements comp.lang.java.api JavaBeans Java and CORBA Java and Databases comp.lang.java.developer Java GUI Toolkits Java Help comp.lang.java.javascript JVM, native methods and hardware comp.lang.java.misc Java Language Programming Security and Java comp.lang.java.setup Java Software Tools comp.lang.java.tech ibm.software.java.linux  Non-English Java Newsgroups----------------Java Language (German)Language Java (Français)Java (Italian)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.