Re: Using pointers in Java
From: chris (chris_at_kiffer.eunet.be)
Date: 01/31/04
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Date: Sat, 31 Jan 2004 17:35:27 +0100
Funny, I was just saying to someone yesterday: "Java is not a replacement
for Fortran". For the project in question we took it as understood that the
number-crunching would be performed in C.
One alternative to your "arrays of one element" proposal would be to put
all three parameters into a single array:
void add(double[] a) {
a[0] = a[1] + a[2];
a[1] = a[0] + a[2];
}
In both cases you'll have array bounds checking overhead, which I don't
think the JIT compiler will be able to optimise away (could be wrong there).
Or alternatively:
class Triple {
double a1, a2, a3;
}
void add(Triple t) {
t.a1 = t.a2 + t.a3;
t.a2 = t.a1 + t.a3;
}
That does at least look object-oriented, and should generate reasonably
efficient bytecode. Whether it really _is_ object-oriented depends on
whether t corresponds to anything recognisable in your problem domain ...
HTH
Chris
S Manohar wrote:
> I need to pass a 'reference' to a double value to a function which
> changes that value. Each time the function is called, it needs to
> change different sets of values. In C I'd simply pass references.
>
> void add(double *a1, double *a2, double *a3){
> *a1 = *a2 + *a3;
> *a2 = *a1 + *a3;
> }
>
> In Java, one solution would be to create a class to represent a
> mutable value:
>
> public class MutableDouble {
> private double value;
> public final void set(double value){ this.value=value; }
> public final double get(){ return value; }
> }
>
>
> But, my this is cumbersome in my code for several reasons:
>
> 1) My code contains many long mathematical equations, which become
> very convoluted using this syntax, with ridiculous numbers of
> parentheses! It is very important that the maths in the code is
> readable, as someone else may have to work on my code. E.g.,
> a = a + 1
> becomes
> a.set(a.get()+1)
> The problem is, of course, that unlike C++, operators cannot be
> defined.
>
> 2) get() and set() will presumably be slower than direct use of
> values. Though I don't know enough about how the javac inlines
> functions.
>
> 3) I need to define a MutableInteger, MutableLong and MutableFloat as
> well.
>
> 4) The size of the compiled code will be, presumably, more bulky.
>
> So, This is the kind of solution I am thinking of adopting:
>
> void test(){
> double[] x=new double[1], y=new double[1];
> add(x,y,y);
> }
>
> void add(double[] a1, double[] a2, double[] a3){
> a1[0] = a2[0] + a3[0];
> a2[0] = a1[0] + a3[0];
> }
>
>
> Using arrays of length 1 as pointers has some advantages, seems
> 'cheap' in terms of code and memory, and the code will look much nicer
> than using get/set. But naturally there are disadvantages, e.g., as
> with pointers, there's the potential for inadvertent null-pointer
> exceptions (which will be ArrayIndexOutOfBounds exceptions).
>
> Has anyone used this method?
>
> Has anyone any comments on its effectiveness and/or problems?
>
> What is your opinion on its methodological purity?
>
> Sanjay
-- Chris Gray chris@kiffer.eunet.be /k/ Embedded Java Solutions
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