Swapping objects in Java

[Christopher Young]

How would I go about this without using an array?

[Ernest Friedman-Hill]

Can you elaborate a bit? Are you trying to answer an interview question like "How would you write a Java method to swap two variables?" or something along those lines?

[Christopher Young]

I've been in debate with someone over the technical differences in C++ and Java when it comes to parameter passing. He said something about trying to swap objects in Java without using an array.

I tried something like

swap(Obj a, Obj b) {
Obj c = a;
b = a;
c = b;
}

but no luck, though. When I read out the values for the objects in the caller method they were the same.
[ December 13, 2008: Message edited by: Christopher Young ]

[Henry Wong]

I tried something like
 
swap(Obj a, Obj b) {
    Obj c = a;
    b = a;
    c = b;
}

Unfortunately, if you are arguing that you can exchange the references, without using an array, and anything that "indirects" the references, you lost the debate. Your friend is right.

Henry
[ December 13, 2008: Message edited by: Henry Wong ]

[Steve Luke]

If you read This JavaRanch article you may get an understanding of why what Henry said is true.

[Christopher Young]

Thanks.

[Campbell Ritchie]

Originally posted by Christopher Young:
swap(Obj a, Obj b) {
Obj c = a;
b = a;
c = b;
}

Please use the code tags to maintain indentation.

What you would use in C is like this

void swap(Obj *a, Obj *b)
{
   Obj *c = *a;
   *b = *a;
   *c = *b;
}

I have used stars to mark the difference; the stars themselves are the difference. Only that still won't work. I'll leave you to work out why!

[Charles Lyons]

He said something about trying to swap objects in Java without using an array.

I'd like to add that C++ always passes by value. It has a "pass by reference" concept, but it's a compiler trick and not a fundamental concept (like pointers are). The difference with C/C++ is that you can use (and pass the value of) pointers to data types/objects, and even pointers-to-pointers. Doing the latter allows you to swap two object references over. Like this:

void swap(void** a, void** b) {
  void* tmp = *a;
  *a = *b;
  *b = tmp;
}

Java can't have "pointers-to-pointers", hence why this doesn't work. Sometimes, but not often, that's a disadvantage too, because Java can't have "out parameters". But you could simulate it like this (it is pointless by the way!):

public class Reference<T> {
    public T obj;
}
 
void <T> swap(Reference<T> a, Reference<T> b) {
    T tmp = a.obj;
    a.obj = b.obj;
    b.obj = tmp;
}

But then all your variables are of type Reference<T>, which is just confusing :roll: You can obviously do the same with an Object[] as the argument, because you can re-reference the elements in the array to do the swap, just like I re-referenced the 'obj' member in Reference<T>.

References in C++ (denoted by myclass& for example) are essentially equivalent to "pass by reference" in Java because a reference is really a single pointer (like void*) and not a pointer-to-pointer. So using a reference in C++ also wouldn't allow you to swap two variables over... you'd still need a pointer to the reference

[Martijn Verburg]

Originally posted by Charles Lyons:
I'd like to add that C++ always passes by value. It has a "pass by reference" concept, but it's a compiler trick and not a fundamental concept (like pointers are). The difference with C/C++ is that you can use (and pass the value of) pointers to data types/objects, and even pointers-to-pointers. Doing the latter allows you to swap two object references over. Like this:

void swap(void** a, void** b) {
  void* tmp = *a;
  *a = *b;
  *b = tmp;
}

Java can't have "pointers-to-pointers", hence why this doesn't work. Sometimes, but not often, that's a disadvantage too, because Java can't have "out parameters". But you could simulate it like this (it is pointless by the way!):

public class Reference<T> {
    public T obj;
}
 
void <T> swap(Reference<T> a, Reference<T> b) {
    T tmp = a.obj;
    a.obj = b.obj;
    b.obj = tmp;
}

But then all your variables are of type Reference<T>, which is just confusing :roll: You can obviously do the same with an Object[] as the argument, because you can re-reference the elements in the array to do the swap, just like I re-referenced the 'obj' member in Reference<T>.

References in C++ (denoted by myclass& for example) are essentially equivalent to "pass by reference" in Java because a reference is really a single pointer (like void*) and not a pointer-to-pointer. So using a reference in C++ also wouldn't allow you to swap two variables over... you'd still need a pointer to the reference

Ah that brings back memory of the *** declarations in my Undergraduate code, I've learned better now

[Campbell Ritchie]

Originally posted by Martijn Verburg:


Ah that brings back memory of the *** declarations in my Undergraduate code, I've learned better now

I thought *** was what you put in the middle of a naughty word. I now know where it comes from

[Martijn Verburg]

Originally posted by Campbell Ritchie:


I thought *** was what you put in the middle of a naughty word. I now know where it comes from

Well it would provoke that reaction from the Graduate marker, e.g. "Why did you *** use ***void in your code!".

[William Brogden]

In my less than humble opinion, the whole gefuffle is due to an incorrect statement of the problem:

Swapping objects in Java

A correct statement of the problem as:
Swapping references to objects in Java
automatically leads you in the right direction.

Bill

[Campbell Ritchie]

I have been shown a C program with function pointers in which is supposed to work exactly the same if you write f() *f() **f() or ******************f().

I know * is a pointer and ** is a handle. But is there a name for ***?

[Rob Spoor]

Pointer to handle? Pointer to pointer to pointer? Ridiculous construction?

[Martijn Verburg]

We always called *** triple dereferenced pointers, our markers just called it crap