How to code a swap() method, or alternatives?

Question

I know all of you here are very knowledgeable and generally quite skilled at explaining, so I'd like to ask for your input.

I've seen a few posts and articles on the distinction between pass-by-value and pass-by-reference languages and they've made me very curious as to how I might implement a swap method. This is of particular interest because both a school project and a personal project are/will be using a swap method in order to work. I did find a rather novel solution by Marcus in this question that I'll use if I cannot find another one.

My understanding is that it is not possible to implement the following and have x = 5, y = 10 as a result:

int x = 10; int y = 5; 

swap(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}

public static void main(String[] args) {
    swap(x, y);
    //x = 10, y = 5
    System.out.println("x was 10 and y was 5");
    System.out.println("Now x is " + x + ", and y is " + y);
}

However, as far as I have gathered, the following will work because x and y are not passed:

int x = 10; int y = 5;

swap() {
    int temp = x;
    x = y;
    y = temp;
}

public static void main(String[] args) {
    swap();
    //x = 5, y = 10
}

How do I swap two primitives? What about two instances of the same object (I only need the fields/variables swapped)? Are there any special considerations I must make for swapping in arrays?

score 1 · Answer 1 · answered Mar 08 '14 at 09:38

1

Swapping primitives like that can't be done in Java. The closest you can get is swapping information contained in objects, whether arrays or regular objects that hold values.

answered Mar 08 '14 at 09:38

chrylis -cautiouslyoptimistic-

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Rami · Answer 2 · 2014-03-08T09:44:29.460

1

You can swap 2 primitives by storing them in an Object like for example an array list. Objects live on the heap space, so basically passing an array list to the swap method is like passing a reference to that array list.

Hope that helps.

EDIT: It is important to note that all parameter passing in JAVA is By Value, but when passing an object you are basically passing the object reference value which points to the object allocated on the Heap.

edited Mar 08 '14 at 09:44

answered Mar 08 '14 at 09:39

Rami

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That is a rather dirty workaround to do the trick. – Engin Kayraklioglu Mar 08 '14 at 09:42
Ah, interesting solution. Sadly, the values I would need to swap usually would reside in different classes or variables, so in most cases this solution is not viable. It is perfect though for my school project which simply swaps values in an ArrayList. Thanks! +1 – Aarowaim Mar 08 '14 at 09:45
@EnginKayraklioglu, It is a tricky question that suits interviews and needs a tricky answer as well. In fact I was asked that question in an interview a while ago. – Rami Mar 08 '14 at 09:46

score 1 · Accepted Answer · answered Mar 08 '14 at 09:40

1

As you understand, the only way to swap to primitives by a method works only if they are in global scope or class scope.

As for arrays and objects, they are passed by reference in Java. So any changes that you make changes the actual argument to the method. On the other hand if you are looking for a neat way to swap two objects, just implement a swap method as a method in the class and call it like:

anObject.swap(anotherObject);

This way you can implement the swap however you want.

answered Mar 08 '14 at 09:40

Engin Kayraklioglu

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Alright, I'll double check that primitives are always declared globally if I absolutely need to swap them. I'll definitely use your Object.swap(Object) suggestion. You answered my question perfectly, so +1 and best answer! – Aarowaim Mar 08 '14 at 09:48
Arrays and objects are not passed by reference in Java. Instead, references to these are passed by value. – Andy Thomas Oct 15 '15 at 21:36

How to code a swap() method, or alternatives?

3 Answers3