Java - Re-reference an array by method

Question

:)

The problem I'm having: It's the very simplified version of what I'm trying to do, as I actually need to recount a large array of colors (taken from Color Object), and need to sort them, and remove and count duplicates in 2 arrays. (Or whatever data model I should use, I chose 2 arrays). The problem I'v having is that I cant re-reference the array within a method the way it stays there afterwards.

public class Algos2 {

  private int[] b1 = null;

  public void relink(int[] a1, int[] a2) {
    a2 = a1;
  }

  public Algos2() {
    int[] a1 = { 0, 5, 5, 5, 3, 3, 0, 0, 1, 2, 1, 5};
    relink(a1, b1);
    System.out.println("a1 " + a1); // whatever, working
    System.out.println("b1 " + b1); // still null 
}

  public static void main(String[] args) {
      new Algos2();

  }
}

b1should return the same pointer as a1 afterwards. And I need to do it within a method. :/

Edit: I also can't do it directly, returning an array by a method. So no: public int[] relink(int[] a1) {return a1;} as I also would neeed to call that from within another method, which also results in null

Well, kinda. But I'm actually struggling with the easy part of it. Homework is the explanation and the code I left out. — Cilice
– Cilice, Commented Oct 23, 2011 at 14:37
Have you tried using other data types such as ArrayList<E> as jon suggests in his answer? — luketorjussen
– luketorjussen, Commented Oct 23, 2011 at 14:49
@luketorjussen Unfortunately, I haven't yet. Maybe it's the key to solve the whole problem. Thanks then. — Cilice
– Cilice, Commented Oct 23, 2011 at 14:50

Jon Skeet · Accepted Answer · 2011-10-23 14:32:51Z

2

This doesn't work because Java passes arguments by value (even if that value is a reference, such as a reference to an array). It can't change the value of b1 within relink - because all it receives is a copy of the value of b1. (Well, it could assign directly to b1 in this case because it's an instance method and b1 is an instance variable, but I suspect that's just a coincidence which may not be valid in your real code.)

If you want the two variables to refer to the same array, why don't you just use the assignment operator within the calling method?

b1 = a1;

Alternatively if you have other work to do, you could potentially make the method return a reference to the appropriate array:

b1 = relink(a1);

It's hard to know exactly what's most appropriate as you haven't really told us about what you're trying to do. If you want to be able to remove elements, then arrays may very well be the wrong data structure to use, precisely because you can't remove elements from them. Have you considered using a List<E> implementation instead, such as ArrayList<E>?

answered Oct 23, 2011 at 14:32

Jon Skeet

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4 Comments

Cilice Over a year ago

I've tried that, it's just I need to split 1 array into 2 different ones. I'll copy all the code here: gist.github.com/1307427

Jon Skeet Over a year ago

@Cilice: Showing all the code doesn't really explain what you're trying to achieve. But fundamentally, you'll need to work with the fact that Java passes arguments by value. You can't get away from that.

Cilice Over a year ago

I'll try it with the ArrayList, seems to solve the problem by the looks of it. Thank you.

Eng.Fouad Over a year ago

@Cilice see my answer for more details.

Vlad · Accepted Answer · 2011-10-25 16:55:22Z

If it the swap has to be done using a method, you can wrap the handles to your arrays in 'holder' objects, in order to simulate passing them by reference. However, this adds another layer of indirection, which will degrade performance.

Example:

class Ref<T>{
    public T obj;
    public Ref(T obj){this.obj = obj;}

    public static <T> void swap(Ref<T> a, Ref<T> b){
        T aux = a.obj;
        a.obj = b.obj;
        b.obj = aux;
    }
}

class test{
    public static void main(String[]args){
        Ref<int[]> a1, b1;
        a1 = new Ref<int[]>(new int[]{1,2,3});
        b1 = new Ref<int[]>(new int[]{4,5,6});

        System.out.println(java.util.Arrays.toString(a1.obj));
        System.out.println(java.util.Arrays.toString(b1.obj));

        System.out.println("Swap.");
        Ref.swap(a1, b1);

        System.out.println(java.util.Arrays.toString(a1.obj));
        System.out.println(java.util.Arrays.toString(b1.obj));
    }
}

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