Why doesn't this for-each loop work?

Question

In this code, why isn't my array initialised as I want it to? Is the for-each loop not designed to do that, or am I just not using it correctly?

 int[] array = new int[5];

 //initialise array -> Doesn't work! Array still full of 0's
 for(int i : array)
  i = 24;

Answer 1

the int is a primitive type in the system, so you're actually getting a copy of the value of the cell in the array, rather than a reference to that cell in the array that you can assign into....

Let me try and explain this. If you had an array of Xs, where X is a class that has data members, you would be getting a reference to a different cell in each iteration, and would be able to change its state by calling functions on it (e.g., setValue).

When you have ints it's a different story, your int is not a reference, it is an actual value on the stack since it's a primitive type, so in each iteration, the for copies a value from the array into the i. You then update your i, but that has no effect on the array.

Answer 2

i is just a copy of the int at that point in the array, not a reference to it. The for-each loop doesn't work in this case.

Answer 3

The for-each loop will not work for this case. You cannot use a for-each loop to initialize an array. Your code:

int[] array = new int[5];
for (int i : array) {
    i = 24;
}

will translate to something like the following:

int[] array = new int[5];
for (int j = 0; j < array.length; j++) {
    int i = array[j];
    i = 24;
}

If this were an array of objects, it would still fail. Basically, for-each assigns each entry in the collection or array, in turn, to the variable you provide, which you can then work with. The variable is not equivalent to an array reference. It is just a variable.

For-each cannot be used to initialize any array or Collection, because it loops over the current contents of the array or Collection, giving you each value one at a time. The variable in a for-each is not a proxy for an array or Collection reference. The compiler does not replace your "i" (from "int i") with "array[index]".

If you have an array of Date, for example, and try this, the code:

Date[] array = new Date[5];
for (Date d : array) {
    d = new Date();
}

would be translated to something like this:

Date[] array = new Date[5];
for (int i = 0; i < array.length; i++) {
    Date d = array[i];
    d = new Date();
}

which as you can see will not initialize the array. You will end up with an array containing all nulls.

NOTE: I took the code above, compiled it into a .class file, and then used jad to decompile it. This process gives me the following code, generated by the Sun Java compiler (1.6) from the code above:

int array[] = new int[5];
int ai[];
int k = (ai = array).length;
for(int j = 0; j < k; j++)
{
    int i = ai[j];
    i = 5;
}

Answer 4

Do this instead :

int[] array = new int[5];

// initialise array -> Will work now
for(int i = 0 ; i< array.length ; i++)
    array[i] = 24 ;

Answer 5

Use

java.util.Arrays.fill(array, 24)

if you're going to be initializing to the same value. Other than that, Eddie is pretty much spot on in his translation of the for-each construct.