Why doesn't this for-each loop work? [duplicate]

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 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;
}