Java Modifying Elements in a foreach

问题:

I'm learning Java on my own; and therefore the code below has no function other than for learning/testing.

Essentially I'm trying to modify the elements of an Integer array (namely, halving them) whilst in a foreach loop.

I should note that I'm not re-ordering, adding, or deleting elements; simply changing their values.

Here is my code:

Logger.describe("Now copying half of that array in to a new array, and halving each element");
Integer[] copyArray = new Integer[DEFAULT_SAMPLE_SIZE / 2];     
System.arraycopy(intArray, 0, copyArray, 0, DEFAULT_SAMPLE_SIZE / 2);
for (Integer x : copyArray) x /= 2;
Logger.output(Arrays.deepToString(copyArray));

However, the original array (intArray) is this:

[47, 31, 71, 76, 78, 94, 66, 47, 73, 21]

And the output of copyArray is:

[47, 31, 71, 76, 78]

So although the array has been halved in size, the elements (Integers) haven't also been halved in value. So what am I doing wrong?

Thank you

回答1:

You can't do that in a foreach loop.

for (int i=0; i<copyArray.length;i++)
    copyArray[i] /= 2;

Else you are not assigning it back into the array. Integer objects are immutable by the way so can't modify them (creating new ones though).

Updated from comment: Beware though that there are a few things going on, autoboxing/unboxing for example, roughly:

copyArray[i] = Integer.valueOf(copyArray[i].intValue()/2);

回答2:

for (int i = 0; i< copyArray.length; i++) {
    copyArray[i] = new Integer(x /2);
}

should work.

回答3:

int counter = 0;
for(int x : copyArray)
{
        x /= 2;
        copyArray[counter++] = x;
}

Your program just modified the value of variable x , not the values within the blocks of array copyArray

回答4:

I think that you can NOT use the foreach loop construct in order to modify the elements of the array you are iterating. Instead, you need to use a classic for loop like so:

Logger.describe("Now copying half of that array in to a new array, and halving each element");
Integer[] copyArray = new Integer[DEFAULT_SAMPLE_SIZE / 2];     
System.arraycopy(intArray, 0, copyArray, 0, DEFAULT_SAMPLE_SIZE / 2);
    for (int i = 0; i < copyArray.length; i++) {
        copyArray[i] /= 2;
    }
Logger.output(Arrays.deepToString(copyArray));