Java: Foreach loop not working as expected in int array?

Question

I have a pretty simple loop:

int[] positions = {1, 0, 0} //print content of positions for (int i : positions) { if (i <= 0) i = -1; } //print content of positions 

Now, what I expect to get:

 array: 1, 0, 0 array: 1, -1, -1 

but instead i get

 array: 1, 0, 0 array: 1, 0, 0 

Just ... why?

Regards, Jellyfish

Answer 1

Because " i " is a copy of an array element, not a reference to it :) You are changing a local variable, not an array element

this code is equivalent

 for(int index = 0; index < array.length; index++) { int i = array[index]; ... } 

Answer 2

It's simple. If you write

 int i = positions[0]; 

Then you copy positions[0] by value, not by reference. You cannot change the original value in positions[0] of i . The same goes for assignment i inside the foreach loop.

The solution does not have a foreach loop

 for (int i = 0; i < positions.length; i++) { if (positions[i] <= 0) positions[i] = -1; } 

Answer 3

This happens behind the scenes if we use the extended for loop with arrays:

 int[] array = {1,2,3,4,5}; for($i = 0; $i<array.length; $i++) { int i = array[$i]; // your statements if (i <= 0) i = -1; } 

$i is just a placeholder for an unnamed inner loop variable. See what happens: you assign a new value to i , but i loaded with the next element in the array in the next iteration.

So, practically speaking, we cannot use the variable declared in the extended loop to modify the underlying array.

^{Link: JLS 3.0, 14.14.2}