why does this particular program print me five?

Question

Please help me with this one guys! why are there arguments given to a method that is void??

public class C {   
    public static void main(String[] argv) {     
        int k = 1;     
        int[] x = {0, 1, 2, 3};     
        int[] y = x;     
        lurig(x, y, k);     
        System.out.println(x[0] + k + y[0]);   
    } 

    public static void lurig(int[] p, int[] q, int r) {     
        p[0] = 1;     
        q[0] = 2;     
        r = 3;   
    } 
}

I have a question back for you: Why can't a void method take arguments? — Martin, Mar 20 '20 at 09:37
Java is call by value (see https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value) therefore `r = 3` does not modify `k`. Also, `x` and `y` refer to the same object so `q[0] = 2;` overwrites the value written by `p[0] = 1;`. — Stephen C, Mar 20 '20 at 09:38
@Stephen C ahh i see, but why in this case is this program giving me 5, from what?!??!! — Mr Miyagi, Mar 20 '20 at 09:41
@MrMiyagi Your program returns 5 due to 'System.out.println(x[0] + k + y[0]);' line. This one prints to standard output. — n1t4chi, Mar 20 '20 at 09:46
right im asking how in the whole world can the system give me 5 when nohing is being returned from the method and k=1 whilst x and y is 0? — Mr Miyagi, Mar 20 '20 at 09:49

score 0 · Answer 1 · answered Mar 20 '20 at 09:41

Your x and y array actually refer to the same location in memory. This means that after calling lurig(x, y, k) both x[0] and y[0] are 2.

If you want x and y to refer to seperate memory locations, you need to do it like this

int[] x = {0, 1, 2, 3};     
int[] y = new int[4];
System.arraycopy(x, 0, y, 0, x.length);

score 0 · Accepted Answer · answered Mar 20 '20 at 09:44

x[0] is 0 k is 1 y[0] is 0

-> lurig() changes x[0] to 1 and y[0] to 2 but since y has the same reference as x it also changes x[0] to 2 k is a primitive type so it remains as 1 (completely unaffected)

-> 2 + 1 + 2 gives 5

5 is NOT returned it's being written out to the console. You don't have a return statement you have a System.out out stands for output.

why does this particular program print me five?

2 Answers2