For a challenge, a fellow code golfer wrote the following code:
import java.util.*;
public class Main {
public static void main(String[] args) {
int size = 3;
String[] array = new String[size];
Arrays.fill(array, "");
for (int i = 0; i <= 100;) {
array[i++ % size] += i + " ";
}
for (String element: array) {
System.out.println(element);
}
}
}
When running this code in Java 8, we get the following result:
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 86 89 92 95 98 101
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99
When running this code in Java 10, we get the following result:
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100
The numbering is entirely off using Java 10. So what is happening here? Is it a bug in Java 10?
Follow ups from the comments:
The issue appears when compiled with Java 9 or later (we found it in Java 10). Compiling this code on Java 8, then running in Java 9 or any later version, including Java 11 early access, gives the expected result.
This kind of code is non-standard, but is valid according to the spec. It was found by Kevin Cruijssen in a discussion in a golfing challenge, hence the weird use case encountered.
Didier L simplified the issue with this much smaller and more understandable code:
class Main { public static void main(String[] args) { String[] array = { "" }; array[test()] += "a"; } static int test() { System.out.println("evaluated"); return 0; } }Result when compiled in Java 8:
evaluatedResult when compiled in Java 9 and 10:
evaluated evaluatedThe issue seems to be limited to the string concatenation and assignment operator (
+=) with an expression with side effect(s) as the left operand, like inarray[test()]+="a",array[ix++]+="a",test()[index]+="a", ortest().field+="a". To enable string concatenation, at least one of the sides must have typeString. Trying to reproduce this on other types or constructs failed.
array[i++] += nis enough to triggeri++twice? – Disentangle+=applied to indirectStringreferences. So first, your array must be aString[]. The issue doesn't occur withint[],long[]and friends. But yes, you're basically right! – ParatrooperString[]. If it isObject[]and you doarray[expression] += "foo";, it’s the same. But yes, it doesn’t apply to primitive arrays, as it must be able to hold references of typeString(Object[],CharSequence[],Comparable[], …), to store the result of string concatenation. – Demantoidvaris not recognized (such as invar x = "";. – Paratrooper