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: evaluated Result when compiled in Java 9 and 10: evaluated evaluated
The issue seems to be limited to the string concatenation and assignment operator (+=) with an expression with side effect(s) as the left operand, like in array[test()]+="a", array[ix++]+="a", test()[index]+="a", or test().field+="a". To enable string concatenation, at least one of the sides must have type String. Trying to reproduce this on other types or constructs failed.
+= applied to indirect String references. So first, your array must be a String[]. The issue doesn't occur with int[], long[] and friends. But yes, you're basically right!
String[]. If it is Object[] and you do array[expression] += "foo";, it’s the same. But yes, it doesn’t apply to primitive arrays, as it must be able to hold references of type String (Object[], CharSequence[], Comparable[], …), to store the result of string concatenation.
This is a bug in javac starting from JDK 9 (which made some changes with regard to string concatenation, which I suspect is part of the problem), as confirmed by the javac team under the bug id JDK-8204322. If you look at the corresponding bytecode for the line:
array[i++%size] += i + " ";
It is:
21: aload_2
22: iload_3
23: iinc 3, 1
26: iload_1
27: irem
28: aload_2
29: iload_3
30: iinc 3, 1
33: iload_1
34: irem
35: aaload
36: iload_3
37: invokedynamic #5, 0 // makeConcatWithConstants:(Ljava/lang/String;I)Ljava/lang/String;
42: aastore
Where the last aaload is the actual load from the array. However, the part
21: aload_2 // load the array reference
22: iload_3 // load 'i'
23: iinc 3, 1 // increment 'i' (doesn't affect the loaded value)
26: iload_1 // load 'size'
27: irem // compute the remainder
Which roughly corresponds to the expression array[i++%size] (minus the actual load and store), is in there twice. This is incorrect, as the spec says in jls-15.26.2:
A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.
So, for the expression array[i++%size] += i + " ";, the part array[i++%size] should only be evaluated once. But it is evaluated twice (once for the load, and once for the store).
So yes, this is a bug.
Some updates:
The bug is fixed in JDK 11 and was back-ported to JDK 10 (here and here), but not to JDK 9, since it no longer receives public updates.
Aleksey Shipilev mentions on the JBS page (and @DidierL in the comments here):
Workaround: compile with -XDstringConcat=inline
That will revert to using StringBuilder to do the concatenation, and doesn't have the bug.
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IntStream.range(0, 10) .peek(System.out::println).boxed().toArray()[0] += "";…test().field += "sth".array[index++] += "x";will read fromarray[index+1]and write toarray[index]…-XDstringConcat=inlineas a workaround, for those who need it.