This is a bug in
javac starting with JDK 9 (which made some changes regarding string concatenation, which I suspect is part of the problem), as confirmed by the team
javac with bug id JDK-8204322 . If you look at the corresponding bytecode for the line:
array[i++%size] += i + " ";
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 matrix. 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), it’s in there twice. This is incorrect, as the specification says in jls-15.26.2 :
A compound assignment expression of the shape
E1 op= E2equivalent to
E1 = (T) ((E1) op (E2))where is it
Tis the kind of
E1, except that
E1it is evaluated only once.
So, for the expression
array[i++%size] += i + " ";the part
array[i++%size] it only needs to be evaluated once. But it is rated twice (once for the cargo and once for the store).
So yes, this is a bug.
The bug was fixed in JDK 11 and there will be a back-port to JDK 10 (but not JDK 9, since no longer receives public updates ).
Aleksey Shipilev mentions on the JBS page (and @DidierL in the comments here):
Solution: compile with
This will return to use
StringBuilder to do the concatenation and does not have the bug.