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 there will be a back-port to JDK 10 (but not 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