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.