Java: "+=" applied to String operands can provoke side effects

Wow. This is a pretty big bug. The official bug report: <a href="https:&#x2F;&#x2F;bugs.openjdk.java.net&#x2F;browse&#x2F;JDK-8204322" rel="nofollow">https:&#x2F;&#x2F;bugs.openjdk.java.net&#x2F;browse&#x2F;JDK-8204322</a> as Stuart Marks, a jvm architect, posted. I&#x27;m surprised it stayed uncaught for so long.<p>For the unfamiliar, the way string concatenation is compiled changed with invokedynamic.<p>Previously, it would compile to either Sting.concat calls or transform into a StringBuilder, but with invoke dynamic we can leave it to the runtime to optimize when it first runs.

&gt; Operator += is broken for Strings in Java 9 and 10<p>The title [Edit: since changed] is a little bit fear-monger-y. It&#x27;s only broken when the left hand side is not safe to evaluate more than once.<p>It can be prevented with a compile-time flag.

Adding side effects to your array index expression and depending on order of their evaluation is a bad idea anyway.<p>Unfortunately, the `++` &#x2F; `--` made it to Java, and are still used sometimes, despite JVM not being PDP-11, and definitely not trying to conserve every precious byte of RAM.

I used that operator in a tight loop before the compiler was smart enough to automatically convert the code to use a StringBuilder. Boy was that slow...

Time to rewrite Java in Rust.

Lisp macros get this right.<p><pre><code> [2]&gt; (macroexpand &#x27;(incf (aref a (incf i)) x)) (LET* ((#:G3214 A) (#:G3215 (INCF I)) (#:G3217 X) (#:G3216 (+ (AREF #:G3214 #:G3215) #:G3217))) (SYSTEM::STORE #:G3214 #:G3215 #:G3216)) ; T </code></pre> You can&#x27;t just blindly expand the += by copying the expressions&#x2F;AST node on the left. Or in general; you don&#x27;t take expressions in the program and blindly duplicate them. How can someone working on compilers not know that???

Yea... what was the Bloch&#x27;s explanation for not having operator overloading in Java? I think it was something like &quot;it&#x27;s not allowed because I&#x27;ve seen too many programmers who did that in a wrong way&quot; :)<p>On the other hand this explanation is stupid, as a programmer can implement `add()` in a wrong way.

&gt; &quot;side effects&quot;<p>A side effect would be like submitting an order for am Amazon Echo by mistake.<p>This is a bug, a serious bug, that leads to variables being assigned the wrong value.