How to demonstrate Java instruction reordering problems?

Question

With Java instruction reordering the execution order of the code is changed by the JVM at compile time or run time, possibly causing unrelated statements to be executed out-of-o

Answer 1

For single thread executions reordering is not a problem at all, because of Java Memory Model (JMM) (guarantee that any reads actions related to writes are total ordered) and cannot lead to unexpected results.

For concurrent execution, rules are completely different and things gets more complicated to understand (even by providing simple example which will raise even more questions). But even this are totally described by JMM with all corner cases, so, unexpected results also forbidden. Generally, forbidden if all barriers are placed right.

For better understanding reordering I strongly recommend this subject with plenty examples insides.