If variables in Java are accessed from multiple threads, one must ensure that they are safely published. This usually means using synchronizedor volatile. I have got the impression, that some of my colleagues do not take this issue seriously, since they "never heard of volatile before and their programs have worked for years". So my question is: Can someone provide an example Java program/snippet, that reliably shows data visibility problems. I think running a program and seeing the unexpected NPE or stale variable value would help more, than just theoretic explanations, that cannot be demonstrated. Thanks a lot for your help!