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Java has had annotations since version 5, which wikipedia says is 2004, and they are also heavily used in C#, where they are called attributes. They are heavily used in many places, notably in Hibernate, Spring and JUnit/TestNG. Introducing custom attributes/annotations is a very powerful way of extending a language.

It seems to me that Cache method keywords are the most similar thing to Java method annotations. Expanding them to match annotations would require a mechanism to introduce custom method keywords, to be able to fetch keywords for a method at runtime, and also to have custom keywords on properties, and for keywords to have arbitrary objects as values instead of being limited to strings. But I don't think it would be easy to do.

if the package A uses package's C version 0.0.1, and package B uses package's C version 0.1.0 (which behaves differently and is not back-and-forth compatible!)

In the Maven/Java world, there is a mechanism to exclude the dependency of Package A on Package C from Package A's dependency list. This results in both Package A and B to use the same version of Package C. If Package A really cannot use the new version of Package C, then you have no choice but to make a patched version of Package A, but at least you are alerted to the potential conflict.

Good point Rini. Also, by recognizing software patterns you can influence the development of language features. For example, because functional programming has become more common in the Javascript community, language features such as "fat arrow" notation have been added to ES6 make it less cumbersome/strange/etc. 

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