Are Coding competitions good?

As the title states, are coding competitions good/effective to learn proper learning skills?

It depends on the objective of the competition, I wouldn't say that code golf challenges are particularly useful for programming "real" applications for instance.

Your question is quite open-ended and depends on the context, but I assume you're referring to writing good quality code which is what really matters in the long-term. I wouldn't be too concerned about language features or frameworks as these change over time and any good development manager won't really mind too much; a good programming interview will look at how you approach a problem and how you structure your code.

Here's some general tips:

• Keep your source code files succinct i.e. don't have files which are 3000+ lines long
• Since you're learning Java and OOP, then I would generally have a source file per a class/interface
• Keep your methods/functions purposeful with a name which clearly shows what they do
• Don't pass too many arguments to a single function, I try to keep at 5 or less
• Follow the KISS, DRY and SOLID principles which will usually mean using generics and templates when appropriate
• Follow the programming language style conventions where possible
• Write for readability over performance at first, refactoring can always be done later after benchmarking your code

I would say that the teacher's grades are more effective than competitions.  Obviously, if the competitors don't share their code, there is nothing for you to learn from.

If you wanna test your skills compared to others then yes,

Depending on the competition and if you connect with other contestants and they're willing to share their way of coding then it a huge boost to your knowledge if you find the right people.

It al boils down to how you are aproching the contest, to learn? to win? to network?