Congrats on the internship! I also did my internship on backend development using Java and I learned a lot.
The leadership skills I focused on a the time includes: ownership, customer obsesseion, and delivering results. Learn more about Amazon's leadership principles here: amazon.jobs/en/principles
In terms of technical skills, writing clean code (using the SOLID principles), testable code, and testing helped me a lot to get a return offer.
I'd also emphasize the importance of writing reusable documentation. By this, I mean an engineer from another team who is trying to do something similar to your project should be able to follow your wiki or documentation as a good starting point.
Good to know. I majored in English, so documentation is already something I care about.
I definitely need to work on testing. Any resources to get better?
Sure! I think knowing one unit testing framework and one mocking framework will meet 80% of what you'll need.
Most of the frameworks share similar ideas so if later you find that your team is using something different, it's very easy to transition to the next framework.
As a side note, the book Effective Java is a gold mine full of Java best practices. There are a lot of book summaries available online, but I ended up buying the book to support the author.
Good luck and wish you all the best!
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