4. Learn APIs and Libraries
If you have worked with great Java developers, you might have noticed their overall knowledge of the Java ecosystem, and APIs form a major part of it.
Java is the world’s most popular and mature programing language, and there are tons of libraries and APIs available for doing almost everything.
Of course, you are not expected to know all of them but you should be familiar with some key APIs, e.g. JSON processing APIs like Jackson and Gson, XML processing APIs like JAXB and Xerces, and unit testing libraries like Mockito and JUnit.
If you don’t know them, you can learn or at least get an overview of them in 2018. To start with, you can check out my list of 20 Java libraries every Java developer should know, which covers libraries from key areas like parsing, bytecode manipulation, concurrency, collections, etc.