"Focus on the data"
Good advice is often surprisingly simple - to some it's unsatisfactory. I believe this advice fits is in that category. It's like "Invest in Index funds" - brilliant investment advice for so many reasons (e.g. diversification, lower taxes, lower costs, avoid bad management) but often people expect a solution much more intricate and involved . . . . . but I digress.
Within "data" there is a lot of stuff in Java - let's start with how data is often stored and represented
Representation of data and accessing it
- RDBMS -- There's so much to SQL - the DDL and DML - and of course there's JDBC (CallableStatements, PreparedStatements etc.). Then there's the differences between Sybase, Oracle, MSS etc.
- XML -- Parsing it (DOM, SAX, XPP), formatting it (XSLT), accessing it (XPath), describing it (XML Schema)
Getting it out of the "persistence device" and into Java. That is:
- Object-Relational Mapping (e.g. Hibernate, iBatis, Toplink)
- XML-Object Mapping (e.g. Castor, Digester)
Transmission of data
Now we probably have to send that data to somewhere - partners, other business units, end customers etc.
- Web Services (JAX-WS and Axis) -- then of course there's SOAP and REST but let's not go there now :-)
- Messaging (JMS -- check out ActiveMQ)
These are the hot architectures of the moment but they really put the emphasis on data and services to access that data.
I could also go into the display of data too but that's really getting a little too broad . . .
Either way, that's a lot of stuff - just to address business data-related functionality - but hopefully your career as a developer will be long and successful so you've got time to plug away at it. My advice - start with JDBC and XML as everything builds on that foundation. Know those already? Web Services and Messaging and so forth.
In the end we build most applications to access, update, manipulate and display data because that's what customers and employers will pay for. And if your "value add" as a developer aligns with the needs of your customer / employer then your career as a developer will be bright.
And yet as with all such apparently simple advice - the follow-through, persistence and effort - week-in and week-out is the hard part and it's what often distinguishes the best from the rest.