Market Knowledge

• While not essential, hiring a developer who understands how global financial markets work can save a lot of time in debugging the system or making functional revisions. While the developer is expected to take instructions and ideas from global market experts to translate into Java code, a lot can be lost in translation. It is impossible for the financial experts to convey everything they know and might need to the programmer. A developer who understands market mechanics is better prepared to head off problems before they occur.
  
  

Extensibility

• A successful trading system is used by numerous employees across a variety of companies. Hiring a Java developer experienced in making feature-rich advanced programming interfaces (APIs) that allow third-party programs to use the system can benefit the system's overall quality.
  
  

Network Security

• Successful trading systems have lots of money and sensitive information flowing through them constantly. Consequently, they are prime targets for hackers to farm account numbers, buy stocks for free or divert money into unauthorized accounts. Java developers with a strong background in hardening Java distributed systems against attacks are highly desirable.
  
  

Technical Writing

• A program is never truly complete, especially one as complex as a trading system. As time goes on, security issues may arise, software components will need updating, and the system will need to change with advancing operating system technology. Consequently, it is not enough for a single developer to understand the system -- entire teams need to be knowledgeable. Because of the highly social nature of trading systems, the ability to write quality technical documentation to allow others to understand how the system works is very useful for a Java developer.