A Java UDF (User-Defined Function) developer designs and implements custom functions in Java that extend the functionality of database systems. These UDFs perform complex computations, data transformations, or business logic that are not readily available in the system. The developer is responsible for ensuring the UDFs' seamless integration, performance, and reliability. They also write test cases to validate the UDFs' accuracy and effectiveness. Understanding of SQL and database structures, strong coding skills in Java, and problem-solving abilities are essential for this role. Additionally, they may collaborate with other team members to troubleshoot and optimize UDFs.