A Java Concurrency API developer designs and implements multithreaded applications using Java's Concurrency API. They manage and control thread execution to maximize the utilization of CPU and improve application performance. Their tasks include creating thread pools, synchronizing shared resources, handling thread interruptions, and processing data in parallel with the Fork/Join framework. They also deal with thread safety, deadlocks, and race conditions to ensure that applications function correctly in a concurrent environment. Additionally, they troubleshoot and resolve any issues related to multithreading in Java applications. They strive to optimize code for scalability and efficiency.