Note that the Java API source code is likely in a file named src.jar or src.zip in your J2SE SDK installation folder.
For a decent introduction to and as a resource for building a better understanding of the subject of how to implement not just an expandable array, but other data structures as well, let me suggest bookmarking Bruno Preiss' free on-line book,
Data Structures and Algorithms with Object-Oriented Design Patterns in Java, which includes step-by-step explanations of the aforementioned topics.