javaPlex - an extensible platform for persistence. javaPlex is the newest member of the Stanford-based Plex family of persistent homology and cohomology software packages. Written in Java, it is optimized for easy interfacing with Matlab as well as anything running in the Java ecosystem - including Processing, Scala, Jython, and Mathematica. javaPlex was written to create a platform easier to extend and experiment with than its predecessor jPlex. In its last release, the platform support absolute and relative persistence (co)homology, as well as hom-complexes of simplicial complexes, cellular and simplicial complex constructions and fast Vietoris-Rips and witness complex constructions. We will demonstrate the architecture, usage, development and extension features of javaPlex, focusing on interoperability with Matlab as an interface to the library.