There is a pure Java implementation of 2D alpha shapes on Github at the Tinfour software project in the class AlphaShape.java under package org.tinfour.utils.alphashape. An example application showing how to use the AlphaShape class is also available at Tinfour (see AlphaShapeDemoImage.java).

A set of web articles describing the methods used is available at Implementation details for a 2D alpha shape utility

And, finally, here's a sample image of an alpha shape created from a set of scattered points which were generated for test purposes from the lowercase letter g.