To use the following examples you need to specify the dependency in your Maven config correctly.
You'll first need to build off an existing Graphhopper instance for routing.
See this example code how to build and traverse a shortest path tree, which means enumerating, in order, all nodes that can be reached within a given time limit.
See IsochroneResource.java to see how we use the shortest path tree to construct an isochrone (or other isoline, depending on the weighting).