JtsGeometryUnionCollector.java

    @Getter
    @Setter
    @AllArgsConstructor
    private static class GeometryHandle
    {
        Geometry geometry;
    }

    public static Collector<Geometry, GeometryHandle, Geometry> collectorUnion(@NonNull GeometryFactory factory)
    {
        return new Collector<Geometry, GeometryHandle, Geometry>()
        {
            @Override
            public Supplier<GeometryHandle> supplier()
            {
                return () -> new GeometryHandle(factory.createGeometryCollection(new Geometry[0]));
            }
            
            @Override
            public BiConsumer<GeometryHandle, Geometry> accumulator()
            {
                return (a, b) -> a.geometry = a.geometry.union(b);
            }
            
            @Override
            public BinaryOperator<GeometryHandle> combiner()
            {
                return (a, b) -> new GeometryHandle(a.geometry.union(b.geometry));
            }
            
            @Override
            public Function<GeometryHandle, Geometry> finisher()
            {
                return GeometryHandle::getGeometry;
            }
            
            @Override
            public Set<Characteristics> characteristics()
            {
                return Sets.immutableEnumSet(Characteristics.UNORDERED);
            }
        };
    }

Java Stream API Collector implementation for the union operation on JTS geometries.

Depends on JTS, Lombok and Guava, but the latter two can be factored out trivially.