Structure API has dependencies on a few open-source libraries that are transitively included in your project when you add a dependency on Structure API.
You don't need to explicitly add dependencies on these libraries.
Integers and HPPC
The open source library Integers provides collections of primitive types with java.util-like interfaces. When working with Forest, you will typically use LongList and LongArray (an implementation of LongList).
It comes with another primitive type collection library, HPPC , which provides specific implementations of these collections.
See API Usage Samples to get the idea how to work with those interfaces.
JetBrains Annotations
Annotations library from JetBrains provides @Nullable and @NotNull annotations, used throughout the API.