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. 

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.