Java Collections and Streams

Three interfaces, many implementations, and which name to declare a variable with.

Two lists with the same methods and opposite costs.

Collections that refuse duplicates, and collections that decide what comes out next.

What a hash map does with a key, and what it costs when two keys collide.

Teaching Java an order, for one natural ordering and for many.

Type parameters, what the compiler checks, and what is gone by runtime.

Passing behaviour as an argument, and the interface it is compiled against.

A pipeline over a source, what runs lazily, and what comes out of the end.

A return type that makes "there might be nothing" impossible to ignore.