JCF vs DataBase...

The JCF has nothing to do with databases. HashSet, ArrayList etc. are data structures, not databases. They exist only at runtime. They exist to organize data, not to store it. There is no persistence happening (when the program ends, the data is gone).

Databases are there to persist data (to store it). Databases speak their own language (for example SQL), and have their own structure (tables and rows).

When a Java app works with a database, the Java app needs to send queries to the database in a language the database can understand. When the database sends data back, the Java app needs to translate the rows back into Java objects. There is a specification on how to deal with this called the Java Persistence API (JPA). There are a lot of tools you can use that help with this.

Here on Treehouse you can learn about one of those tools called Hibernate in this course: https://teamtreehouse.com/library/hibernate-basics I recommend taking it if the topic interests you. But there are many other tools out there that take different approaches. For example jOOQ is pretty interesting, but it requires you to have a better understanding of SQL than an ORM like Hibernate.