Take a moment to see how far you've come. You started with zero Java and now you can write classes, create object hierarchies with inheritance, define contracts with interfaces, and handle errors with try-catch.
Here is what you have in your toolkit:
Variables, operators, and control flow to express logic.
Methods to organize reusable code.
Arrays, ArrayLists, HashMaps, and Sets to store and retrieve data.
Full OOP: encapsulation, inheritance, polymorphism, and abstraction.
Exception handling to write code that fails gracefully.
Every topic in this section builds directly on those foundations.