Java runs enterprise systems, Android apps and many banking and ERP back ends. Eight lessons from installing the JDK to classes, collections, exceptions and an invoice calculator project.

1. 1Java Lesson 1: Install the JDK and Run Hello WorldInstall a JDK (Temurin or Oracle), check java -version, write and run Hello.java from the terminal and in VS Code or IntelliJ…
2. 2Java Lesson 2: Variables and TypesJava’s static types: int, double, boolean, char and String, declaring variables, var, arithmetic and integer division, casting and printf formatting.
3. 3Java Lesson 3: Conditions and LoopsUse if/else, switch expressions, comparison and logical operators, for, enhanced for and while loops in Java.
4. 4Java Lesson 4: MethodsWrite static methods with parameters and return types, overloading, and keeping methods small and testable.
5. 5Java Lesson 5: Classes and ObjectsModel real things with classes: fields, constructors, methods, private data with public methods, and records for simple data.
6. 6Java Lesson 6: Collections — List and MapStore groups of objects with ArrayList and HashMap/TreeMap, loop over them, merge totals by key, and use streams for sums.
7. 7Java Lesson 7: Exceptions and FilesHandle errors with try/catch/finally, throw meaningful exceptions, and read and write text files with java.nio.file.Files.
8. 8Java Lesson 8: Project — An Invoice CalculatorBring it together: an Invoice class with validated lines, GST calculation, item summaries with a Map and formatted output, plus next steps…

Related: Java streams, read and write Excel with Apache POI.