Java syntax, classes and interfaces

Java is statically typed, which means every variable has a fixed type checked before the program runs. Everything lives inside a class, so learn the syntax and the basics of objects together. Start with primitive types versus objects, and classes with constructors and fields.

Then learn access modifiers such as private, static members, and interfaces, which describe what a type can do. Learn modern Java, version 17 or later, from the start. That gives you var for local variables, switch expressions, text blocks and pattern matching with instanceof.

Compare objects with equals, not ==, because == only checks whether two variables point at the very same object. Spring builds almost everything around interfaces and constructor injection, so understanding interfaces now pays off later.