Your first class

Classes model things

Classes were invented to solve a scaling problem: in a big program, the data describing one thing (a user's name, email, balance) and the code that operates on it drift apart into different files, and every change breaks something. A class fuses them into one unit — this is the core move of object-oriented programming, and it is how essentially all large Java systems (and the entire standard library you have been using) are organized. String with its .length() is a class; now you build your own.

So far Main was just a wrapper. A class becomes interesting when it describes a kind of thing: what data each one carries (fields) and what each one can do (methods).

class Dog {
  String name;   // field
  int age;       // field

  void bark() {  // instance method, no static
    System.out.println(name + " says woof");
  }
}

An object (or instance) is one concrete dog built from the blueprint:

Dog rex = new Dog();
rex.name = "Rex";
rex.bark();

new Dog() allocates a fresh object. Each object gets its own copy of the fields. Notice bark has no static: it belongs to one dog, and inside it, name means this dog's name.

Fields start with default values

A local variable must be assigned before use — the compiler refuses otherwise. Fields are different: every field gets an automatic default value the moment new runs, because the object must be in some defined state before the constructor or your code fills it in. The defaults are zero-like: 0 for numbers, false for boolean, and null for every object type, including String.

Dog stray = new Dog();
stray.age    // 0
stray.name   // null — no String here yet

null means "this reference points at nothing". Calling a method through it (stray.name.length()) throws a NullPointerException, the most famous crash in Java. This is the practical reason the next lesson exists: constructors make sure no object ever leaves new half-empty.