Java is an object-oriented programming language where every program has at least one class. Programs are often built from many classes and objects, which are the instances of a class.

Classes define the state and behavior of their instances. Behavior comes from methods defined in the class. State comes from instance fields declared inside the class.

Classes are modeled on the real-world things we want to represent in our program. Later we will explore how a program can be made from multiple classes. For now, our programs are a single class.

public class Dog {
  // instance field
  String breed;
  // constructor method
  public Dog(String dogBreed) {
    /* 
    value of parameter dogBreed 
    assigned to instance field breed
    */
    breed = dogBreed;
  }
  public static void main(String[] args) {
    /* 
    create instance: 
    use 'new' operator and invoke constructor
    */
    Dog fido = new Dog("poodle");
    /* 
    fields are accessed using: 
    the instance name, `.` operator, and the field name.
    */
    fido.breed;
    // "poodle"
  }
}