Classes, Objects & References

A class is a blueprint; an object is one concrete thing built from it. The class says what data (fields) and behavior (methods) every object of that kind has:

class Dog {
    String name;
    int age;

    void bark() {
        System.out.println(name + " says woof");
    }
}

Dog a = new Dog();     // object 1
a.name = "Rex";
a.age = 3;
a.bark();              // Rex says woof

Dog b = new Dog();     // object 2: own name, own age, SAME behavior
b.name = "Lucky";
b.bark();              // Lucky says woof

new Dog() allocates a fresh object with its own copy of every field. The dot accesses what the object owns: a.name is object a's name. Two objects are independent — renaming Rex never touches Lucky.

State + behavior in one place is the point of classes. bark() uses name without a parameter, because it automatically operates on the object it was called on. Inside a method, this names that object: this.name is "the name field of the object I was called on". When a parameter shadows a field, this disambiguates:

void rename(String name) {
    this.name = name;   // field = parameter
}

Object references. Dog d2 = a; does NOT copy the dog — it copies the reference (the arrow pointing at the object). Now a and d2 are two names for one object; changing fields through either is visible through both. This is the object half of Module 5's "parameters are copies": the copy is of the arrow, not the dog.

Next: constructors — controlling how objects are born.