Lesson 6: Putting It Together - Your First Class

Putting It Together A constructor, two fields and a method: a real class.

1class Dog {

2 String name;

3 int age;

5 Dog(String dogName, int dogAge) {

6 name = dogName;

7 age = dogAge;

8 }

10 void introduce() {

11 System.out.println("I am " + name + ", age " + age);

12 }

13}

15public static void main(String[] args) {

16 Dog rex = new Dog("Rex", 3);

17 Dog bella = new Dog("Bella", 5);

18 rex.introduce();

19 bella.introduce();

20}

Terminal

$ javac Main.java

$ java Main

I am Rex, age 3

I am Bella, age 5

Every line, explained

class Dog {: class Dog { ... } defines a class: a blueprint describing what every Dog has and can do. A blueprint on its own does nothing; it is a plan, waiting to be built from.
String name;: A variable declared inside a class is called a field. Every Dog built from this blueprint gets its OWN copy: its own little name box.
int age;: Second field: every Dog has an age too.
Dog(String dogName, int dogAge) {: A constructor can take several values, one for each field it needs to fill in.
name = dogName;: Stores the first value into the name field.
age = dogAge;: Stores the second value into the age field.
}: This } closes the constructor.
void introduce() {: A method every Dog can do: introduce itself.
System.out.println("I am " + name + ", age " + age);: Uses BOTH fields of whichever dog is introducing itself.
}: This } closes the method.
}: This } closes the class.
public static void main(String[] args) {: The main method: where the program starts, exactly as in Intro to Java.
Dog rex = new Dog("Rex", 3);: Builds rex with both values in one go.
Dog bella = new Dog("Bella", 5);: And a second, completely separate dog.
rex.introduce();: rex introduces himself...
bella.introduce();: ...and bella introduces herself. Same method, different objects, different answers: that is the magic of classes.
}: This } closes the main method.