Classes & Objects

1. Classes & Objects Representin’ real-world things in code-space Brian Camodeca, Mercyhurst College

2. What is a class? • Defines, conceptually, some real-world thing and how the computer can build such a thing; a blueprint. • Consists of state (stuff describing the thing) and behavior (stuff the thing does).

3. Building a class • Classes have built-in mechanisms for state and behavior • Instance variables represent state • Methods represent behaviors • Consider this…

4. Release the hounds • What if our application needs to work with dogs? • Built-in types like integers, strings, and arrays alone can’t really help us here. • But, maybe we can unify them in this new class thingie! • Consider things about a dog…

5. A Dog State Behavior • Name • Breed • Fur Color • Weight • Temperament • Hungry • Tired • Bark • Beg • Eat • Chase Tail • Fetch • Sleep

6. A Dog…in code • Name : String • Breed : String • Fur Color : String • Weight : int • Temperament : String • Hungry : boolean • Tired : boolean

7. Building a class • For example, Dog.java • public class Dog { String name, breed; int weight; public void bark() { System.out.println(“Woof”); } }

8. Building a class • Creating a class creates a new type • If we name our class “Dog”, we can now create variables of type Dog • Dog fido;

9. Creating an object • An object is a given instance of a class • A class is the abstract idea, an object is the concrete example • Dog fido = new Dog(); “fido” is the object, an instance of the Dog class

10. Manipulating the object • We can change the state of the object by manipulating its instance variables directly (for now) • For example, • Dog someDog = new Dog(); someDog.name = “Fido”; someDog.weight= 35; • System.out.println(someDog.name);

11. Using the object • We can invoke behaviors of the object by calling its methods by name • For example, • someDog.bark(); • Prints “Woof!” to the console

12. Manipulating the object II • Before, we changed the values directly. • DON’T ALLOW THIS • Set instance variables as “private” and create public “getter” and “setter” methods • For example…

13. Manipulating the object II • public class Dog { private String name, breed; private int weight; public void setName(String name) { this.name = name; } public String getName() { return this.name; } }

14. Encapsulation • But why? • Encapsulation! Also known as “information hiding” • Consider this…

15. Encapsulation • (Assuming variables are still public) • Dog someDog = new Dog(); someDog.name= “Fido”; someDog.weight= -7; • No control over what values get assigned to the instance variables. • If only we had a way to make sure-

16. “Setters” • Write methods to set instance variables! • public void setWeight (int weight) { if (weight > 0) { // phew this.weight= weight; } else { // AHHHHH! PANIC!!! // Throw exception } }

17. “Getters” • Uh-oh, only other members of the class can “see” private members. We need a liaison! • public int getWeight() { return this.weight; }

18. The power of instance variables • Make the class dynamic! • Consider the correlation between a dog’s size and the sound of its bark…

19. The power of instance variables • public void bark() { if (this.weight > 50) { System.out.println(“Woof!”); } else if (this.weight > 15) { System.out.println(“Ruff!”); } else { System.out.println(“Yip!”); } }

20. The constructor • Special function that is invoked upon object instantiation • Java convention: named the same as the class’ name, and is that class’ return type • For example…

21. The constructor • public Dog() { // Do Something }

22. The constructor • public Dog(String name) { this.name = name; } • public Dog(String name, String breed) { this.name = name; this.breed = breed; }

23. The toString() method • A method that returns a string representation of that object. • By default it’s not very helpful, but we can implement our own toString() method! • @Override

24. Where is “static”? • Static members of a class can be accessed without an instantiation, like our readLine() method. • All our variables pertain to the particular instance of the dog, therefore they are non-static.