Objects and classes object : An entity that combines state and behavior. Java classes object-oriented programming (OOP) : Writing programs that perform most of their behavior as interactions between objects. class : 1. A program/module. or, 2. A blueprint/template for an object. classes you may have used so far: Savitch, ch 5 String , Scanner , File We will write classes to define new types of objects. 2 Abstraction Class = blueprint, Object = instance Music player blueprint abstraction : A distancing between ideas and details. state : current song Objects in Java provide abstraction: volume battery life We can use them without knowing how they work. behavior : power on/off change station/song change volume choose random song You use abstraction every day. Example: Your portable music player. Music player #1 Music player #2 Music player #3 state: state: state: You understand its external behavior (buttons, screen, etc.) song = ”Feels like rain" song = "Code Monkey" song = "Thriller" volume = 17 volume = 9 volume = 24 You don't understand its inner details (and you don't need to). battery life = 3.41 hrs battery life = 1.8 hrs battery life = 2.5 hrs behavior: behavior: behavior: power on/of power on/of power on/of change station/song change station/song change station/song change volume change volume change volume choose random song choose random song choose random song 4 3 How often would you expect Snake Eyes to get snake eyes? public class SnakeEyes { public static void main(String[] args){ int ROLLS = 100000; If you’re unsure on how to int count = 0; compute the probability then Die die1 = new Die(); Need to write the Die class! Die die2 = new Die(); you write a program that for (int i = 0; i < ROLLS; i++){ simulates the process. if (die1.roll() == 1 && die2.roll() == 1){ count++; } Can do this with short bit of code (google it) in a } main method, but let's say you want to reuse System.out.println(”snake eyes probability: " + (float)count / ROLLS); this code in multiple game development } projects. }
Die object The Die class The class (blueprint) knows how to create objects. State (data) of a Die object: Die class Instance variable Description state: int numFaces numFaces the number of faces for a die int faceValue behavior: the current value produced by rolling the die faceValue roll() getFaceValue() Behavior (methods) of a Die object: Die object #1 Die object #2 Die object #3 Method name Description state: state: state: numFaces = 6 numFaces = 6 numFaces = 10 roll the die (and return the value rolled) roll() faceValue = 2 faceValue = 5 faceValue = 8 getFaceValue() retrieve the value of the last roll behavior: behavior: behavior: roll() roll() roll() getFaceValue() getFaceValue() getFaceValue() Die die1 = new Die() ; 7 8 Die class The following code creates a new class named Die . Object state: public class Die { instance variables public int numFaces; declared outside of any method public int faceValue; } Save this code into a file named Die.java . Each Die object contains two pieces of data: an int named numFaces , an int named faceValue No behavior (yet). 10 9 Instance variables Instance variables Each Die object maintains its own numFaces instance variable : A variable inside an object that holds and faceValue variable, and thus its own part of its state. state Each object has its own copy . Declaring an instance variable: Die die1 = new Die(); Die die2 = new Die(); <type> <name> ; die1 numFaces 5 public class Die { 2 faceValue public int numFaces; public int faceValue; } die2 numFaces 6 3 faceValue 11
Accessing instance variables Client code Die.java can be made executable by giving it a main … Code in other classes can access your object's We will almost always do this…. WHY? To test the class Die before it is used by other classes instance variables. or can be used by other programs stored in separate .java files. Accessing an instance variable: dot operator client code : Code that uses a class <variable name> . <instance variable> Roll.java (client code) Modifying an instance variable: main(String[] args) { Die die1 = new Die(); Die.java die1.numFaces = 6; <variable name> . <instance variable> = <value> ; die1.faceValue = 5; public class Die { Examples: Die die2 = new Die(); public int numFaces; die2.numFaces = 10; public int faceValue; die2.faceValue = 3; System.out.println(”you rolled " + die.faceValue); } ... die.faceValue = 20; } 13 14 Instance methods Classes combine state and behavior. Object behavior: methods instance variables: define state instance methods : define behavior for each object of a class---the way objects communicate with each other and with users. instance method declaration, general syntax: public <type> <name> ( <parameter(s)> ) { <statement(s)> ; } 16 15 Rolling the dice: instance methods public class Die { Object initialization: public int numFaces; public int faceValue; constructors public int roll (){ faceValue = (int)(Math.random() * numFaces) + 1; return faceValue; } } Die die1 = new Die(); Think of each Die object as having its own die1.numFaces = 6; copy of the roll method, which operates int value1 = die1.roll(); on that object's state Die die2 = new Die(); die2.numFaces = 10; int value2 = die2.roll(); 18
Initializing objects Die constructor public class Die { When we create a new object, we can assign public int numFaces; Die die1 = new Die(6); public int faceValue; values to all, or some of, its instance variables: Die die1 = new Die(6); public Die (int faces) { numFaces = faces; How do we make that happen? faceValue = 1; } public int roll (){ faceValue = (int)(Math.random()*numFaces) + 1; return faceValue; } } 19 Constructors Multiple constructors are possible public class Die { constructor : creates and initializes a new object int numFaces; Die die1 = new Die(5); int faceValue; Die die2 = new Die(); public <type> ( <parameter(s)> ) { <statement(s)> ; public Die () { } numFaces = 6; For a constructor the <type> is the name of the class faceValue = 1; A constructor runs when the client uses the new keyword. } A constructor implicitly returns the newly created and initialized object. public Die (int faces) { If a class has no constructor, Java gives it a default constructor numFaces = faces; faceValue = 1; with no parameters that sets all the object's fields to 0 or null. } we did this in Recap.java } 21 The Student class Let’s write a class called Student with the Encapsulation following state and behavior: Student state: String name String id int[] grades behavior: Constructor – takes id and name numGrades – returns the number of grades addGrade – adds a grade getAverage – computes the average grade 24
Encapsulation Implementing encapsulation Instance variables can be declared private to indicate encapsulation : that no code outside their own class can access or change them. Hiding implementation details of an object from clients. Declaring a private instance variable: Encapsulation provides abstraction ; private <type> <name> ; Examples: we can use objects without knowing how they work. private int faceValue; private String name; The object has: an external view (its behavior) Once instance variables are private, client code cannot an internal view (the state and methods that access them: accomplish the behavior) Roll.java:11: faceValue has private access in Die System.out.println(”faceValue is " + die.faceValue); ^ 25 26 Instance variables, encapsulation and access Accessors and mutators We provide accessor methods to examine their values: In our previous implementation of the Die class we used the public access modifier: public int getFaceValue() { public class Die { return faceValue; public int numFaces; } public int faceValue; This gives clients read-only access to the object's fields. } We can encapsulate the instance variables using private: Client code will look like this: System.out.println(”faceValue is " + die.getFaceValue()); public class Die { private int numFaces; If required , we can also provide mutator methods: private int faceValue; } public void setFaceValue(int value) { But how does a client class now get to these? faceValue = value; } Often not needed. Do we need a mutator method in this case? 28 Benefjts of encapsulation Access Protection: Summary Protects an object from unwanted access by clients. Access protection has three main benefits: Example: If we write a program to manage users' bank It allows you to enforce constraints on an object's state. accounts, we don't want a malicious client program to be able to arbitrarily change a BankAccount object's balance. It provides a simpler client interface. Client programmers don't need to know everything that’s in the class, only the Allows you to change the class implementation later. public parts. As a general rule, all instance data should be modified only It separates interface from implementation, allowing by the object, i.e. instance variables should be declared them to vary independently. private 29
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