Encapsulation & Abstraction

Encapsulation & Abstraction • Defining Abstraction • Levels of Abstraction • Class as Abstraction • Defining a Java Class • Instantiating a Class • Class Members • Class Modifiers • Member Modifiers • Accessibility Scope • Defining Encapsulation • Principles of Encapsulation • Encapsulating a Class • Setters & Getters • Constructors

Objectives • Define abstraction • Identify levels of abstraction • Understand that a class is a form of abstraction • Define a Java class • Learn how to create objects by instantiating their class • Identify class members • Identify and define each modifier applicable to a class • Identify and define each modifier applicable to class members • Define access scope for class and its members • Know the purpose of constructors and how to create one for a class

Objectives (continued) • Define encapsulation • Describe the principles of encapsulation • Learn how to encapsulate a class • Learn how to use setters and getters

Defining Abstraction • Abstraction is the process of extracting common features from specific examples • Abstraction is a process of defining the essential concepts while ignoring the inessential details

Different Types of Abstraction • Data Abstraction Programming languages define constructs to simplify the way information is presented to the programmer. • Functional Abstraction Programming languages have constructs that ‘gift wrap’ very complex and low level instructions into instructions that are much more readable. • Object Abstraction OOP languages take the concept even further and abstract programming constructs as objects.

Everything is an Object Anything that you can describe can be represented as an object, and that representation can be created, manipulated and destroyed to represent how you use the real object that it models.

information an object must know: • identity – uniqueness • attributes – structure • state – current condition behavior an object must do: • methods – what it can do • events – what it responds to Defining an Object An objectis a self-contained entity with attributes and behaviors

Person name sex age tellSex() tellAge() Class as Abstraction • A class is an abstraction of its instances. It defines all the attributes and methods that its instances must also have.

Defining a Class • A Classacts as the template from which an instance of an object is created. The class defines the properties of the object and the methods used to control the object's behavior. • A Class specifies the structure of data as well as the methods which manipulate that data. Such data and methods are contained in each instance of the class. • A Class is a model or template that can be instantiated to create objects with a common definition, and therefore common properties, operations and behavior. • A Class provides a template for defining the behavior of a particular type of object. Objects are referred to as “instances” of a class.

Defining a Java Class • A Java Class denotes a category of objects, and acts as a blueprint for creating such objects. • It defines its members referred to as fields and methods. • The fields (also known as variables or attributes) refer to the properties of the class. • The methods (also known as operations) refer to behaviors that the class exhibits. class Person { String name; char sex; int age; void tellSex() { if (sex=='M') System.out.println("I'm Male."); else if (sex=='F') System.out.println("I'm Female."); else System.out.println("I don't know!"); } void tellAge() { if (age<10) System.out.println("I'm just a kid."); else if (age<20) System.out.println("I'm a teenager."); else System.out.println("I'm a grown up."); } }

Class Members • A class member refers to one of the fields or methods of a class. • Static members are variables and methods belonging to a class where only a single copy of variables and methods are shared by each object. • Instance members are variables and methods belonging to objects where a copy of each variable and method is created for each object instantiated. class Person { static int maleCount; static int femaleCount; String name; char sex; int age; static void showSexDistribution() { if (maleCount>femaleCount) System.out.println("Majority are male."); else if (femaleCount>maleCount) System.out.println("Majority are female."); else System.out.println("Equal number of male and female."); } void tellSex() { if (sex=='M') System.out.println("I'm Male."); else if (sex=='F') System.out.println("I'm Female."); else System.out.println("I don't know!"); } void tellAge() { if (age<10) System.out.println("I'm just a kid."); else if (age<20) System.out.println("I'm a teenager."); else System.out.println("I'm a grown up."); } }

Instantiating a Class & Accessing its Members • Instantiating a class means creating objects of its own type. • The new operator is used to instantiate a class. Create Person objects using the new operator. class MainProgram { public static void main(String[] args) { // instantiating several objects Person p1 = new Person(); Person p2 = new Person(); Person p3 = new Person(); // accessing instance variables p1.name = "Vincent"; p1.sex = 'M'; p1.age = 8; p2.name = "Janice"; p2.sex = 'F'; p2.age = 19; p3.name = "Ricky"; p3.sex = 'M'; p3.age = 34; // accessing static variables Person.maleCount = 2; Person.femaleCount = 1; // accesssing instance methods p1.tellSex(); p1.tellAge(); p2.tellSex(); p2.tellAge(); p3.tellSex(); p3.tellAge(); // accessing static method Person.showSexDistribution(); } } class Person { static int maleCount; static int femaleCount; String name; char sex; int age; static void showSexDistribution() { // method body here } void tellSex() { // method body here } void tellAge() { // method body here } } Sample Output: I'm Male. I'm just a kid. I'm Female. I'm a teenager. I'm Male. I'm a grown up. Majority are male. Access class variables by setting their values Access class methods by invoking their names

Class Modifiers • Class modifiers change the way a class can be used. • Access modifiers describe how a class can be accessed. • Non-access modifiers describe how a class can be manipulated.

Access Modifiers • Member modifiers change the way class members can be used • Access modifiers describe how a member can be accessed

public default All classes may access public features of the Sample class. Only classes that are in the package may access default features of classes that are in the package Class Class Class Class Class Class Sample protected private Classes that are in the package and all its subclasses may access protected features of the Sample class. Private features of the Sample class can only be accessed from within the class itself. Access Modifiers Package * Default is not a modifier; it is just the name of the access level if no access modifier is specified.

Member Modifiers • Member modifiers change the way class members can be used • Non-access modifiers describe how a member can be manipulated

Accessibility Scope • Accessibility scope defines the boundary of access to a class and its members

Defining Encapsulation • Encapsulation is the process of hiding an object’s implementation from another object, while presenting only the interfaces that should be visible.

Principles of Encapsulation “Don’t ask how I do it, but this is what I can do” - The encapsulated object “I don’t care how, just do your job, and I’ll do mine” - One encapsulated object to another

Encapsulating a Class • Members of a class must always be declared with the minimum level of visibility. • Provide setters and getters (also known as accessors/mutators) to allow controlled access to private data. • Provide other public methods (known as interfaces ) that other objects must adhere to in order to interact with the object.

private char sex; public void setSex(char s) { // validate here sex = s; } public char getSex() { // format here return sex; } Setters and Getters allow controlled access to class data Setters are methods that (only) alter the state of an object Use setters to validate data before changing the object state Getters are methods that (only) return information about the state of an object Use getters to format data before returning the object’s state Setters and Getters

Encapsulation Example class Person { // set variables to private private static int maleCount; private static int femaleCount; private String name; private char sex; private int age; /* * setters & getters, set to public */ public int getAge() { return age;} public void setAge(int a) { age = a;} public String getName() { return name;} public void setName(String n) { name = n;} public char getSex() { return sex;} public void setSex(char s) { sex = s;} /* * set other methods as interfaces */ public static void showSexDistribution() { // implementation here } public void tellSex() { // implementation here } public void tellAge() { // implementation here } } public static void main(String[] args) { // instantiate several objects Person p1 = new Person(); Person p2 = new Person(); Person p3 = new Person(); // access instance variables using setters p1.setName("Vincent"); p1.setSex('M'); p1.setAge(8); p2.setName("Janice"); p2.setSex('F'); p1.setAge(19); p3.setName("Ricky"); p3.setSex('M'); p3.setAge(34); // access static variables directly Person.maleCount=2; Person.femaleCount=1; // access instance methods p1.tellSex(); p1.tellAge(); p2.tellSex(); p2.tellAge(); p3.tellSex(); p3.tellAge(); // access static method Person.showSexDistribution(); } I'm Male. I'm just a kid. I'm Female. I'm a teenager. I'm Male. I'm a grown up. Majority are male.

Constructors • Constructors are methods which set the initial state of an object • Constructors are called when an object is created using the new operator • A default constructor is a constructor with no parameters, it initializes the instance variables to default values • Restrictions on constructors • constructor name must be the same as the class name • constructor cannot return a value, not even void • only an access modifier is allowed

Key Points • Abstraction is the process of formulating general concepts by extracting common properties of instances. • A class is an abstraction of its instances. • A Java Class denotes a category of objects. • Class members refer to its fields and methods. • Static members are variables and methods belonging to a class. • Instance members are variables and methods belonging to objects. • Instantiating a class means creating objects of its own type. • Class modifiers include: (no modifier), public, abstract, finalandstrictfp. • Member modifiers include: (no modifier), public, protected, private, static, final, abstract, strictfp, synchronized, native, transientandvolatile.

Key Points (Continued) • Encapsulation hides implementation details of a class. • Encapsulating a class means declaring members with minimum level of visibility. • Setters are methods whose only function is to alter the state of an object in a controlled manner. • Getters are methods which only function is to return information about the state of an object. • Constructorsare methods which set the initial state of an object upon creation of the object.

Encapsulation & Abstraction