Encapsulation Abstraction Defining Abstraction Levels of Abstraction Class
Encapsulation & Abstraction • • • Defining Abstraction Levels of Abstraction Class as Abstraction Defining a Java Class Instantiating a Class Members Class Modifiers Member Modifiers Accessibility Scope • Defining Encapsulation • Principles of Encapsulation • Encapsulating a Class • Setters & Getters • Constructors 1
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 2
Objectives (continued) • • Define encapsulation Describe the principles of encapsulation Learn how to encapsulate a class Learn how to use setters and getters 3
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 4
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. 5
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. 6
Defining an Object An object is a self-contained entity with attributes and behaviors 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 7
Class as Abstraction • A class is an abstraction of its instances. It defines all the attributes and methods that its instances must also have. Person name sex age tell. Sex() tell. Age() 8
Defining a Class • A Class acts 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. 9
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 tell. Sex() { 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 tell. Age() { 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. "); } 10
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 male. Count; static int female. Count; String name; char sex; int age; } static void show. Sex. Distribution() { if (male. Count>female. Count) System. out. println("Majority are male. "); else if (female. Count>male. Count) System. out. println("Majority are female. "); else System. out. println("Equal number of male and female. " ); } void tell. Sex() { 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 tell. Age() { 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. "); } 11
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 Main. Program { public static void main(String[] args) { class Person { static int male. Count; static int female. Count; String name; char sex; int age; // instantiating several objects Person p 1 = new Person(); Person p 2 = new Person(); Person p 3 = new Person(); static void show. Sex. Distribution() { // method body here } // accessing instance variables p 1. name = "Vincent"; p 1. sex = 'M'; p 1. age = 8; p 2. name = "Janice"; p 2. sex = 'F'; p 2. age = 19; p 3. name = "Ricky"; p 3. sex = 'M'; p 3. age = 34; // accessing static variables Person. male. Count = 2; Person. female. Count = 1; // accesssing p 1. tell. Sex(); p 2. tell. Sex(); p 3. tell. Sex(); instance methods p 1. tell. Age(); p 2. tell. Age(); p 3. tell. Age(); // accessing static method Person. show. Sex. Distribution(); } } void tell. Sex() { // method body here } void tell. Age() { // method body here } } Sample Output: I'm Male. I'm just a kid. I'm Female. Access class variables I'm a teenager. by setting their values Access class methods I'm Male. by invoking their names I'm a grown up. Majority are male. 12
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. Modifier Description (no modifier) class is accessible within its package only public class is accessible by any class in any package abstract class cannot be instantiated (a class cannot be abstract and final at the same time) final class cannot be extended strictfp class implements strict floating-point arithmetic 13
Access Modifiers • Member modifiers change the way class members can be used • Access modifiers describe how a member can be accessed Modifier Description (no modifier) member is accessible within its package only public member is accessible from any class of any package protected member is accessible in its class package and by its subclasses private member is accessible only from its class 14
Access Modifiers private Private features of the Sample class can only be accessed from within the class itself. default Only classes that are in the package may access default features of classes that are in the package Clas s Package Sample Clas s protected public Classes that are in the package and all its subclasses may access protected features of the Sample class. All classes may access public features of the Sample class. Clas s * Default is not a modifier; it is just the name of the access level if no access modifier is specified. 15
Member Modifiers • Member modifiers change the way class members can be used • Non-access modifiers describe how a member can be manipulated Modifier Description static member belongs to a class final declares a constant variable or method abstract method is declared with no implementation (applied to methods, cannot be combined with other non-access modifiers ) strictfp method implements strict floating-point arithmetic (applied to methods) synchronized method is executed by only one thread at a time (applied only to methods) native method implementation is written in other language (applied only to methods) transient an instance variable is not saved when its object is persisted or serialized (applied only to variables) volatile variable is modified asynchronously by concurrently running threads (applied only to variables) 16
Accessibility Scope • Accessibility scope defines the boundary of access to a class and its members Scope Access static code can access static members but not instance members non-static code can access both static members and instance members package a class and its members can be accessed within the package they are declared class members can be accessed within the class block local variables can be accessed only within a block 17
Defining Encapsulation • Encapsulation is the process of hiding an object’s implementation from another object, while presenting only the interfaces that should be visible. 18
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 19
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. 20
Setters and Getters • 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 private char sex; public void set. Sex(char s) { // validate here sex = s; } public char get. Sex() { // format here return sex; } 21
Encapsulation Example public static void main(String[] args) { // instantiate several objects Person p 1 = new Person(); Person p 2 = new Person(); Person p 3 = new Person(); // access instance variables using setters p 1. set. Name("Vincent"); p 1. set. Sex('M'); p 1. set. Age(8); p 2. set. Name("Janice"); p 2. set. Sex('F'); p 1. set. Age(19); p 3. set. Name("Ricky"); p 3. set. Sex('M'); p 3. set. Age(34); class Person { // set variables to private static int male. Count; private static int female. Count; private String name; private char sex; private int age; // access static variables directly Person. male. Count=2; Person. female. Count=1; // access instance methods p 1. tell. Sex(); p 1. tell. Age(); p 2. tell. Sex(); p 2. tell. Age(); p 3. tell. Sex(); p 3. tell. Age(); // access static method Person. show. Sex. Distribution(); } } /* * setters & getters, set to public */ public int get. Age() { return age; } public void set. Age(int a) { age = a; } public String get. Name() { return name; } public void set. Name(String n) { name = n; } public char get. Sex() { return sex; } public void set. Sex(char s) { sex = s; } /* * set other methods as interfaces */ public static void show. Sex. Distribution() { // implementation here } public void tell. Sex() { // implementation here I'm Male. } I'm just a kid. public. I'm void tell. Age() { Female. // implementation I'm a teenager. here } I'm Male. I'm a grown up. Majority are male. 22
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 23
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, final and strictfp. • Member modifiers include: (no modifier), public, protected, private, static, final, abstract, strictfp, synchronized, native, transient and volatile. 24
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. • Constructors are methods which set the initial state of an object upon creation of the object. 25
- Slides: 25