Object Oriented Programming with Java

Transcription

1 CSC System Development with Java Object Oriented Programming with Java Department of Statistics and Computer Science 1

2 Advantages of OOP Provides a clear modular structure for programs OOP makes it easy to maintain and modify existing code (new objects can be created with small differences to existing ones) OOP provides a good framework for code libraries Software components can be easily adapted and modified by the programmer. 2

3 Concepts of OOP Objects Classes Data Abstraction Encapsulation Inheritance Polymorphism 3

4 What is an Objects? Object is a software bundle of related state and behavior Characteristics: state and behavior 4

5 What is a class? A Java class is a group of Java methods and variables Object is an instance of a class 5

6 Object Oriented Concepts 6

7 Constructors Very similar to C++ You can create multiple constructors, each must accept different parameters. If you don't write any constructor, the compiler will (in effect) write one for you: classname() {} If you include any constructors in a class, the compiler will not create a default constructor! 7

8 Multiple Constructors One constructor can call another. You use "this", not the classname: class Foo { int i; Foo() { } this(0); Foo( int x ) { } i = x; A call to this must be the first statement in the constructor! 8

9 Destructors No! There is a finalize() method that is called when an object is destroyed. you don't have control over when the object is destroyed (it might never be destroyed). The JVM garbage collector takes care of destroying objects automatically (you have limited control over this process). 9

10 Class Modifiers Public Abstract Final Default (none) 10

11 Class Modifiers public: anyone can create an object of the defined class. only one public class per file, must have same name as the file (this is how Java finds it!). default: is non-public (if you don't specify "public"). 11

12 Class Modifiers Abstract: modifier means that the class can be used as a superclass only. no objects of this class can be created. Final: if its definition is complete and no subclasses are desired or required a final class never has any subclasses, the methods of a final class are never overridden 12

13 Field Modifiers public protected private none or package or default Static Final 13

14 Field Modifiers public: any method (in any class) can access the field. protected: any method in the same package can access the field, or any derived class. private: only methods in the class can access the field. Default: is that only methods in the same package can access the field. 14

15 Field Modifiers Static: Fields declared static are called class fields (class variables). others are called instance fields. There is only one copy of a static field, no matter how many objects are created. Final:class and instance variables (static and non-static fields) may be declared final. The keyword final means: once the value is set, it can never be changed static final int BUFSIZE=100; final double PI= ; 15

16 Method Modifiers public protected none or package or default private final abstract static native synchronized 16

17 Method Modifiers private/protected/public: same idea as with fields. abstract: no implementation given, must be supplied by subclass. the class itself must also be declared abstract 17

18 Method Modifiers static: the method is a class method, it doesn't depend on any instance fields or methods, and can be called without first creating an object. final: the method cannot be changed by a subclass (no alternative implementation can be provided by a subclass). 18

19 Method Modifiers native: the method is written in some local code (C/C++) - the implementation is not provided in Java. synchronized: only one thread at a time can call the method. 19

20 Method Overloading You can overload methods: same method name, different parameters. you can't just change return type, the parameters need to be different. Method overloading is resolved at compile time. int CounterValue() { return counter; } double CounterValue() { } return (double) counter; Won't Work! 20

21 All Possible Combinations of Features and Modifiers Modifier Class Variable Method Constructor public yes yes yes yes protected no yes yes yes None (default) yes yes yes yes private no yes yes yes final yes yes yes no abstract yes no yes no static no yes yes no native no no yes no transient no yes no no volatile no yes no no synchronized no no yes no strictfp yes no yes yes 21

22 Java Modifier Summary Modifier Used on Class Meaning Contains unimplemented methods and cannot be instantiated abstract interface All interfaces are abstract. Optional in declarations method No body, only signature. The enclosing class is abstract 22

23 Java Modifier Summary Modifier Used on Class Meaning Cannot be subclassed Final method field Cannot be overridden and dynamically looked up Cannot change its value. static final fields are compile-time constants. variable Cannot change its value. 23

24 Java Modifier Summary Modifier Used on Meaning Native method Platform-dependent. No body, only signature Modifier Used on Meaning Private member Accessible only in its class Modifier Used on protected member Meaning Accessible only within its package and its subclasses 24

25 Java Modifier Summary Modifier Used on Meaning Class Accessible only in its package None Interface Accessible only in its package Member Accessible only in its package 25

26 Java Modifier Summary Modifier Used on Class Meaning Accessible anywhere Public interface Accessible anywhere Member Accessible anywhere. 26

27 Java Modifier Summary class Make an inner class top-level class method A class method, invoked through the class name. static field initializer A class field, invoked through the class name one instance, regardless of class instances created. Run when the class is loaded, rather than when an instance is created. 27

28 Packages 28

29 Package defined as a grouping of related types existing packages in Java are: java.lang - bundles the fundamental classes java.io - classes for input, output functions are bundled in this package 29

30 Creating a Package Put a package statement with the package name At the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package The package statement should be the first line in the source file There can be only one package statement in each source file, and it applies to all types in the file 30

31 Example /* File name : Animal.java */ package animals; interface Animal { public void eat(); } package animals; /* File name : MammalInt.java */ public class Mammal implements Animal { public void eat() { System.out.println("Mammal eats"); } Now you compile these two files and put them in a sub-directory called animals 31

32 The Import Keyword If a class wants to use another class in the same package, the package name does not need to be used. Classes in the same package find each other without any special syntax The fully qualified name of the class can be used. import animals.mamal The package can be imported using the import keyword and the wild card (*) import animals.*; 32

33 Abstraction 33

34 Abstraction Refers to the ability to make a class abstract in OOP Abstract class Cannot be instantiated Other functionality of the class still exists Cannot create an instance of the abstract class 34

35 Abstract Class Use the abstract keyword to declare a class abstract public abstract class Employee { } private String name; private String address;... Cannot use Employee e = new Employee(); Employee.java: xx: Employee is abstract; cannot be instantiated Employee e = new Employee(); ^ 1 error1 35

36 Extending Abstract Class public class Salary extends Employee { } private double salary; Salary(String name, String address, { }... int number, double salary) super(name, address, number); setsalary(salary); Call Employee class Extend Employee class 36

37 Abstract Methods Can declare the method in the parent class as abstract Abstract methods consist of a method signature, but no method body public abstract class Employee { private String name; private String address; private int number; public abstract double computepay(); //Remainder of class definition } 37

38 Declaring a Method as Abstract The class must also be declared abstract Any child class must either override the abstract method or declare itself abstract A child class that inherits an abstract method must override it If they do not, they must be abstract,and any of their children must override it 38

39 Salary is Extending Employee class it is required to implement computepay() method public class Salary extends Employee { private double salary; //Annual salary public double computepay() { System.out.println("Computing " + getname()); return salary/52; } //Remainder of class definition } 39

40 Example 40

41 Example(1) Employee Name Address Number computepay() mailcheck() tostring() getname() 41

42 Salary class Employee Salary 42

43 Abstract Demonstration Employee Salary 43

44 Exercise Remove computepay() methods in the Salary class and re-run the program. Change the computepay() as public abstract double computepay() ; And re-run the program Identify the correct usage of the Abstract class Abstract methods 44

45 Encapsulation 45

46 Encapsulation Information Hiding. Don't need to know how some component is implemented to use it. Implementation can change without effecting any calling code. "protects us from ourselves" 46

47 Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods is also referred to as data hiding Benefit is the ability to modify our implemented code without breaking the code of others who use our code Encapsulation gives maintainability, flexibility and extensibility to our code 47

48 Example public class Encap { private String name; private String idnum; private int age; public int getage() { return age; } public String getname() { return name; } public String getidnum() { return idnum; } public void setage( int newage) { age = newage; } public void setname(string newname) { name = newname; } public void setidnum( String newid) { idnum = newid; } } 48

49 Example public class RunEncap { public static void main(string args[]) { EncapTest encap = new EncapTest(); encap.setname("james"); encap.setage(20); encap.setidnum("12343ms"); } } System.out.println("Name : " + encap.getname()+ " Age : "+ encap.getage()); 49

50 Benefits of Encapsulation: The fields of a class can be made read-only or write-only. A class can have total control over what is stored in its fields. The users of a class do not know how the class stores its data. A class can change the data type of a field, and users of the class do not need to change any of their code. 50

51 Inheritance 51

52 Inheritance On the surface, inheritance is a code re-use issue. we can extend code that is already written in a manageable manner. Inheritance is more, it supports polymorphism at the language level! Information is made manageable in a hierarchical order 52

53 Inheritance cont. With Inheritance, new class can be derived from existing classes as a building block New class Inherit properties and methods from the existing class New class can also added Its own properties and methods Keyword Extends Implements 53

54 Inheritance cont. Take an existing object type (collection of fields and methods) and extend it. create a special version of the code without rewriting any of the existing code (or even explicitly calling it!). End result is a more specific object type, called the sub-class / derived class / child class. The original code is called the super class / parent class / base class. 54

55 IS-A Relationship (Example) public class Animal { } extends Animal public class Mammal extends Animal { } public class Reptile extends Animal { } Mamal public class Dog extends Mammal { } 55

56 IS-A Relationship (Example) public class Dog extends Mammal { public static void main(string args[]) { Animal a = new Animal(); Reptile r = new Reptile(); Mammal m = new Mammal(); Dog d = new Dog(); System.out.println(m instanceof Animal); System.out.println(d instanceof Mammal); System.out.println(d instanceof Animal); } } Animal Mamal Dog Reptile 56

57 IS-A Relationship In Object Oriented terms following are true: Animal is the superclass of Mammal class. Animal is the superclass of Reptile class. Mammal and Reptile are sub classes of Animal class. Dog is the subclass of both Mammal and Animal classes. 57

58 IS-A Relationship IS-A relationship we can say: Mammal IS-A Animal Reptile IS-A Animal Dog IS-A Mammal Hence : Dog IS-A Animal as well Use of the extends keyword the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass 58

59 Accessing superclass methods Can use super() to access all (non-private) superclass methods. even those replaced with new versions in the derived class. Can use super() to call base class constructor. 59

60 Single Inheritance You can't extend more than one class! the derived class can't have more than one base class. You can do multiple inheritance with interface inheritance. 60

61 Inheritance Example cont. Employee: name, , phone FulltimeEmployee: also has salary, office, benefits, Manager: CompanyCar, can change salaries, rates contracts, offices, etc. Contractor: HourlyRate, ContractDuration, A manager a special kind of FullTimeEmployee, which is a special kind of Employee. The relationship modeled by inheritance is often referred to as a is a relationship 61

62 Inheritance Example Employee Name Phone FulltimeEmployee Salary Office Manager CompanyCar Phone Contractor HourlyRate, ContractDuration 62

63 Inheritance Example Is a Employee Name Phone Is a FulltimeEmployee Salary Office Manager CompanyCar Phone Is a Contractor HourlyRate, ContractDuration 63

64 Inheritance Example Manager is a full time employee Full time employee is a employee 64

65 HAS-A Relationship relationships are mainly based on the usage determines whether a certain class HAS-A certain thing helps to reduce duplication of code as well as bugs 65

66 Example public class Vehicle {} public class Speed {} public class Van extends Vehicle { private Speed sp; } class Van HAS-A Speed Reuse the Speed class in multiple applications Vehicle Van sp 66

67 Interface 67

68 Interfaces is a collection of abstract methods An interface is not a class A class describes the attributes and behaviors of an object An interface contains behaviors that a class implements 68

69 Interfaces cont. Interfaces have the following properties: An interface is implicitly abstract. You do not need to use the abstract keyword when declaring an interface. Methods in an interface are implicitly public. 69

70 Interfaces cont. An interface is a definition of method prototypes and possibly some constants (static final fields). An interface does not include the implementation of any methods, it just defines a set of methods that could be implemented. 70

71 Implement an interfaces A class can implement an interface, this means that it provides implementations for all the methods in the interface. Java classes can implement any number of interfaces (multiple interface inheritance). 71

72 Interfaces Creation (definition) of interfaces can be done using inheritance: one interface can extend another. Sometimes interfaces are used just as labeling mechanisms: Look in the Java API documentation for interfaces like Cloneable. 72

73 Interfaces An interface is similar to a class in the following ways: An interface can contain any number of methods. An interface is written in a file with a.java extension, with the name of the interface matching the name of the file. The bytecode of an interface appears in a.class file. Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name. 73

74 Interfaces An interface is different from a class in several ways, including: You cannot instantiate an interface. An interface does not contain any constructors. All of the methods in an interface are abstract. An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final. An interface is not extended by a class; it is implemented by a class. An interface can extend multiple interfaces. 74

75 Declaring Interfaces The interface keyword is used to declare an interface public class MammalInt implements Animal { public void eat() { System.out.println("Mammal eats"); } public static void main(string args[]) { MammalInt m = new MammalInt(); m.eat(); } } interface Animal { public void eat(); } 75

76 Extending Interfaces An interface can extend another interface public interface Sports { public void sethometeam(string name); } Sport public interface Football extends Sports { public void hometeamscored(int points); } Football Hockey public interface Hockey extends Sports { public void homegoalscored(); } 76

77 Multiple Interfaces A Java class can only extend one parent class. Multiple inheritance is not allowed An interface can extend more than one parent interface Extends keyword is used once, and the parent interfaces are declared in a comma-separated list. public interface Hockey extends Sports, Event Sports Event Hockey 77

78 The instanceof Keyword the instanceof operator to check determine whether Mammal is actually an Animal, and dog is actually an Animal 78

79 Polymorphism 79

80 Polymorphism is the ability of an object to take on many forms OOP occurs when a parent class reference is used to refer to a child class object 80

81 Polymorphism Create code that deals with general object types, without the need to know what specific type each object is. Generate a list of employee names: all objects derived from Employee have a name field! no need to treat managers differently from anyone else. 81

82 Polymorphism The real power comes with methods/behaviors. A better example: shape object types used by a drawing program. we want to be able to handle any kind of shape someone wants to code (in the future). we want to be able to write code now that can deal with shape objects (without knowing what they are!). 82

83 Example public interface Vegetarian {} public class Animal{} public class Deer extends Animal implements Vegetarian{} Deer class is considered to be polymorphic since this has multiple inheritance A Deer IS-A aanimal A Deer IS-A Vegetarian A Deer IS-A Deer A Deer IS-A Object 83

84 Types of Polymorphism Overloading Overriding Dynamic method binding 84

85 Example public static void main ( String ary[ ] ) { Box x; Box b1 =new Box( ); WoddenBox wb=new WoddenBox( ); SteelBox s1=new SteelBox( ); LargeWoddenBox p1=new LargeWoddenBox( ); b1.info( ); wb.info( ); s1.info( ); p1.info( ); System.out.println(" "); Box b[]=new Box[5]; b[1]=new Box(); b[2]=new WoddenBox(); b[3]=new SteelBox(); b[4]=new LargeWoddenBox(); for(int i=1;i<5;i++) b[i].info(); } WoddenBox LargeWoddenBox Box SteelBox 85