Inheritance CSC9Y4. Inheritance

Transcription

1 Inheritance CSC9Y4 1 Inheritance We start with a superclass from which a subclass can be derived. New attributes and methods can be defined in a subclass and existing methods re-defined. How do we know which version to use? We shall look at inheritance and dynamic binding in C++ and Java. In C++, a superclass is known as a base class and a subclass is known as a derived class. 2 1

2 is-a vs. has-a is-a represents inheritance subclass object is an example of the superclass object a Car is a Vehicle Car is subclass, and Vehicle is superclass keywords: extends has-a represents composition object contains one or more objects of other classes as members Car has a Steering Wheel 3 Composition Dog has-a owner, mother, father, leash public class Dog { private String name; private int age; private Person owner; private Dog mother, father; private Leash dogleash; } 4 2

3 Inheritance Dog extends ( is-a ) Animal Animal Animal s stuff Dog Animal s stuff Dog s stuff 5 An inheritance (Class) hierarchy CommunityMember Employee Student Alumnus Faculty Staff Administrator Teacher 6 3

4 Draw an Inheritance Hierarchy for these classes: WalkingBird Owl Ostrich Parrot Bird FlyingBird Chicken TalkingParrot Bird WalkingBird FlyingBird Ostrich Chicken Parrot Owl TalkingParrot 7 Strategy Design classes for objects Identify characteristics classes have in common Abstraction: focus on commonalities among objects in a system Design superclasses to store common characteristics 8 4

5 Access control in Java Access control keyword set access levels for classes, variables, methods, and constructors public class classname { //... } private boolean myflag; double weeks = 9.5; protected int width = 42; Modifier Class Package Subclass World public Y Y Y Y private Y N N N none Y Y N N protected Y Y Y N 9 protected members Variables or methods available to subclasses Accessible to superclass members subclass members class members in the same package Use super. to access a superclass method that has been overridden by a subclass method. ClassC ClassA.m1() ClassA.m3() is-a ClassA public m1() private m2() protected m3() has-a ClassB ClassA.m1() 10 5

6 Java class Person public class Person { protected String thename; public Person(String name) { thename = name; } // constructor public String getinfo() { return thename; } // getinfo public String detailinfo() { return "Details are: + this.getinfo(); } // detailinfo } // Person 11 Calls Java: Person woman = new Person("Sue"); String st = woman.getinfo(); C++: Person man("jim"); Person* woman = new Person("Sue"); printf(woman->getinfo()); 12 6

7 Java class Student public class Student extends Person { private int theregnum; public Student(String name, int reg) { super(name); theregnum = reg; } // constructor public int getregnum() { return theregnum; } // getregnum public String getinfo() { return thename + "," + theregnum; } // getinfo } // Student 13 Inheritance: type checking An object of a subclass can always be used when an object of its superclass is expected. Consider the following Java declarations: Person p1 = new Person("Fred"); Person p2 = new Student("George",974503); The type of both p1 and p2 is reference to Person. Can I write: Student p3 = new Person( James"); 14 7

8 Inheritance Person p2 = new Student("George",974503); What happens in the call: String st = p2.getinfo(); Which definition of getinfo is called? As p2 is currently pointing at a Student object, it is the definition of getinfo defined in class Student that is called. So we will produce name and reg number. 15 Dynamic binding Consider now the call p2.detailinfo(); The only definition of detailinfo is in class Person and so that is the method that is called. During its execution we have the call this.getinfo(); What is the result? As p2 is currently pointing at a Student it is the version of getinfo defined in Student which is executed. 16 8

9 Type of p2 Consider now the call: p2.getregnum(); Method getregnum is defined in class Student and p2 is currently pointing at a Student object. What result does this have? The type of p2 is reference to Person and so this call is not legal and would give a syntax error. 17 Overloading vs. Redefinition In Java, when a method is redefined in a subclass with exactly the same parameter profile as the original method in the superclass, then we have redefinition and the binding of method calls occurs at run time, i.e. it is dynamic. If the new method has the same name, but a different parameter profile, then we have overloading and the binding occurs at compile time, i.e. it is static. 18 9

10 Overloading Two procedures, functions or methods in C++, Java or Ada may have the same name and different parameter profiles: void swap(int& a, int& b) {... } void swap(double& a, double& b) {... } Separate code is generated for each of the overloaded routines, and the compiler decides at compile time which one to use. We therefore have static binding. 19 Overloading In Java, we often have several constructors: class BankAcc { public BankAccount(String id) { } public BankAccount(String id, int amount) { } } This is an example of overloading

11 Redefinition in C++ C++ supports both static and dynamic binding of methods. When a method is defined in the superclass to be virtual as in: virtual String getinfo() {... } then it may be redefined in a subclass giving us dynamic binding. If the reserved word virtual is omitted then the binding is always static. This is more efficient. 21 Advantages: software re-use Summary faster software development Inheritance should only be used when there is a logical relationship between a superclass and a subclass. This is the is_a relationship. We have: Student is_a Person 22 11