{small lecturenumber - hepage :} Inheritance Review{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} More examples: Java I/O{small lecturenumber - hepage :} Abstract classes{small lecturenumber - hepage :} Abstract classes{small lecturenumber - hepage :} Abstract classes{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} Dynamic binding{small lecturenumber - hepage :} Dynamic binding{small lecturenumber - hepage :} Constructors and Inheritance{small lecturenumber - hepage :} Constructors and Inheritance{small lecturenumber - hepage :} Constructors and Inheritance{small lecturenumber - hepage :} More on super(){small lecturenumber - hepage :} More on super(){small lecturenumber - hepage :} Exercise{small lecturenumber - hepage :} Next timeIntro to Programming IIMore InheritanceChris BrooksDepartment of Computer ScienceUniversity of San FranciscoDepartment of Computer Science — University of San Francisco – p. 1/??14-2: Inheritance Review•Inheritance allows us to reuse existing code.•Allows us to define a hierarchy of classes.•Base class has the most general behavior•Derived classes have more specific behavior.Department of Computer Science — University of San Francisco – p. 2/??14-3: Examplepublic class Person{public String lastName;public String id;public void eat() { };public void sleep() { };}Department of Computer Science — University of San Francisco14-4: Examplepublic class Professor extends Person{public String officeNum;public void teach() { };public void grade() { };public void forget() { };}Department of Computer Science — University of San Francisco – p. 4/??14-5: Example•What if we wanted to make a Student class that was a subclassof Person?◦methods attendClass, doHomework()•What if we also wanted to make a GradStudent that was asubclass of Student?◦New instance variable: public Professor advisor.•Note that Student is an is-a relationship, and advisor is a has-arelationship.Department of Computer Science — University of San Francisco – p. 5/??14-6: More examples: Java I/O•The Java I/O package provides some nice examples ofinheritance.•InputStream is a base class◦Provides read(), skip(), close() methods.◦Very basic functionality.•FileInputStream and FilterInputStream subclass InputStream◦They override the behavior of InputStream for particulardata sources.Department of Computer Science — University of San Francisco14-7: Abstract classes•On Monday, you did a lab in which you created a Shape class◦This had an area() method•You then subclassed it with Circle and Rectangle classes.•One problem with this: you may not want users to ever createShapes◦You just want anything that inherits from Shape to have anarea() method.Department of Computer Science — University of San Francisco – p. 7/??14-8: Abstract classes•Solution: define Shape as an abstract classpublic abstract class Shape{public int locX;public int locY;public abstract double area(); // note: semi-colon, no method body}Department of Computer Science — University of San Francisco – p. 8/??14-9: Abstract classes•An abstract class is one that has one or more abstract methods.◦Can also have concrete methods.•Classes that subclass from an abstract class must override allabstract methods.•An abstract class therefore provides a common interface for aset of subclassesDepartment of Computer Science — University of San Francisco14-10: Example•How would we rewrite Shape as an abstract class?Department of Computer Science — University of San Francisco – p. 10/??14-11: Dynamic binding•What if we have this situation:public class A {public void m1() { }public void m2() { }}public class B extends A {public void m2() { }}B bex = new B();A aex = new B();bex.m1();aex.m1();bex.m2()•Which methods are called?Department of Computer Science — University of San Francisco – p. 11/??14-12: Dynamic binding•Java resolves this via dynamic binding◦The actual type of an object is determined at runtime.◦That object’s class is searched for the correspondingmethod.◦If the method doesn’t exist in that class, the parent class ischecked.•What is the advantage of dynamic binding?•What is the disadvantage?Department of Computer Science — University of San Francisco14-13: Constructors and Inheritance•Suppose that the Person constructor looks like this:public Person(String lname) {lastName = lname;}•How can we still set the last name in derived classconstructors?Department of Computer Science — University of San Francisco – p. 13/??14-14: Constructors and Inheritance•We could do something like this:public Professor(String lname, String office) {lastName = lname;officeNum = office;}•Except:◦We just had to cut and paste code. (we hate that!)◦If the base class changes, all derived classes need tochange.Department of Computer Science — University of San Francisco – p. 14/??14-15: Constructors and Inheritance•Instead, let’s just indicate that the parent class’ constructorshould be called.•We do this with super()public Professor(String lname, String office) {super(lname);officeNum = office;}•Now we don’t need to worry about what the base class’constructor does anymore.Department of Computer Science — University of San Francisco14-16: More on super()•We can also use super to explicitly call a superclass’ method.•This lets us extend a method, rather than overriding it.•For example, let’s say that Person has the following method:/*in Person.java*/public void greet() {System.out.println(‘‘Nice to meet you’’);}Department of Computer Science — University of San Francisco – p. 16/??14-17: More on super()•We’d like for professors to do this greeting, plus a little extra.(They’re long-winded.) So we can do this:/*in Professor.java*/public void greet() {System.out.print(‘‘I do say, old chap,’’);super.greet();}Department of Computer Science — University of San Francisco – p. 17/??14-18: Exercise•Write a base class called Animal. Give it two instance variables(name and furColor). Give it a constructor that sets both ofthese.•Give Animal a printSelf method that prints out the following:◦My name is (name). My fur is (furColor).•Now create a class called Cat that inherits from Animal. Catshould have one instance variable: age.•Write a constructor for Cat that
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