IS-A / HAS-A. I IS-A Teacher and I HAS-A Laptop

Transcription

1 IS-A / HAS-A I IS-A Teacher and I HAS-A Laptop 1

2 Corresponding Book Sections Pearson Custom Computer Science: Chapter 1, Sections 9, 12 2

3 IS-A A class has an IS-A relationship to a class type if it is that class type, or inherits from that class type. A Dog IS-A Animal is a true statement A class does not have an IS-A relationship to subclasses of a given class An Animal IS-A Dog is a false statement 3

4 HAS-A A class has a HAS-A relationship with another class if it has an object of that class type as a class field. HAS-A refers to the composition of a class, or what the class is made up of A Dog HAS-A Bone 4

5 IS-A vs. HAS-A Example public class Soldier {! Weapon theweapon;! public void march() { public class Marine extends Soldier {! public Marine() {!! theweapon = new M16();!! public void motto() {!! System.out.println("Semper Fidelis!");! public class Weapon {! public void fire() {! public void reload() { public class M16 extends Weapon {! public void noobtube() { public class M40A3 extends Weapon {! public void zoom() { public class ScoutSniper extends Marine {! public ScoutSniper() {!! theweapon = new M40A3();!! public void motto() {!! System.out.println("One shot, one kill!");! Marine IS-A Soldier Marine HAS-A M16 Marine HAS-A Weapon ScoutSniper IS-A Soldier Can you name some other IS-A/HAS-A relationships? 5

6 Another Example public class Person {! protected String name;! protected Computer computer;! private String ssn;! public void eat() { public class Student extends Person {!private String mnumber;! public Student(String idnum) {!! mnumber = new String(idNum);!! computer = new Laptop();!! public void study() {!! System.out.println("Study Study Study!");! public class Alumnus extends Person {! public String job;! public Alumnus(String newjob) {!! job = newjob;!! computer = new Desktop();!! public void work() {!! System.out.println("Work work work!");! public class Computer {! protected String processor;! public void reboot() {! public void shutdown() { public class Laptop extends Computer {! public void playfarmville() { public class Desktop extends Computer {! public void playsolitaire() { Student IS-A Person Person HAS-A Computer Student HAS-A Laptop Alumnus IS-A Person Alumnus HAS-A job Can you name some other IS-A/HAS-A relationships? 6

7 IS-A/HAS-A Breakdown Situation A Square IS-A Rectangle, right? Rectangle square = new Square(); square.setheight(10); square.setwidth(5); System.out.println( Height is + square.getheight()); What should be printed? 7

8 IS-A/HAS-A Breakdown Situation Liskov Substitution Principle (LSP) - Subtypes must be substitutable for their base types Inheritance should not be blindly applied when an IS-A relationship applies, but when the subclass behaves in the same manner that the superclass would 8

9 instanceof Keyword Sometimes you have the situation where you want to use functionality of the base class of an object but the only reference you have to the object is of a superclass type because you are utilizing polymorphism Java has the keyword instanceof that you can use to check that an object is of a specific class type so you can do safe typecasting. 9

10 instanceof Example public class Rectangle extends Shape {! protected double ratio = 1.6;! // width to height ratio! protected Color color = Color.yellow;! // rectangles will be yellow! public void setratio(double newratio) {!! ratio = newratio;! // end of setratio method!! public void draw(graphics g) {!! g.setcolor(color);!! g.fillrect(xpos, ypos,!!!! (int) (size * ratio), size);! // end of setposition method class Display extends JPanel {! private Shape[] theshapes = new Shape[8];! public Display() {!! // create a bunch of Shape objects //...!!! // position the shapes in the applet!! for (int num = 0; num < theshapes.length; num += 1) {!!! int newx = (int) (Math.random() * 500);!!! int newy = (int) (Math.random() * 500);!!! theshapes[num].setposition(newx, newy);!!! int newsize = (int) (Math.random() * );!!! theshapes[num].setsize(newsize);!!! // adjust the ratio of rectangles if (theshapes[num] instanceof Rectangle) {!!!! ((Rectangle) theshapes[num]).setratio(2.0);!!!!!! // end of constructor 10

11 getclass() Defined in the Object class Inherited by all other classes Specifies actual base type of the object 11

12 getclass() Example public class Weapon {! public void fire() {! public void reload() { public class M16 extends Weapon {! public void noobtube() { public class M40A3 extends Weapon {! public void zoom() { public class getclasstest { public static void main(string[] args) { Weapon noobweapon = new M16(); Weapon sniperweapon = new M40A3(); if (noobweapon instanceof Weapon) { System.out.println("noob IS-A Weapon"); if (sniperweapon instanceof Weapon) { System.out.println("sniper IS-A Weapon"); System.out.println(noobWeapon.getClass()); System.out.println(sniperWeapon.getClass()); if (noobweapon.getclass().equals(sniperweapon.getclass())) { System.out.println("Same Class"); 12

13 TO THE CODE! 13