Week 11: Class Design

Transcription

1 Week 11: Class Design 1

2 Most classes are meant to be used more than once This means that you have to think about what will be helpful for future programmers There are a number of trade-offs to consider More methods means more flexibility Too many methods can be confusing 2

3 Document thoroughly Write documentation clearly in a way that describes what every method does Input Output Expectations The user probably will not be able to see your code String API example: /String.html 3

4 Try to provide only the methods that are absolutely essential Example: The Java Math library provides sin() and cos() methods but no sec() or cosec() methods String has lots of methods, and they are great Usually, less is more 4

5 How do you make sure that your class does everything it will need to? If you design a class to keep records about human beings, should you check to make sure that the age is in a certain range? What range? Make sure it s less than 130 years? Make sure it s not negative? 5

6 Y2K Bug 2 digits for the date was not enough, now 4 It s all just going to get messed up in Y10K Y2038 Bug Unix machines often use a 32-bit integer to represent seconds since January 1, 1970 Zip Codes (5+4) Vehicle Identification Numbers IP Addresses 6

7 IPv4 uses n1:n2:n3:n4 where each n is (like ) That allows 2 32 IP addresses (8.6 billion) There are approximately 6.8 billion people on earth IPv6 uses n1:n2:n3:n4:n5:n6:n7:n8 8 groups of 4 hexadecimal digits each f7c3:0db8:85a3:0000:0000:8a2e:0370:7334 How many different addresses is this? is enough to have 500 trillion addresses for every cell of every person s body on Earth Will that be enough?! 7

8 Members are the data inside objects But, sometimes, wouldn t it be nice if some data were linked to the class as a whole, rather than just one object? What if you wanted to keep track of the total number of a particular kind of object you create? 8

9 Static members are stored with the class, not with any particular object public class Item private static int count = 0; // only one copy (in class) private String name; public Item( String s ) name = s; count++; // one copy per object // updates class counter public String getname() return name; public static int gettotalitems() return count; 9

10 Static members are also called class variables Static members can be accessed by either static methods or regular Static members can be either public or private 10

11 Sometimes a value will not change after an object has been created: Example: A ball has a single color after it is created You can enforce the fact that the value will not change with the final keyword A member declared final can only be assigned a value once Afterwards, it can never change 11

12 Using final, we can fix the value of a member It will only be set once, usually when the constructor is called It is customary to use all caps for constant names public class Human private String name; private final String BIRTHPLACE; private int age; // never changes public Human( String s, String whereborn ) name = s; BIRTHPLACE = whereborn; age = 0; public void birthplacechange(string locn) BIRTHPLACE = locn; // compile-time error! 12

13 It is possible to set a static member to be constant using the final keyword Usually, this is used for global constants that will never ever change Making these values public is reasonable Since they never change, we never have to worry about a user corrupting the data inside of the object 13

14 The number of sides of a pentagon is always 5 Other code can access this information by using the value Pentagon.SIDES Exactly like Math.PI or Math.E public class Pentagon private double x; private double y; public static final int SIDES = 5; // never changes public Pentagon( double newx, double newy ) x = newx; y = newy; public double getx() return x; public double gety() return y; 14

15 Always think about future uses of your objects and classes Hide as much data as possible (using private) Make as much data as possible constant (using final) Try to use the right number of methods Short, powerful, intuitive methods Don t use redundant methods that do similar things Check the input of all methods for errors 15

16 The idea of inheritance is to take one class and generate a child class This child class has everything that the parent class has (members and methods) But, you can also add more functionality to the child The child can be considered to be a specialized version of the parent 16

17 The key idea behind inheritance is safe code reuse You can use old code that was designed to, say, work with Vehicles, and apply that code Cars All that you have to do is make sure that Car is a subclass (or child class) of Vehicle 17

18 All this is well and good, but how do you actually create a subclass? Let s start by writing the Vehicle class public class Vehicle public void travel(string destination) System.out.println( Traveling to + destination); 18

19 We use the extends keyword to create a subclass from a superclass public class Car extends Vehicle private String model; public Car(String s) model = s; public String getmodel() return model; public void startengine() System.out.println( Vrooooom! ); A Car can do everything that a Vehicle can, plus more 19

20 There is a part of the Car class that knows all the Vehicle members and methods Car car = new Car( Camry ); System.out.println( car.getmodel() ); //prints Camry car.startengine(); //prints Vrooooom! car.travel( New York City ); //prints Traveling to New York City 20

21 Each Car object actually has a Vehicle object buried inside of it If code tries to call a method that isn t found in the Car class, it will look deeper and see if it is in the Vehicle class Car Vehicle travel() model getmodel() startengine() 21

22 Sometimes you want to do more than add You want to change a method to do something different You can write a method in a child class that has the same signature as a method in a parent class The child version of the method will always get called This is called overriding a method 22

23 We make the Boat class override the travel() method public class Boat extends Vehicle public void travel(string destination) System.out.println( Traveling across + the water to + destination); In use: Boat b = new Boat(); b.travel( Portugal ); // prints Traveling across the water to Portugal // even though v is a Vehicle reference 23

24 We can define the Mammal class as follows: public class Mammal public void makenoise() System.out.println( Grunt! ); 24

25 From there, we can define the Dog, Cat, and Human subclasses, overriding the makenoise() method appropriately public class Dog extends Mammal public void makenoise() System.out.println( Woof ); public class Cat extends Mammal public void makenoise() System.out.println( Meow ); public class Human extends Mammal public void makenoise() System.out.println( Hello ); 25

26 We made a Student class last week Each Student contained First Name: Last Name: GPA: ID: String String double int We could make a GraduateStudent class adding: Degree : Advisor: String String 26

27 public class Student private String firstname; private String lastname; private double gpa; private int id; public Student() // default constructor firstname = ; lastname = ; gpa = 0.0; id = 0; No matter how complex a program is, inside this method, only x, y, and z variables exist 27

28 public Student(String first, String last, double grades, int num) // constructor firstname = first; lastname = last; gpa = grades; id No = matter num; how complex a program is, inside this method, only x, y, and z variables exist public double getgpa() // accessor return gpa; 28

29 public int getid() // accessor return id; public String getname() // accessor No matter how complex a program is, inside return lastname +, + firstname; this method, only x, y, and z variables exist 29

30 public void setgpa(double grades) // mutator if (grades>=0.0 && grades<=4.0) gpa = grades; else System.out.println( Incorrect GPA ); No matter how complex a program is, inside this method, only x, y, and z variables exist public void setid(int num) // mutator if (num>=0 && num<=99999) id = num; else System.out.println( Incorrect ID ); 30

31 public class GraduateStudent extends Student private String degree; private String advisor; public GraduateStudent() // default No matter how complex a program is, inside super(); this method, only x, y, and z variables exist degree = ; advisor = ; 31

32 public GraduateStudent(String first, String last, double grades, int num, String deg, String adv) // constructor super (first, last, grades, num); degree = deg; advisor No matter = adv; how complex a program is, inside this method, only x, y, and z variables exist public String getdegree() // accessor return degree; 32

33 public String getadvisor() // accessor return advisor; public void setdegree(string deg) // mutator No matter how complex a program is, inside if (deg== PhD deg== MS ) this method, only x, y, and z variables exist degree = deg; else System.out.println( Incorrect Degree ); 33