CS#61B#Data#Structures#and#Programming#Methodology##July#2,#2008#David#Sun#Announcements#• Project#1#spec#and#code#is#available#on#the#course#website.#Due#July#15th.#Start#early!#• Midterm#I#is#next#Wed#in#class#from#11:00#–#1:00p.m.#Open#book:#lecture#notes,#lab#notes,#but#no#laptop!#Today#• Constants#• Preventing#Inheritance#• Abstract#Classes#• Interfaces#Static#Fields#and#Methods#class Parent { int x = 0; static int y = 1; static void f() { System.out.printf "Ahem!%n"); } static int f(int x) { return x+1; } } class Child extends Parent { String x = "no"; static String y = "way"; static void f() { System.out.printf ("I wanna!%n"); } } Child dave = new Child(); Parent pDave = dave; dave.x => no pDave.x => 0 dave.y => way pDave.y => 1 dave.f() => ? pDave.f() = ? dave.f(1) => ? pDave.f(1) => 2 Fields#hide#inherited#fields#of#same#name;#static#methods#hide#methods#of#the#same#signature.#Constants • Adding final#in#front#of#a#variable#means#that#the#value#of#the#variable#can#never#be#changed.#– You)must)initialize)the)value)of)the)final)variable)when)it)is)declared.)– Use)final variables)to)create)constants.)class Math { public static final double PI = 3.141592653…; } public class System { public static final PrintStream out = … ; }Constants#(cont.)#• Numerical#values#that#are#used#repeatedly#should#be#turned#into#a#final#constant.#if (month == 3) { . . .} //BAD public static final int MARCH = 3; if (month == MARCH) { . . . } • Not#all#constants#need#to#be#static. – For)an)array)x, x.length is)a final field.)Preventing#Inheritance#• A#class#declared#to#be#final)cannot#be#extended#by#others#final class Executive extends Manager{ } • A#method#declared#final#cannot#be#overriden#by#subclasses#– All)methods)in)a)final)class)are)automatically)final class Employee { … public final String getName() { return name; } … })Abstract#Classes#• As)we)move)up)the)inheritance)hierarchy,)classes)become)more)general)or)more)abstract.(• At)the)some)point,)the)ancestor)class)becomes)so)general)that)it)becomes)a)basis)for)other)classes)rather)than)a)class)with)specific)instances)that)you)want)to)create.)• Such)ancestor)classes)are)declared)to)be)abstract.)Their)sole)purpose)is)to)be)extended.)Example#Student)Employee)Human)• ###Some#attributes#and#methods#make#sense#for#every#human,#and#these#can#be#factored#out#into#the#Human#class.#public class Human { . . . private int age; private String name; public int getAge() { return age}; public String getName() { return name}; . . . }Example#(cont.)#public class Human { . . . public String introduce() { . . .} //how should we code this? } Student david = new Student( . . . ); david.introduce(); //”I’m David and I major in Computer Science” Employee wendy = new Employee( . . . ); wendy.introduce(); //”I’m Wendy I work for Google” • The#Human#class#knows#nothing#about#the#person#except#the#name#and#age.##• We#can#try:#public String introduce() { return “”; } • A#better#way:#public abstract String introduce(); //no implementation required)Abstract#Class#public abstract class Human { private String name; public Human(String n); public String getName() { return name; } public abstract String introduce(); } • Any#class#with#one#or#more#abstract#methods#must#be#declared#as#abstract. • abstract classes#can#have#concrete#data#and#methods.#• abstract methods#act#as#placeholders#for#methods#that#are#implemented#in#subclasses.public abstract class Human { protected String name; . . . public abstract String introduce(); } public class Student extends Human{ private String major; public student(String name, String major) {. . .} public String introduce() { return “I’m “ + name + “and I major in” + major; } } public class Employee extends Human{ private String company; public student(String name, String company) {. . .} public String introduce() { return “I’m “ + name + “and I work for ” + company; } }Rules#for#abstract#Classes#• The#concrete#classes#Student and Employee must#implement#the#abstract#method)#introduce(). – The)compiler)will)generate)an)error)if)you)don’t.)• If#you#extend#an#abstract#class#without#implementing#an#abstract#method,#the#subclass#must#be#declared#as#abstract.#• You#can#create#a#variable#of#an#abstract#class:# Human student; //OK #• You#cannot#create#an#object#of#an#abstract#class:# //Compile time ERROR!## Human student = new Human(“David”); //OK! Human student = new Student(“David”); student.introduce();Human[] human = new Human[2]; human[0] = new Employee( ... ); human[1] = new Student( ... ); for (int i = 0; i < human.length; i++) { human[i].introduce(); } • The)call)human[i].introduce(); is)well)defined)because human[i] never)refers)to)an)object)of)the)abstract)human class,)it)must)refer)to)a)concrete)subclass.))Example##What#are#Interfaces?#• Describing#what#classes#should#do,#without#specifying#how#they#would#do#it.##• Think#of#it#as#a#contract#for#a#set#of#classes.##“If)your)class)conforms)to)the)requirements)set)in)this)contract)(or)this)interface),)then)I’ll)perform)these)services)for)the)objects)of)your)class”)Example#• The#sort method#of#the#Array class#promises#to#sort#an#array#of#objects,#but#under#one#condition:##the#objects#in#the#array#must#implement#the#Comparable#interface:#public interface Comparable { int compareTo(Object other); } Like(an(abstract(method,(no(implementation(is(provided(Example#public interface Comparable { int compareTo(Object other); } public class Employee implements Comparable { public int compareTo(Object other) { ... } } The(Employee(class(must(implement(the(compareTo(method.(i.e.,(define(a(method(named(compareTo(that(takes(an(Object(and(returns(an(int.((compareTo)• When#we#call#x.compareTo(y),#the#method#returns#an#indication#whether#x#or#y#is#larger:##– return)a)negative)number)if)y)is)larger)– a)positive)number)if)x)is)larger,))– zero)otherwise.))• Suppose#we#want#to#sort#by#salary:#public class Employee implements Comparable { public int compareTo(Object other) { Employee otherEmployee = (Employee) other; if (salary < otherEmployee.salary) return -1; if (salary > otherEmployee.salary) return 1; return 0; }
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