Methods & Classes COMP163

Transcription

1 Methods & Classes COMP163

2 People think that computer science is the art of geniuses, but the actual reality is the opposite, just many people doing things that build on each other, like a wall of mini stones. Donald Knuth

3 Convocation The Fall Convocation will be on Thursday, November 1, at 10:00am in Harrison Auditorium All classes are cancelled on November 1 from 10:00 to noon Students in section 5, the 10:00 lab section, will not have lab this week Section 5 students do not have to do the lab assignment The other sections WILL have lab as scheduled

4 ZyBooks Read chapter 10 except section 10.9 of the ZyBooks online textbook and answer all of the participation questions Due by midnight on Sunday, November 4, 2018

5 Static Variables Class variables can be specified as static public class electric { static int cow; // static class variable int goat; // non- static class variable There is only one copy of a static variable shared by all objects of that class There is a separate copy of non-static variables for each object

6 Multiple Objects public class Electric { static int cow = 1; int goat = 2; Electric possum = new Electric(); Electric rat = new Electric(); Electric mouse = new Electric(); There are three Electric objects Each object has its own copy of goat There is only one cow variable

7 Static and Instance Variables There is only one cat variable, but there is a dog for every object Electric class cow possum rat mouse Electric object goat 1 Electric object goat Electric object goat 2 2 2

8 Using Instance Variables public class Electric { static int cow = 1; int goat = 2; public void cow2() { cow = cow + 2; public void goat3() { goat = goat + 3; public void prtcow() { System.out.println(cow); public void prtgoat() { System.out.println(goat); Electric volt = new Electric(); Electric watt = new Electric(); volt.cow2(); watt.goat3(); displays volt.prtcow(); 3 volt.prtgoat(); 2 watt.prtcow(); 3 watt.prtgoat(); 5

9 public class SandD { static int snake = 1; int dog = 5; void plusboth() { snake++; dog++; System.out.println(snake+" "+dog); static public void main(string[] x) { SandD goat = new SandD(); SandD pig = new SandD(); goat.plusboth(); pig.plusboth(); What is displayed? A B C D

10 Data in Every Object When you create an object of a class, the object contains its own copy of each non-static variable Static variables are not separate for each object

11 Making Multiple Objects public class Wombat { public int koala; private double roo; public Wombat( int cat, double dog) { koala = cat; roo = dog; goat cow // in another class Wombat goat = new Wombat( 3, 1.5 ); Wombat cow = new Wombat( 2, 4.7 ); koala 3 roo 1.5 koala 2 roo 4.7

12 final variables A variable declared as final cannot be changed If you want to define a constant value, it should be declared final final int MAXTHINGS = 10; Variables that are final must be initialized with a value It is good programming practice to put constants at the beginning of the program instead of using a number throughout the code

13 final is final final variables must be initialized when they are declared You will get a compiler error if you try to change a final variable final double dragon = 47.0; dragon = -3.5; // this will get an error

14 static Constants Typically a program constant would apply to all objects in a class Therefore it is useful to make constants static so there is only one copy public class Both { private static final int MAXSIZE = 12; Variable attributes can be in any order final static private int MAXSIZE = 12;

15 Review of Variable Attributes public variable can be used anywhere private variable can only be used in the same class static variable belongs to the class not objects. Only one copy final variable cannot be changed

16 Attributes on Instance Variables public class Modified { public static int dog; private final double cat; public void doit( int bull) { public int cow; // not allowed final int GERBIL = 7; dog = (double)bull + GERBIL;

17 What is displayed? Wombat cat = new Wombat( 3, 4.0 ); Wombat dog = new Wombat( 5, 7.0 ); dog.koala = cat.koala; cat.koala = 2; System.out.println(cat.koala+" "+dog.koala); A. 2 3 B. 3 5 C. 3 2 D. 2 5 E. none of the above

18 Only One Copy of Static Variables public class Egg { static int yolk; int whites; public Egg(int frog, int toad) { yolk = frog; whites = toad; // In another class Egg bird = new Egg( 2, 4 ); Egg lizard = new Egg( 3, 5 ); frog toad Egg yolk bird

19 Only One Copy of Static Variables public class Egg { static int yolk; int whites; public Egg(int frog, int toad) { yolk = frog; whites = toad; // In another class Egg bird = new Egg( 2, 4 ); Egg lizard = new Egg( 3, 5 ); bird.whites Egg.yolk 2 frog toad 2 4

20 Only One Copy of Static Variables public class Egg { static int yolk; int whites; public Egg(int frog, int toad) { yolk = frog; whites = toad; // In another class Egg bird = new Egg( 2, 4 ); Egg lizard = new Egg( 3, 5 ); bird.whites 4 Egg.yolk 2 frog toad 2 4

21 Only One Copy of Static Variables public class Egg { static int yolk; int whites; public Egg(int frog, int toad) { yolk = frog; whites = toad; // In another class Egg bird = new Egg( 2, 4 ); Egg lizard = new Egg( 3, 5 ); bird.whites 4 Egg.yolk 2 lizard

22 Only One Copy of Static Variables public class Egg { static int yolk; int whites; public Egg(int frog, int toad) { yolk = frog; whites = toad; // In another class Egg bird = new Egg( 2, 4 ); Egg lizard = new Egg( 3, 5 ); bird.whites 4 Egg.yolk 3 frog lizard.whites toad 3 5

23 Only One Copy of Static Variables public class Egg { static int yolk; int whites; public Egg(int frog, int toad) { yolk = frog; whites = toad; // In another class Egg bird = new Egg( 2, 4 ); Egg lizard = new Egg( 3, 5 ); bird.whites 4 Egg.yolk 3 frog lizard.whites 5 toad 3 5

24 Static and Non-static A non-static variable can only be accessed by non-static methods Non-static variables belong to the object A static variable can be accessed by any method Static variables belong to the class

25 Static and Non-static Example public class Program { static int statdata = 3; int dydata = 1; void inc() { dydata++; statdata++; // non-static method public static void main(string[] x) { Program thing = new Program(); Program other = new Program(); thing.inc(); statdata = 2; dydata = 5; thing.dydata = 7; // not Allowed // this is allowed

26 Static or Non-static? If a method uses class instance variables, it cannot be static If a method does not use any class instance variables, it can be static

27 Hashtable This week's programming assignment requires you to use a java.util.hashtable object A hash table is a data structure that stores information with a keyword You can later retrieve the information by using that keyword Hash tables make it very easy to save data and find it later

28 Creating a HashTable java.util.hashtable<keyword type, data type> dog = new java.util.hashtable<>(maxsize); java.util.hashtable<string, WordValue> cat = new java.util.hashtable<>(22000); java.util.hashtable<string, String> goat = new java.util.hashtable<>();

29 Using a Hashtable The Hashtable class has two useful methods mytable.put( keyword, data) Adds the data to the table under the specified keyword mytable.get( keyword ) Returns the data previously added under that keyword or null if there is no such data goat.put("anteater", "aardvark"); String frog = goat.get("anteater");

30 Instance and Local Example public class PermTemp { static int aardvark = 1; 1 public static void main(string[] uu) { addto( 2 ); addto( 1 ); static void addto( int ant ) { int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark); aardvark termite ant Output

31 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

32 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

33 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

34 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { 9 3 int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

35 Instance and Local Example public class PermTemp { static int aardvark = 1; 3 public static void main(string[] uu) { addto( 2 ); addto( 1 ); static void addto( int ant ) { int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark); aardvark termite ant Output 9 3

36 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { 9 3 int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

37 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { 9 3 int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

38 Instance and Local Example public class PermTemp { aardvark termite ant static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); Output static void addto( int ant ) { 9 3 int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark);

39 Instance and Local Example public class PermTemp { static int aardvark = 1; public static void main(string[] uu) { addto( 2 ); addto( 1 ); static void addto( int ant ) { int termite = 7; aardvark = aardvark + ant; termite = termite + ant; System.out.println( termite+" "+ aardvark); aardvark termite ant Output

40 Static or Not If all objects are going to share the variable, make it static If each object should have its own value, do not make it static If a method uses instance variables, it cannot be static If a method does not use any instance variables, it can be static

41 What is displayed? public class Teach { public int cat = 2; public Teach(int dog) { cat = dog; public void prtcat() { System.out.print(cat); public class MyProg { public static void main( ) { Teach mine = new Teach(5); Teach yours = new Teach(3); mine.prtcat(); yours.prtcat(); A. 2 2 B. 3 3 C. 5 3 D. 5 5

42 Queuing Theory Queuing theory is the mathematics of waiting lines It is extremely useful in predicting and evaluating system performance

43 Single or Multiple Queues M/M/N Bank Teller model λ S S S M/M/1 Grocery Store model λ /2 λ /2 S S

44 Queuing Theory The probability that all n servers are busy in an M/M/N system (bank teller model) is C where N sk K C 1 1 N i i s N i i s i i K 0! ) ( 1 0! ) (

45 Example Method Write a method that returns the value C given the three input parameters lambda(λ), s and n The method will have the header double proballbusy( double lambda, double s, int n)

46 Solution Outline In the equation for K, the denominator is a sum that is the same as the numerator except that it is one term longer If we have a method to calculate the sum, the rest of the method is simple

47 proballbusy method double proballbusy( double lambda, double s, int n) { double k = ksum(lambda, s, n-1) / ksum(lambda, s, n); return (1.0 - k) / (1.0 lambda*s*k/n); 1 K C sk 1 N K N 1 ( s) i 0 i! N i ( s) i 0 i! i

48 ksum method The ksum method calculates this sum given the parameters lambda, s and n N i ( 0 i! s i ) We will need a method to calculate factorials

49 factorial method The factorial function in mathematics, n!, is 1 * 2 * 3 * 4 * * n For the special case of zero, 0! = 1

50 The header for factorial should be A. int factorial( int n ) B. void factorial( int n, int fac ) C. static factorial( int n ) D. none of the above

51 Write the factorial method With your team, write a method with one integer parameter, n, that returns n! Don t forget the special case for zero You might not have to do anything special

52 Possible Solution int factorial( int n ) { int fac = 1; for (int i =1; i <= n; i++) { fac = fac * i; return fac;

53 Write the ksum method Write a method to calculate this sum given the parameters lambda, s and n N i ( 0 i! s i ) double ksum( double lambda, double s, int n )

54 Possible Solution double ksum( double lambda, double s, int n ) { double sum = 0.0; for (int i = 0; i <= n; i++) { sum += Math.pow( lambda*s, (double)i ) / factorial( i ); return sum;

55 It looks Greek to me double ksum( double λ, double s, int n ) { double sum = 0.0; for (int i = 0; i <= n; i++) { sum += Math.pow( λ*s, (double)i ) / factorial( i ); return sum;

56 Small Steps The value of using methods is that you can break a larger problem into small steps The proballbusy method called the ksum method twice ksum called factorial Note that we wrote the program using methods we had not yet written, but knew we could write

57 Recursion (N-1)! is 1 * 2 * 3 * 4 * * N-1 N! is 1 * 2 * 3 * 4 * * N-1 * N or (N-1)! * N If you have a factorial method that computes (N-1)!, you can calculate N! by multiplying the result of the method by N You need to be mindful that N-1 might drop to zero

58 Recursive Solution int factorial( int n ) { if (n <= 1) return 1; return n * factorial( n-1 );

59 Methods and Object Data Methods belong to a class or object Methods can use all instance variables, parameters and local variables In a typical program, you keep the data in instance variables which is accessed and modified by methods

60 Convocation The Fall Convocation will be on Thursday, November 1, at 10:00am in Harrison Auditorium All classes are cancelled on November 1 from 10:00 to noon Students in section 5, the 10:00 lab section, will not have lab this week Section 5 students do not have to do the lab assignment The other sections WILL have lab as scheduled

61 ZyBooks Read chapter 10 except section 10.9 of the ZyBooks online textbook and answer all of the participation questions Due by midnight on Sunday, November 4, 2018