1. COMP 110: Introduction to Programming Tyler Johnson Mar 16, 2009 MWF 11:00AM-12:15PM Sitterson 014

2. COMP 110: Spring 20092 Questions?

3. COMP 110: Spring 20093 Program 3 Example solutions

4. COMP 110: Spring 20094 Today in COMP 110 Constructors Static Variables & Methods

5. COMP 110: Spring 20095 Constructors Section 6.1 in text

6. COMP 110: Spring 20096 Creating Objects Student jack = new Student(); Why does this look like a method call? Because it is This is a call to a special method called a constructor

7. COMP 110: Spring 20097 Constructors A constructor is a special method that is called when an object is created using new The purpose of a constructor is to perform initializing actions e.g. initialize the instance variables of an object

8. COMP 110: Spring 20098 Constructors The purpose of a constructor is similar to that of a mutator (setter) Used to set the value of variable(s) However, constructors create an object in addition to initializing it

9. COMP 110: Spring 20099 Example: Pet class public class Pet { private String name; private int age; private double weight; public Pet() { name = No name yet.; age = 0; weight = 0; } public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; }

10. COMP 110: Spring 200910 Constructors A constructor must have the same name as its class The constructor for the class Pet is Pet() The constructor for the class Student is Student() Constructors do NOT specify a return type Not even void

11. COMP 110: Spring 200911 Constructors The classes you have used up to this point use a constructor created automatically by Java Gives default values to instance variables May not be what you want You can specify how instance variables should be initialized by creating your own constructors

12. COMP 110: Spring 200912 Constructors w/ Parameters Like methods, constructors can have parameters public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; }

13. COMP 110: Spring 200913 Default Constructor A constructor that takes no arguments is called a default constructor public Pet() { name = No name yet.; age = 0; weight = 0; } Java automatically defines a default constructor if you do not define any constructors

14. COMP 110: Spring 200914 Multiple Constructors You can define multiple constructors All have the same name, but different parameters Group their definitions together

15. COMP 110: Spring 200915 Constructors You cannot call a constructor on an existing object Pet myPet = new Pet(); myPet.Pet("Roberto", 1, 150.0); //error Must use mutators on objects that have already been created myPet.setPet("Roberto", 1, 150.0); //ok

16. COMP 110: Spring 200916 Calling Methods within Constructors Just like calling methods within methods /*constructor*/ public Pet(String initName, int initAge, double initWeight) { setPet(initName, initAge, initWeight); //have the mutator perform the set } /*mutator*/ public void setPet(String newName, int newAge, double newWeight) { name = newName; age = newAge; weight = newWeight; } 16

17. COMP 110: Spring 200917 Static Variables & Methods Section 6.2 in text

18. COMP 110: Spring 200918 The Keyword Static The keyword static is a specifier that can be applied to instance variables and methods in Java You are already familiar with some other specifiers such as public, private, final public class Student { private double gpa; public double getGPA() { return gpa; }

19. COMP 110: Spring 200919 The Keyword Static The keyword static is used to indicate that only ONE copy of the instance variable or method should exist for the entire class public class UnitsAndMeasures { //static, all objects share the SAME copy of this variable public static final int FEET_PER_YARD = 3; //NOT static, all objects have their OWN copy of this variable private int feet; }

20. COMP 110: Spring 200920 Example: Static Instance Variables A class that counts the number of method calls to ALL of its objects public class StaticExample { //static, all objects share the SAME copy of this variable private static numberOfCalls = 0; public void method() { numberOfCalls++; } public class StaticExampleTester { public static void main(String[] args) { StaticExample se = new StaticExample(); StaticExample se2 = new StaticExample(); se.method(); //changes numberOfCalls to 1 se2.method(); //changes numberOfCalls to 2 }

21. COMP 110: Spring 200921 The Keyword Static The keyword static can also be used with methods The main method public static void main() { StaticExample se = new StaticExample(); StaticExample se2 = new StaticExample(); se.method(); //changes numberOfCalls to 1 se2.method(); //changes numberOfCalls to 2 }

22. COMP 110: Spring 200922 Using the Keyword Static When should you use the keyword static with a method? When a method does not access instance variables public int pow(int x, int y) { int result = 1; for(int i = 0; i < y; i++) { result *= x; } return result; } Does this method access any instance variables? No. Should be declared static

23. COMP 110: Spring 200923 Example public class DimensionConverter { public static final int INCHES_PER_FOOT = 12; public static final double convertFeetToInches(double feet) { return feet*INCHES_PER_FOOT; } public static double convertInchesToFeet(double inches) { return inches / INCHES_PER_FOOT; }

24. COMP 110: Spring 200924 Accessing Static Variables From outside the class, static variables that are declared public can be accessed using the name of the class int inchesPerFoot = DimensionConverter.INCHES_PER_FOOT; Class Name Static Variable Name No Object is Specified!

25. COMP 110: Spring 200925 Calling Static Methods From outside the class, static methods that are declared public can be accessed using the name of the class int inches = DimensionConverter.convertFeetToInches(12); Class Name Static Variable Name No Object is Specified!

26. COMP 110: Spring 200926 Restrictions on Static Methods Static methods CANNOT Access non-static instance variables Call non-static methods Static methods CAN Be called by any method, static or non-static

27. COMP 110: Spring 200927 Restrictions on Static Methods Access Non-Static Variables Access Static Variables Call Non-Static Methods Call Static Methods Static methods XX Non-Static Methods

28. COMP 110: Spring 200928 Example public class Circle { public static final double PI = 3.14159; private double area; public static double area(double radius) { area = PI * (radius * radius); return area; } Will this code compile?No. Cannot access non-static instance variables in static methods

29. COMP 110: Spring 200929 Example public class Circle { public static final double PI = 3.14159; private void printArea(double area) { System.out.println(area); } public static void area(double radius) { printArea(PI * (radius * radius)); } Will this code compile?No. Cannot call non-static methods inside static methods

30. COMP 110: Spring 200930 Example public class Circle { public static final double PI = 3.14159; private void printArea() { System.out.println(area(3.0)); } public static double area(double radius) { return PI * (radius * radius); } Will this code compile?Yes. CAN call static methods inside non-static methods

31. COMP 110: Spring 200931 Programming Demo Grade Distribution A class to display the distribution of letter grades in a class Given the number of A,B,C,D, and Fs, compute the percentage of each type of grade e.g. 15% As, 30% Bs, 30% Cs, 15% Ds, 10% Fs Include accessors and mutators for each type of grade Draw a bar graph of the grade distribution

32. COMP 110: Spring 200932 Programming Demo Output Each * == 2 percent 0 10 20 30 40 50 60 70 80 90 100 | | | | | | | | | | | ************************************************** **** A ************** B *********C *****D ***F

33. COMP 110: Spring 200933 Wednesday Math class Wrapper class Writing & Testing Methods