Today in COMP 110

1. Today in COMP 110 • MouseBrain.java sample solutions • Regrade for Program 3 due Wednesday • Max grade: 75 from here on • Constructors (Review) • Static Variables & Methods • Today is Undergraduate Drop Date

2. Questions? • Objects / References • Exam Material • Constructors

3. Program 3 • Example solutions • left/right loop then up/down loop • Modification: do while for each • all directions each iteration • step forward, if scent is weaker step back • lawnmower • (not a good solution) • left/right find max scent then up/down max scent

4. 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

5. 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

6. 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

7. 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; } }

8. 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

9. 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

10. Constructors w/ Parameters • Like methods, constructors can have parameters public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; }

11. 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

12. Multiple Constructors • You can define multiple constructors • All have the same name, but different parameters • Group their definitions together

13. 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

14. 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; } 14

15. Static Variables & Methods

16. 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; } }

17. The Keyword Static: Variables • 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; }

18. 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 } }

19. The Keyword Static • The keyword static can also be used with methods • The main method 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 }

20. 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

21. Example public class DimensionConverter { public static final int INCHES_PER_FOOT = 12; public static double convertFeetToInches(double feet) { return feet*INCHES_PER_FOOT; } public static double convertInchesToFeet(double inches) { return inches / INCHES_PER_FOOT; } }

22. 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; No Object is Specified! Class Name Static Variable Name

23. 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); No Object is Specified! Class Name Static Method Name

24. 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

25. Restrictions on Static Methods

26. Example public classCircle { 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

27. Example public classCircle { public static final double PI = 3.14159; private voidprintArea(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

28. Example public classCircle { public static final double PI = 3.14159; private voidprintArea() { 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

29. 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 F’s, compute the percentage of each type of grade • e.g. 15% A’s, 30% B’s, 30% C’s, 15% D’s, 10% F’s • Include accessors and mutators for each type of grade • Draw a bar graph of the grade distribution

30. Programming Demo • Output • Each * == 2 percent 0 10 20 30 40 50 60 70 80 90 100 | | | | | | | | | | | ************************************************** **** A ************** B *********C *****D ***F

31. Wednesday • Math class • Wrapper class • Writing & Testing Methods