Intro to CS – Honors I Classes and Methods

1. Intro to CS – Honors IClasses and Methods Georgios Portokalidis gportoka@stevens.edu

2. Object-Oriented Programming • Objects • They are all around us • Cars, people, trees… • Each object can perform certain actions • Independently or interacting with other objects • These actions are called methods • Each objects has characteristics or attributes • Example: a car has static attributes like its color and dynamic attributes like its speed • A class defines the type or kind of object • Example: all car objects could belong to the automobile class • A blueprint • Objects belonging to a particular class have similar attributes, but are not the same!

3. A Class Outline • The Unified Modeling Language can be used describe classes Automobile • fuel: double • speed: double • license: String • accelerate(double pedalPressure) • decelerate(double pedalPressure)

4. A Dog Class • Use UML to describe such a class • What attributes would it have? • What methods?

5. public class Dog • { • public String name; • public String breed; • public int age; • public void writeOutput() • { • System.out.println("Name: " + name); • System.out.println("Breed: " + breed); • System.out.println("Age in calendar years: " + age); • System.out.println("Age in human years: " + getAgeInHumanYears()); • System.out.println(); • } • public intgetAgeInHumanYears() • { • inthumanAge = 0; • if (age <= 2) • { • humanAge= age * 11; • } • else • { • humanAge= 22 + ((age-2) * 5); • } • return humanAge; • } • } Instance variables Variables in the body of the class Can be accessed by all class methods SYNTAX public class Class_Name { Instance_Variable_Declarations ... Method_Definitions }

6. A class does not need to have instance variables • public class DogDemo • { • public static void main(String[] args) • { • Dog balto = new Dog(); • balto.name = "Balto"; • balto.age = 8; • balto.breed = "Siberian Husky"; • balto.writeOutput(); • Dog scooby = new Dog(); • scooby.name = "Scooby"; • scooby.age = 42; • scooby.breed = "Great Dane"; • System.out.println(scooby.name + " is a “ + scooby.breed+ "."); • System.out.print("He is " +scooby.age+ " years old, or "); • inthumanYears = scooby.getAgeInHumanYears(); • System.out.println(humanYears+ " in human years."); • } • } Create a new Dog object using Java’s new operator new allocates a new set of instance variables for the object Public instance variables can be set/read from other classes

7. public class Dog • { • public String name; • public String breed; • public int age; • public void writeOutput() • { • System.out.println("Name: " + name); • System.out.println("Breed: " + breed); • System.out.println("Age in calendar years: " + age); • System.out.println("Age in human years: " + getAgeInHumanYears()); • System.out.println(); • } • public intgetAgeInHumanYears() • { • inthumanAge = 0; • if (age <= 2) • { • humanAge= age * 11; • } • else • { • humanAge= 22 + ((age-2) * 5); • } • return humanAge; • } • } Method not returning any value Method returning an int

8. public class DogDemo • { • public static void main(String[] args) • { • Dog balto = new Dog(); • balto.name = "Balto"; • balto.age = 8; • balto.breed = "Siberian Husky"; • balto.writeOutput(); • Dog scooby = new Dog(); • scooby.name = "Scooby"; • scooby.age = 42; • scooby.breed = "Great Dane"; • System.out.println(scooby.name + " is a “ + scooby.breed+ "."); • System.out.print("He is " +scooby.age+ " years old, or "); • inthumanYears = scooby.getAgeInHumanYears(); • System.out.println(humanYears+ " in human years."); • } • } Defining methods SYNTAX: public Return_TypeMethod_Name(Parameters) { Statements } Invoking a method Invoking a method and saving its return value

9. Defining Methods Method heading • public void writeOutput() • { • System.out.println("Name: " + name); • System.out.println("Breed: " + breed); • System.out.println("Age in calendar years: " + age); • System.out.println("Age in human years: " + getAgeInHumanYears()); • System.out.println(); • } Method body

10. Defining Methods Method arguments Method heading • public void writeOutput(int unusedArgument1, String unusedArgument2) • { • double unusedLocalVariable; • System.out.println("Name: " + name); • System.out.println("Breed: " + breed); • System.out.println("Age in calendar years: " + age); • System.out.println("Age in human years: " + getAgeInHumanYears()); • System.out.println(); • } Local variables Method body

11. Understanding A Method Call Dog balto = new Dog(); balto.name = "Balto"; balto.age= 8; balto.breed= "Siberian Husky"; balto.writeOutput(); Dog balto = new Dog(); balto.name = "Balto"; balto.age= 8; balto.breed= "Siberian Husky"; System.out.println("Name: " + balto.name); System.out.println("Breed: " + balto.breed); System.out.println("Age in calendar years: " + balto.age); System.out.println("Age in human years: " + balto. getAgeInHumanYears()); System.out.println();

12. Defining Methods that Return Values Return statement SYNTAX: return Expression; Method return type • public intgetAgeInHumanYears() • { • inthumanAge = 0; • if (age <= 2) • { • humanAge = age * 11; • } • else • { • humanAge = 22 + ((age-2) * 5); • } • return humanAge; • } TIP Make your code more readable by using only one return statement Return value

13. Using return in void Methods • You can use it without a return value to alter control flow • public void showLandPortion() • { • if (population == 0) • { • System.out.println("Population is zero."); • return; // Ends here to avoid division by zero . • } • double fraction = 6.0 / population; • System.out.println("If the population were spread "); • System.out.println("over 6 continents, each " + "individual"); • System.out.println("would have a fraction of its "); • System.out.println("continent equal to " + fraction); • } TIP Generally should be avoided. Code is cleared when using if .. else

14. Using the this Keyword • Referring to an object’s public instance variables • Object_name . Variable_name • The this keyword can be used to refer to an object’s own instance variables from within its methods • Not necessary in this case, but can help improve readability Dog balto = new Dog(); balto.name = "Balto"; balto.age= 8; balto.breed= "Siberian Husky"; • public void writeOutput() • { • System.out.println("Name = " + this.name); • System.out.println("Population = " + this.population); • System.out.println("Growth rate = " + this.growthRate + "%"); • }

15. Local Variables • Local variables are only valid within the method they are declared • Local variables can also be declared within a block or compound statement (between { }) • Only valid within that block • We have seen local variables and instance variables. Are there any other kind of variables? • public void showNewBalance() • { • { • double newAmount; • newAmount = amount + (rate / 100.0) * amount; • } • System.out.println("With interest added, the new amount is $" + newAmount); • }

16. Method Parameters • Parameters can be used to write generic/customizable methods • Within the method it can be used as a local variable • Supplying arguments when calling methods can be done • Using constants myNumber.exponentiation(2); • Using other variables • int exponent = 4; • myNumber.exponentiation(exponent); • Arguments are automatically cast to the right type, if the parameter is larger than the supplied value • byte→short→int→long→float→double • class Number • { • public long base; • public long exponentiation(int power) • { • long result = 1; • for (inti = 0; i < power; i++) • { • result *= base; • } • return result; • } • }

17. Primitive Types are Passed Using call-by-value • Defining method parameters of primitive types is equivalent to declaring a local variable int exponent = 4; myNumber.exponentation(exponent); System.out.println(exponent); • What will this print? • class Number • { • public long base; • public long exponentiation(int power) • { • long result = 1; • while (power-- > 0) • { • result *= base; • } • return result; • } • }