1. Intro to CS – Honors I Class Types and Object References GEORGIOS PORTOKALIDIS GPORTOKA@STEVENS.EDU

2. A Name and A Value Remember the Java primitive types ◦byte, short, int, long, float, double, boolean, char And class types ◦Like the class Dog we defined in previous lectures Declaring a variable of any type is always the same ◦TYPE VariableName1, VariableName2; But not all names are the same! Back to basics: What is a variable, what is its purpose? Where are data stored? How are data referenced/accessed? Variables are used to store data Data 2 Data 1 Data 3 Data 2 Data 1 Data 3 Data are stored in memory A1 A2 A3 Each piece of data has an address [] accesses the contents of an address

3. Primitive Data Types VariableName1 VariableName2 VariableName3 The names of primitive variables directly access the contents stored in them Data 2 Data 1 Data 3 A1 A2 A3 Memory

4. Object Class Data Types They are declared the same, but … VariableName1 VariableName2 VariableName3 The names of class variables can refer to an object or not Data 2 Data 1 Data 3 A1 A2 A3 Memory Not an object A variable’s data refer can refer to an object This variables are references to objects

5. What Does This Mean For Me? When you declare a variable of a class type ◦Example: Dog scooby; When you create a new object of type Dog ◦Example: new Dog(); When an object is assigned to variable ◦Example: scooby = new Dog(); scooby A1 Memory Data 1 A reference pointing nowhere A new object is created Object

6. Species klingonSpecies, earthSpecies; klingonSpecies.setSpecies("Klingon ox", 10, 15); earthSpecies.setSpecies("Black rhino", 11, 2); earthSpecies = klingonSpecies; earthSpecies.setSpecies("Elephant", 100, 12); System.out.println("earthSpecies:"); earthSpecies.writeOutput(); System.out.println("klingonSpecies:"); klingonSpecies.writeOutput(); - name: String - population: int - growthRate: double Species + setSpecies(String newName, int newPopulation, double newGrowthRate): void + getName(): String + getPopulation(): int + getGrowthRate( ): growthRate + writeOutput(): void

7. References So both variables klingonSpecies and earthSpecies store references to Species objects What does the == then do on references? klingonSpeciesearthSpecies Species object So class type variables essentially contain memory addresses The address is always called a reference in Java SpeciesFourthTry klingonSpecies = new SpeciesFourthTry(); SpeciesFourthTry earthSpecies = new SpeciesFourthTry(); klingonSpecies.setSpecies("Klingon ox", 10, 15); earthSpecies.setSpecies("Klingon ox", 10, 15); if (klingonSpecies == earthSpecies) System.out.println("They are EQUAL."); else System.out.println("They are NOT equal.");

8. Actually Comparing Two Objects How do we compare strings? You can define an equals() method public boolean equals(Species otherObject) { return (this.name.equalsIgnoreCase(otherObject.name)) && (this.population == otherObject.population) && (this.growthRate == otherObject.growthRate); } if (klingonSpecies.equals(earthSpecies)) System.out.println("They are EQUAL."); else System.out.println("They are NOT equal."); Always name the method testing for equality equals

9. Other Methods Returning boolean Use names that make sense in English when used with if..else statements, while loops, etc. public Boolean isExtinct() { return population == 0; } if (s1.isExtinct()) { // Yes, it’s extinct }

10. Class Types are Passed Using call-by-reference Defining method parameters of primitive types is equivalent to declaring a local variable public boolean equals(Species otherObject) { otherObject = new Species(); return (this.name.equalsIgnoreCase(otherObject.name)) && (this.population == otherObject.population) && (this.growthRate == otherObject.growthRate); }

11. Class Types are Passed Using call-by-reference Let’s introduce a new Species class method public boolean eatSpecies(Species unluckySpecies) { unluckySpecies.population = 0; this.growthRate += 5; } Species tyrannosaurus = new Species(), elephant = new Species(); tyrannosaurus.setSpecies(“Dinosaur", 10, 15); elephant.setSpecies(“Mamal", 11, 2); tyrannosaurs.eatSpecies(elephant); elephant.writeOutput(); Which object is updated? Why is this accessible?

13. Lost Objects Species tyrannosaurus = new Species(), elephant = new Species(); tyrannosaurus.setSpecies(“Dinosaur", 10, 15); elephant.setSpecies(“Mamal", 11, 2); tyrannosaurus = elephant;

14. Unit Testing Testing our program with different inputs or even all possible inputs is good! ◦However not possible for larger programs Test smaller parts of your program/class individually ◦AKA Unit Testing Species testSpecies = new Species(); // Test the setSpecies method testSpecies.setSpecies("Tribbles", 100, 50); if (testSpecies.getName().equals("Tribbles") && (testSpecies.getPopulation() == 100) && (testSpecies.getGrowthRate() >= 49.99) && (testSpecies.getGrowthRate() <= 50.01)) { System.out.println("Pass: setSpecies test."); } else { System.out.println("FAIL: setSpecies test."); } Since getGrowthRate returns a double we should not attempt testSpecies. getGrowthRate() == 50