1. CSC 212 – Data Structures Lecture 3: Fields, Methods, & Constructors

2. Problem of the Day Why are manhole covers round? It is the only shape that guarantees that the cover cannot fall in!

3. Announcements Still need a note-taker; please see me if you are interested in the $50 If you feel this Java review is going too fast talk to me, check out the pages from the “Links” page, and/or seek out additional resources

4. Classes vs. Objects Classes are recipes or blueprints which describe a data type  Classes (usually) cannot do anything on their own Objects are the instances of the class  New object created (instantiated) by new  Fields describe object’s state  Methods represent object’s behavior

5. Fields Fields are defined by a class  All instances of class have same fields…  … but fields’ values may differ Fields in a class must have unique name  But same name okay if in different classes Definition must also include a data type  Can be primitive or reference type  Fields behave like variables of that type would

6. Class Example public class Car { /** * Name of company that made the car and name of the car model */ String makeAndModel; /** Color of the car. */ String color; /** How full the gas tank is, expressed as * a percentage. */ float tankLevel; /** Number of miles recorded on the odometer */ int odometerReading; /* Definition continues from here */

7. Using Fields (1) Car profHertzCar = new Car(); profHertzCar.makeAndModel = “BMW Z4”; profHertzCar.color = “Deep Green Metallic”; profHertzCar.tankLevel = 1.0; profHertzCar.odometerReading = 10000; Car shoshannaCar = new Car(); shoshannaCar.makeAndModel = “Ford Taurus Wagon”; shoshannaCar.color = “Silver”; shoshannaCar.tankLevel = 0.0001; shoshannaCar.odometerReading = 175634;

8. Using Fields (2) Car dreamCar = new Car(); dreamCar.makeAndModel = “BMW Z4”; dreamCar.color = “Deep Green Metallic”; dreamCar.tankLevel = 1.0; dreamCar.odometerReading = 10000; Car realCar = dreamCar; realCar.makeAndModel = “Ford Taurus Wagon”; realCar.color = “Silver”; realCar.tankLevel = 0.0001; realCar.odometerReading = 175634;

9. Methods All Java code is defined within a class Define actions & behavior of objects  Also specify how object’s state should change Methods must have unique name and parameter list (“signature”)  Two methods can share name Methods also define return type  Return type is not part of signature, however

10. Method Return Types Return type could be a primitive, reference, array, or void  void methods cannot return data  Other methods must return data Method immediately stops at return Will not compile if:  Code continues after return  Possible to end non-void method without`` return

11. Car Example /** Reset the fuel tank to be full. */ void fillTank() { tankLevel = 1.0; } /** * Change the amount of fuel by some means * @param levelDelta Change in fuel level (as % of tank capacity) */ void adjustFuel(float levelDelta) { tankLevel += levelDelta; /* Check that the tank level makes sense. */ if (tankLevel > 1.0) { tankLevel = 1.0; } else if (tankLevel < 0.0) { tankLevel = 0.0; } }

12. /** * Drive around town and look for the hot spots to hit. * @param distance Number of miles traveled * @param gasUsed Amount of fuel used on this trip */ void crusin(int distance, float gasUsed) { int newDistance = odometerReading + distance; adjustFuel(gasUsed); odometerReading = newDistance; } /** * Find out if the car will start. Assumes that you will not * hotwire the car. * @param haveKey True if we the key to this car; else it is false * @return True if the engine would start, false otherwise */ boolean willStart(boolean haveKey) { if (!haveKey || tankLevel == 0.0) { return false; } else if (makeAndModel.startsWith(“Jaguar”)) { java.util.Random rnd = new Random(); return rnd.nextBoolean(); } else { return true; } }

13. Using Methods Car dreamCar = new Car(); Car realCar = dreamCar; dreamCar.makeAndModel = “BMW Z4”; dreamCar.color = “Deep Green Metallic”; dreamCar.tankLevel = 1.0; dreamCar.odometerReading = 10000; if (dreamCar.willStart(true)) { System.out.println(“Vroom vroom”); } realCar.crusin(400, -1.0); if (dreamCar.willStart(true)) { System.out.println(“Vroom vroom”); } else { System.out.println(“*sigh*”); }

14. Constructors Special methods called only when instantiating an object  Must have identical name as class  Cannot include a return type (include void)  Can define multiple constructors with different signatures Parameters used in new must match at least 1 constructor

15. Car Example /** * Create a new car with the given make, model, color and tankLevel. * @param make Manufacturer of the car being created * @param model Model of the car being instantiated * @param color Color to which the car should be painted * @param pctFull How full the gas tank is (in % of capacity terms) */ Car(String make, String model, String color, float pctFull) { // Set car’s make and model makeAndModel = make + “ “ + model; // Record the color this.color = color; // Set the tank level adjustFuel(pctFull); // New cars have not driven anywhere, yet this.odometerReading = 0; }

16. Using Constructors Car profHertzCar = new Car(); profHertzCar.makeAndModel = “BMW Z4”; profHertzCar.color = “Deep Green Metallic”; profHertzCar.tankLevel = 1.0; profHertzCar.odometerReading = 10000; Car profHertzCar = new Car(“BMW”, “Z4”, “Deep Green Metallic”, 1.0); profHertzCar.odometerReading = 10000; /* This causes an error, since there is not a constructor that takes two String parameters. */ Car badCar = new Car(“BMW”, “Z4”);

17. Before Next Lecture… Finish week #1 homework Wednesday’s lecture will continue with constructors, fields, & methods  Keep thinking & ask any questions you have  May want to review any old notes & handouts