Announcements  If you need more review of Java…  I have lots of good resources – talk to me  Use “Additional Help” link on webpage  Weekly assignments problems due before class  Remove rust from summer and get back into coding  Problems designed to help learn new material, too

Classes vs. Objects  Classes are blueprints describing data type  Classes (usually) cannot do anything on their own  Objects are instances of a class  New objects created (instantiated) using new  Fields describe state of an object  Object’s behavior represented by methods

Instance Variables  All of class's instances have same fields…  … but values can differ between each instance  In a class, each field must have unique name  Different classes can duplicate names of fields  Field declaration also includes data type  Will act like variables of that type  Can be primitive, enum, or reference type

Class Example public class Car { /** What kind of car & who made it */ private String makeAndModel; /** Color of the car. */ private String color; /** Percent full the gas tank is */ private float tankLevel; /** Miles recorded on the odometer */ private int odometerReading; /* Definition continues from here */

Using Fields (1) Car profHertzCar = new Car(); profHertzCar.makeAndModel = “BMW Z4”; profHertzCar.color = “Deep Green Metallic”; profHertzCar.tankLevel = 1.0; profHertzCar.odometerReading = 10000; Car actualCar = new Car(); actualCar.makeAndModel = “Subaru Outback"; actualCar.color = “Brown”; actualCar.tankLevel = ; actualCar.odometerReading = 47634;

Using Fields (2) Car dreamCar = new Car(); dreamCar.makeAndModel = “BMW Z4”; dreamCar.color = “Deep Green Metallic”; dreamCar.tankLevel = 1.0; dreamCar.odometerReading = 10000;

Methods  Define how objects act, behave, & change  Need unique name & parameters (“signature”)  Can share name or parameter list, but not both  Determines which code used at any method call  Methods also define return type  Return type is not part of signature  Cannot help determine which method to call

Method Return Types  Can be primitive, reference, array, or void  void methods cannot return data  All other methods must return data  Method execution stops immediately at return  Will NOT execute further in method  After return, calling method executes immediately  Compilation error occurs when:  Method includes executing code after return  Non- void method can end without return statement

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

Useful Car Methods void crusin(int distance, float gasUsed) { int newDistance = odometerReading + distance; adjustFuel(gasUsed); odometerReading = newDistance; } 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; } }

Calling Methods Car dreamCar = new Car(); 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”); } dreamCar.crusin(400, 1.0); if (dreamCar.willStart(true)) { System.out.println(“Vroom vroom”); } else { System.out.println(“*sigh*”); }

Constructors  Special methods called to instantiate object  Have identical name as class  No return type (not even void ) allowed  If parameters differ, multiple constructors possible  Parameters to new must match 1 constructor  If no constructor defined, implicit one used for class  No parameters included in implicit constructor

Tracing with Objects public class Account { public String owner; public float balance; public String identifier; Account check = new Account(“haxx0r”, 1.25, “0001”); Account richOne = new Account(“billG”, ,“2408”); check.balance -= 1.20; // Bought a coffee check.identifier = “2408”; // 1 st theft attempt richOne.owner = “haxx0r”; // 2 nd theft attempt

Tracing Parameters  Traced like variable created at method call  New variable created each time method called  Variable lives until completion of method  Initialized with value of argument  Works like ran parameter = actual  Primitives copy value into parameter  Parameter is aliased if a reference

Tracing Parameters public void badTransfer(Account source,float amount){ if (amount >= source.balance) { amount = source.balance(); source = null; } else { source.balance -= amount; } balance += amount; } check.badTransfer(richOne, Float.MAX_VALUE);

Locals  Variables declared inside a method  Must be assigned value before using  “Live” only for code block in which declared  Local always better than instance variable  Easy trick to simplify design, algorithm, & code  Reduces opportunities for bugs dramatically  Added bonus, creates fewer comments to write!  Declare local in smallest enclosing block

Your Turn  Get into your groups and complete activity

For Next Lecture  Reading from AF 7.11 for Wednesday  What does the static keyword mean?  How do static fields & static methods work?  Why did 10 out of 12 students get this wrong?  There is weekly assignment problem on Angel  Due before Wednesday’s lecture (via )  Get back into the swing of writing Java code