Question of the Day  What card(s) must you flip to verify the following statement: Cards with a vowel on one side, have an even number on the other side. A A B B

Question of the Day  What card(s) must you flip to verify the following statement: Cards with a vowel on one side, have an even number on the other side. A A B B

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;

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 = “Subaru Outback”; realCar.color = “Silver”; realCar.tankLevel = ; realCar.odometerReading = 45634;

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 = “Subaru Outback”; realCar.color = “Silver”; realCar.tankLevel = ; realCar.odometerReading = 45634;

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 own; public float bal; public String id; Account check = new Account(“haxx0r”, 1.25, “0001”); Account richOne = new Account(“billG”, ,“2408”); check.bal -= 1.20; // Bought a coffee check.id = “2408”; // 1 st theft attempt richOne.own = “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 src,float amt){ if (amt >= src.bal) { amt = src.bal (); src = null; } else { src.bal -= amt; } bal += amt; } 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

Locals  Variables declared inside a method  Must be assigned value before using  “Live” only for code block in which declared  Local variable always better than field  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 Never declare a field when a local will do

Locals  Variables declared inside a method  Must be assigned value before using  “Live” only for code block in which declared  Local variable always better than field  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

Locals  Variables declared inside a method  Must be assigned value before using  “Live” only for code block in which declared  Local variable always better than field  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

Locals  Variables declared inside a method  Must be assigned value before using  “Live” only for code block in which declared  Local variable always better than field  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

Locals  Variables declared inside a method  Must be assigned value before using  “Live” only for code block in which declared  Local variable always better than field  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

Calling Methods  Some methods really need an object  Integer. intValue() is unclear without an object  But other methods independent of an instance  main(String[]) lacks an object to use to call it  Why need Integer to call parseInt(String) ?

static Methods  static methods do not need an instance  Behavior defined by class, not an instance  parseInt(String) based only on Integer  All methods similar in how written & executed  May or may not include parameters  Local variables can be declared and used  Must either be void or declare return type  Mix of methods possible for all classes & enums

Making Method static  Include static keyword in method declaration public static void main(String args[]) public static int parseInt(String s) private static int nextInt()  Behaves like any other method (almost)  Use locals & parameters like normal  As with other code, call (nearly all) other methods  Java operators can be used  Perform any I/O operations

static v. Non- static  Methods that are non- static have this  Aliased to instance on which method called  Can directly use fields & call all methods  No this parameter in static methods  Code directly using non- static members illegal…  … using static fields & methods perfectly legal  As always, can use object to access its members

BadDuck public class BadDuck { private int quackVolume; public static void printVolume() { System.out.println(“Volume is ” + getVolume()); BadDuck duck = new BadDuck(); duck.quackVolume = 11; System.out.println(“Volume is ”+duck.getVolume()); } public int getVolume() { return quackVolume; } }

BadDuck getVolume() public class BadDuck { private int quackVolume; public static void printVolume() { System.out.println(“Volume is ” + getVolume()); BadDuck duck = new BadDuck(); duck.quackVolume = 11; System.out.println(“Volume is ”+duck.getVolume()); } public int getVolume() { return quackVolume; } } Compiler error. Without an object, whose volume are we getting?

BadDuck getVolume() public class BadDuck { private int quackVolume; public static void printVolume() { System.out.println(“Volume is ” + getVolume()); BadDuck duck = new BadDuck(); duck.quackVolume = 11; System.out.println(“Volume is ”+duck.getVolume()); } public int getVolume() { return quackVolume; } } This is good. There is a duck to get the volume.

BadDuck public class BadDuck { public static String duckSpeak() { return “quack”; } } public class UseDuck { public void speak() { System.out.println(BadDuck.duckSpeak()); BadDuck duck = new BadDuck(); System.out.println(duck.duckSpeak()); } }

BadDuck public class BadDuck { public static String duckSpeak() { return “quack”; } } BadDuck.duckSpeak() public class UseDuck { public void speak() { System.out.println(BadDuck.duckSpeak()); BadDuck duck = new BadDuck(); System.out.println(duck.duckSpeak()); } } This compiles. Static methods do not use an object, so this is preferred way to call them.

BadDuck public class BadDuck { public static String duckSpeak() { return “quack”; } } duck.duckSpeak() public class UseDuck { public void speak() { System.out.println(BadDuck.duckSpeak()); BadDuck duck = new BadDuck(); System.out.println(duck.duckSpeak()); } } Also fine, but since method is static, still cannot access instance.

static Fields  Some data belongs to class, not instance Float.MAX_VALUE Citizen.nationalPopulation Instructor.bestProfessor  static fields used for any class-based data  All of the class instances see same value  Do not need an instance to access this value  Can be updated via any instance or via class

static Code for Tracing public class Player { private static int num; private static String champ; private String name; public Player(String newN) { num += 1; name = newN; } public static int getPlayers() { return num; } public static String setChamp(String best) { champion = best; } }

Tracing With Statics public void startGame() { Player player1, player2, player3; player1 = new Player(“Homer”); player2 = new Player(“Joey JoJo”); player3 = player2; Player.champ = “Homer”; player2.setChamp (“Joey JoJo”); }

static v. non- static Review  Difference between static & non- static  static members belong to the class  Accessing & using member possible without instance  Methods lack implicit this parameter no matter what  Instances required for non- static member  Fields’ data instance-specific and not shared by class  Can access all members using implicit this parameter

Your Turn  Get into your groups and complete activity

For Next Lecture  Keep reviewing your Java lessons  Will be doing some fun & different problems Monday  Really need to be back up-to-speed at that point  There is weekly assignment problem on Angel  Due before Monday’s lecture (via )  With this finish removing rust from lazy summer