Problem of the Day  Why are manhole covers round?

Problem of the Day  Why are manhole covers round? No matter how circle rotated, radius is constant… …so only shape that cannot fall into the hole

Announcements  Weekly Assignment #1 scores will be ed  Also updated on Angel (“refresh” to get latest scores)  used for first weeks, better solution soon  Similar process for labs (no s for last week)  Weekly assignments goal to help students learn  Checks that you understand & can apply ideas in code  Due on Tuesdays, but problems mapped to lectures  Start early  Start early enough you can get help; use time wisely

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

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 not  Note that return type not part of signature  Unhelpful finding method to call at any line in code

Return Type  Methods must specify their return type  Primitive or reference type if they will return 1 value double squareIt(float x); int[] primes(int n); Professor getFavoriteTeacher();  Use special return type, void, if no value is returned void goodSoldier(); void doSomethingWithX(int x);

return Examples

 Function ends when return is executed  Once we hit return, method immediately stops  Calling function will resume its execution  If code exists after return, method will not compile return Statement

Methods' Parameters

Are Parameters Variables?  At start of method, assigned value of argument  Works like normal assignment for the parameters type  As would expect, primitives copied; references aliased  Parameters “live” only during call & die once over  Parameter placed in memory at top of frame on stack  Variables also found in frame just at lower addresses

Are Parameters Variables?  At start of method, assigned value of argument  Works like normal assignment for the parameters type  As would expect, primitives copied; references aliased  Parameters “live” only during call & die once over  Parameter placed in memory at top of frame on stack  Variables also found in frame just at lower addresses

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

B is for Bad public class BDuck { public static void main(String[] args) { int quackVolume; System.out.println(“Volume is ”+BDuck.getVolume()); BDuck duck = new BDuck(); quackVolume = 11; System.out.println(“Volume is ”+ getVolume()); } public static int getVolume() { return quackVolume; } }

B is for Bad public class BDuck { public static void main(String[] args) { int quackVolume; System.out.println(“Volume is ”+BDuck.getVolume()); BDuck duck = new BDuck(); quackVolume = 11; System.out.println(“Volume is ”+ getVolume()); } public static int getVolume() { return quackVolume; } } This is fine. It specifies the class in which the method is located.

B is for Bad public class BDuck { public static void main(String[] args) { int quackVolume; System.out.println(“Volume is ”+BDuck.getVolume()); BDuck duck = new BDuck(); quackVolume = 11; System.out.println(“Volume is ”+ getVolume()); } public static int getVolume() { return quackVolume; } } This is also fine, since the method is in the same class

B is for Bad public class BDuck { public static void main(String[] args) { int quackVolume; System.out.println(“Volume is ”+BDuck.getVolume()); BDuck duck = new BDuck(); quackVolume = 11; System.out.println(“Volume is ”+ getVolume()); } public static int getVolume() { return quackVolume; } } Compiler error. quackVolume is local to main() & getVolume() is separate method

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

B is for Bad public class BDuck { public static String duckSpeak() { return “quack”; } } public class UseDuck { public void speak() { System.out.println(BDuck.duckSpeak()); BDuck duck = new BDuck(); System.out.println(duckSpeak()); } } This works. The method is in another class, but we specify which class it is in.

B is for Bad public class BDuck { public static String duckSpeak() { return “quack”; } } public class UseDuck { public void speak() { System.out.println(BDuck.duckSpeak()); BDuck duck = new BDuck(); System.out.println(duckSpeak()); } } Compiler error here, since duckSpeak() is not defined by the UseDuck class

Constructors  Special methods called to instantiate object  Identical name as class; can have 0 or more params  If signatures differ, multiple constructors possible  No return type (not even void ) allowed must  Arguments in new must match 1 constructor  If no constructor defined, implicit one used for class  No parameters included in implicit constructor

Tracing With Methods  At top of stack add frame for every method call  frame is fancy talk for box labeled with method name  Inside frame should include method’s locals & params if  Implicit this in frame if method not static  When method ends, remove frame from stack  Lightly crossing out frame easiest way of doing this  Process is the same no matter how method ends  Goes back to uncrossed out frame at top of stack

Method Trace public class Playa { double static avg(int hits, int pa) { int aB = 3; double retVal = hits/(double)aB; aB = pa; return retVal; } public static void main(String[] args) { Playa h8r = new Playa(); double retVal = avg(10, 100); retVal += avg(0, 10); Playa star = new Playa(); double hits = avg(60, 200); } }

Your Turn  Get into your groups and complete activity

For Next Lecture  Keep reviewing your Java lessons  Should be getting back up-to-speed with Java basics  Friday will discuss how fields in an object work  How to store values between calls to a method?  There are weekly assignment problem on Angel  Due before 5PM on Tuesday as will be the normal case  Should help finish removing rust from lazy summer WILL NOT MEET  Lab WILL NOT MEET tomorrow (sorry!)  Two activities will be posted & are due before next lab