13 X 11 Java Lecture 3 CS 1311 Structure 13 X 11

Types of Java Programs Applications –Stand alone –Requires JVM (java.exe, etc.) on your machine Applets –JVM built in to web browser runs program –Can be downloaded from network or files can be on your machine –Susceptible to browser problems! Servlets –Like an applet but code runs on server when a particular web page is browsed

13 X 11 A View of Memory Class Pool (What you wrote) Stack (Static Area) (Store stuff here) Heap (Dynamic Area) (Store stuff here)

13 X 11 class CokeMachine { int numCans = 3; void vend() { if(numCans > 0) { System.out.println("Have a Coke!"); numCans = numCans - 1; } else { System.out.println("Sorry!"); } void load(int n) { numCans = numCans + 1; System.out.println("Ready to vend!"); } public static void main(String args[]) { CokeMachine cc; cc = new CokeMachine(); cc.vend(); cc.load(); cc.vend(); }

13 X 11 Two Views External CokeMachine cc; cc = new CokeMachine(); cc.vend(); cc.load(5); class CokeMachine { int numCans = 3; void vend() { if(numCans > 0) { System.out.println ("Have a Coke!"); numCans = numCans - 1; } else { System.out.println ("Sorry!"); } void load(int n) { numCans = numCans + 1; System.out.println ("Ready to vend!"); }

13 X 11 javac *.java java Driver Could write... class Driver { public static void main(String args[]) { CokeMachine cc; cc = new CokeMachine(); cc.vend(); cc.load(5); } // main } // Simulation

13 X 11 What happened? java Driver Driver main Class Pool Stack Heap

13 X 11 What happened? Java searches for (and finds) static main method Class Pool Stack Heap Driver main

13 X 11 What happened? Java searches for (and finds) static main method Class Pool Stack Heap Driver main

13 X 11 What happened? CokeMachine cc; Class Pool Stack Heap Driver main CokeMachine numCans vend() load()

13 X 11 What happened? CokeMachine cc; Class Pool Stack Heap Driver main main cc (CokeMachine) CokeMachine numCans vend() load()

13 X 11 What happened? cc = new CokeMachine(); Class Pool Stack Heap Driver main main cc (CokeMachine) CokeMachine numCans vend() load() Obj: CokeMachine numCans vend() load() 3

13 X 11 What happened? cc.vend(); Class Pool Stack Heap Driver main main cc (CokeMachine) CokeMachine numCans vend() load() Obj: CokeMachine numCans vend() load() 2 Have a Coke!

13 X 11 What happened? cc.vend(); Class Pool Stack Heap Driver main main cc (CokeMachine) CokeMachine numCans vend() load() Obj: CokeMachine numCans vend() load() 1 Have a Coke!

13 X 11 What happened? cc.vend(); Class Pool Stack Heap Driver main main cc (CokeMachine) CokeMachine numCans vend() load() Obj: CokeMachine numCans vend() load() 0 Have a Coke!

13 X 11 What happened? cc.vend(); Class Pool Stack Heap Driver main main cc (CokeMachine) CokeMachine numCans vend() load() Obj: CokeMachine numCans vend() load() 0 Have a Coke! Sorry!

13 X 11 As a convenience... We don't really need the driver class We can put a static main in the CokeMachine But the operation is essentially the same! In fact, you should always put a static main in every class you write Use it to test the class for correct operation Will Java get confused?

13 X 11 No Way, Jose! class A main() class B main() class C main() class D main() class E main() class F main() Why?

13 X 11 Because! class A main() class B main() class C main() class D main() class E main() class F main() javac *.java java F

13 X 11 Questions?

13 X 11 Fields Sometimes called attributes In our CokeMachine: numCans Can be –Primitives –References Access cc.numCans = 42; // Direct access cc.vend(); // Via modifier Is direct access a "GoodThing" ® or a "BadThing" ®

13 X 11 Controlling Access Access to fields (and methods) can be controlled public -- Anyone can access private -- Access is only from within class Recall public static void main(String args[])

13 X 11 Example class CokeMachine { int numCans = 3; double cash = 0.0; static final double PRICE = 0.65; void vend() { if(numCans > 0) { numCans--; cash += PRICE; System.out.println("Have a coke!"); } else { System.out.println("Sorry, no Cokes"); }

13 X 11 Let's Test It! class Tester { public static void main(String args[]) { CokeMachine cm;// Make reference cm = new CokeMachine(); // Make a machine cm.numCans--;// Steal a Coke System.out.println("Cash = " + cm.cash); } Where's my money?!?!?!

13 X 11 Let's add some protection class CokeMachine { private int numCans = 3; private double cash = 0.0; private static final double PRICE = 0.65; public void vend() { if(numCans > 0) { numCans--; cash += PRICE; System.out.println("Have a coke!"); } else { System.out.println("Sorry, no Cokes"); }

13 X 11 Warning about Testing class CokeMachine { private int numCans = 3; private double cash = 0.0; private static final double PRICE = 0.65; public void vend() { if(numCans > 0) { numCans--; cash += PRICE; System.out.println("Have a coke!"); } else { System.out.println("Sorry, no Cokes"); } class Tester { public static void main(String a[]) { CokeMachine cm = new CokeMachine(); cm.numCans--; } class CokeMachine { private int numCans = 3; private double cash = 0.0; private static final double PRICE = 0.65; public void vend() { if(numCans > 0) { numCans--; cash += PRICE; System.out.println("Have a coke!"); } else { System.out.println("Sorry, no Cokes"); } public static void main(String a[]) { CokeMachine cm = new CokeMachine(); cm.numCans--; }

13 X 11 Demo

13 X 11 Details Methods –Module defined in a class which effectively becomes a message that an object of that class can understand cc.vend(); –Send a " vend() " message to the object referenced by cc –Almost all code executed in Java will be executed in methods

13 X 11 Anatomy of a method ( ) { } Examples: public static void main(String args[]) { System.out.println("Hello World!"); } void vend(){ /* code goes here */ } private int addEmUp(int a, int b, int c) { return a + b + c; }

13 X 11 Parts is parts public private –There are others we won't discuss static nothing which implies dynamic or instance

13 X 11 Parts is parts –Methods may optionally return a value –May be any valid type Class name Primitive void indicates no return value –If you promise to return something Java will expect you to keep your promise –If you promise to not return anything Java will break your kneecaps with a Louisville Slugger ® if you don't keep your promise

13 X 11 A Promise is a Promise public int max(int a, int b) { if(a >= b) { return a; } if(a < b) { return b; }

13 X 11 A Promise is a Promise public int max(int a, int b) { if(a >= b) { return a; } if(b < a) { return b; } Classname.java: : Return required at end of int max(int, int). public int max(int a, int b)

13 X 11 Method Name... –Method name follows normal identifier rules –Style note: Start method names with a lower case letter but capitalize multiword names: enqueue vend vendACoke load loadCokes

13 X 11 Parameter List –Declare each parameter individually –i.e. int a, float f, CokeMachine cm –Java only supports pass by value (in) parameters. public void swap(int a, int b) { /* This method is worthless! */ int t = a; a = b; b = t; }

13 X 11 Parameter List public static void testMachine(int n, CokeMachine cm) { for(int i=0; i < n; i++) { cm.vend(); } n = 0; // Just to be mean cm = null; // and nasty! } What is the effect on the outside world? Could this be considered (just maybe) a side effect?

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); testMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 3 Object: CokeMachine numCans = 3 main:

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); testMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 3 Object: CokeMachine numCans = 3 main: testMachine: n: cm: 3

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); exerciseMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 3 Object: CokeMachine numCans = 3 main: testMachine: n: cm: 3 public static void testMachine (int n, CokeMachine cm) { for(int i=0; i < n; i++) { cm.vend(); } n = 0; // Just to be mean cm = null; // and nasty! }

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); exerciseMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 0 Object: CokeMachine numCans = 0 main: testMachine: n: cm: 3 public static void testMachine (int n, CokeMachine cm) { for(int i=0; i < n; i++) { cm.vend(); } n = 0; // Just to be mean cm = null; // and nasty! }

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); exerciseMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 0 Object: CokeMachine numCans = 0 main: testMachine: n: cm: 0 public static void testMachine (int n, CokeMachine cm) { for(int i=0; i < n; i++) { cm.vend(); } n = 0; // Just to be mean cm = null; // and nasty! }

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); exerciseMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 0 Object: CokeMachine numCans = 0 main: testMachine: n: cm: 0 public static void testMachine (int n, CokeMachine cm) { for(int i=0; i < n; i++) { cm.vend(); } n = 0; // Just to be mean cm = null; // and nasty! }

13 X 11 Pass by Value class Demo { /* Code for testMachine here*/ public static void Main(String args[]) { int i = 3; CokeMachine cm; cm = new CokeMachine(); exerciseMachine(i, cm); } i: cm: 3 Object: CokeMachine numCans = 0 Object: CokeMachine numCans = 0 main:

13 X 11 Questions?

13 X 11 Sidebar on Methods Factorial (Yes, again) class FactDemo { public static void main(String args[]) { int n = 10; int i; int answer = 1; for(i = 1; i <= n; i++) { answer *= i; } // for System.out.println(n + "! = " + answer); } // main } // factDemo

13 X 11 Sidebar on Methods Make it a method public static void fact(int n) { int i; int answer = 1; for(i = 1; i <= n; i++) { answer *= i; } // for System.out.println(n + "! = " + answer); } // fact

13 X 11 Sidebar on Methods Make it smaller and more efficient? public static void fact(int n) { int i, ans; for(i = 1, ans = 1; i <= n; ans *= i++); System.out.println(n + "! = " + ans); } // fact

13 X 11 Sidebar on Methods Make it more readable? public static void fact(int n) { int i; int ans = 1; for(i = 1; i <= n; i++) ans *= i; System.out.println(n + "! = " + ans); } // fact

13 X 11 Sidebar on Methods Make it more useful? public static int fact(int n) { int i; int ans = 1; for(i = 1; i <= n; i++) ans *= i; return ans; } // fact

13 X 11 Sidebar on Methods Make it more useful? class FactDemo { public static int fact(int n) { int i; int ans = 1; for(i = 1; i <= n; i++) ans *= i; return ans; } // fact public static void main(String args[]) { System.out.println("10! = " + fact(10)); }

13 X 11 Sidebar on Methods Do it recursively! class FactDemo { public static int fact(int n) { if(n == 0) return 1; else return n * fact(n - 1); } // fact public static void main(String args[]) { System.out.println("10! = " + fact(10)); }

13 X 11 Sidebar on Methods Do it recursively! class FactDemo { public static int fact(int n) { if(n == 0) return 1; else return n * fact(n - 1); } // fact public static void main(String args[]) { for(int i = 1; i < 18; i++) System.out.println(i + "! = " + fact(i)); }

13 X 11 Shaky Results 1! = 1 2! = 2 3! = 6 4! = 24 5! = 120 6! = 720 7! = ! = ! = ! = ! = ! = ! = ! = ! = ! = ! = Correct value: n! = 6,227,020,800 > ?

13 X 11 Primitive Data Type Ranges TypeSizeMinDefault booleanfalse Max 1false*true* char'\u0000' (null)16 byte(byte) short(short) ,76832,767 int032 -2,147,483,6482,147,483,647 long0L64 -9,223,372,036,854,775,8089,223,372,036,854,775,807 float0.0F32 Approx ±3.4E+38 with 7 significant digits double0.0D64 Approx ±1.7E+308 with 15 significant digits void * Not truly min and max.

13 X 11 Notice! Notice how Java gave you a helpful warning message to help keep you spacecraft from plunging into the Sun NOT.

13 X 11 Sidebar on Methods Fix the problem (forever?)! class FactDemo { public static long fact(long n) { if(n == 0) return 1; else return n * fact(n - 1); } // fact public static void main(String args[]) { for(int i = 1; i < 18; i++) System.out.println(i + "! = " + fact(i)); }

13 X 11 Pop Quiz class Widget { int a, b; public int sum() { return a + b; } public static void main(String a[]) { a = 5; b = 10; System.out.println(sum()); }

13 X 11 Pop Quiz class Widget { int a, b; public int sum() { return a + b; } public static void main(String a[]) { Widget w; w = new Widget(); w.a = 5; w.b = 10; System.out.println(w.sum()); }

13 X 11 Questions?

13 X 11 Final Thought Just what is a class? class declarations initialization modules* methods

13 X 11 Questions?