decisions, decisions, decisions

Decisions, decisions, decisionsChapter 5Spring 2005CS 101Aaron Bloomfield

BackgroundOur problem-solving solutions so far have the straight-line propertyThey execute the same statements for every run of the program

public class DisplayForecast // main(): application entry pointpublic static void main(String[] args) { System.out.print("I think there is a world"); System.out.print(" market for maybe five "); System.out.println("computers. ); System.out.print(" Thomas Watson, IBM, ); System.out.println("1943.);}}

BackgroundFor general problem solving we need more capabilitiesThe ability to control which statements are executedThe ability to control how often a statement is executed

We will concentrate first on controlling which statements are executed

Java provides the if and switch conditional constructs to control whether a statement list is executedThe if constructs use logical expressions to determine their course of action

Examination begins with logical expressions

Logical expressionsThe branch of mathematics dealing with logical expressions is Boolean algebraDeveloped by the British mathematician George Boole

Logical expressionsA logical expression has either the value logical true or logical false

Some expressions whose values are logical trueThe year 2004 is a leap yearA meter equals 100 centimeters

Some expressions whose values are logical falseA triangle has four sidesThe area of square is always equal to twice its perimeter

Logical expressionsThere are three primary logical operators for manipulating logical valuesLogical andLogical orLogical not

The operators work as most of us would expect

Quick surveyIve seen Boolean logic beforeLots and lots and lotsA fair amountSomeNot really

More demotivators

Truth tablesWe use truth tables to give formal specifications of the operatorsIt works as most of us would expect allows for ambiguity of interpretationJim is smiling or Patty is smilingCan both Jim and Patty both be smiling? Truth tablesLists all combinations of operand values and the result of the operation for each combination

Or and not truth tables

Boolean algebraCan create complex logical expressions by combining simple logical expressions

not (p and q)

DeMorgans lawsnot (p and q) equals (not p) or (not q)

DeMorgans lawsnot (p or q) equals (not p) and (not q)

DeMorgans lawsIf you remember nothing else about the Boolean operators, remember that:

not (a and b) == (not a) or (not b)not (a or b) == (not a) and (not b)

A boolean typeJava has the logical type boolean

Type boolean has two literal constantstruefalse

OperatorsThe and operator is &&Dont use &The or operator is ||Dont use |The not operator is !

Defining boolean variablesLocal boolean variables are uninitialized by default

boolean isWhitespace;boolean receivedAcknowledgement;boolean haveFoundMissingLink;

Defining boolean variablesLocal boolean variables with initialization

boolean canProceed = true;boolean preferCyan = false;boolean completedSecretMission = true;

Defining boolean variablesInstance boolean variables are initialized to false by default

boolean isWhitespace;boolean receivedAcknowledgement;boolean haveFoundMissingLink;

DeCSS: The program#include typedef unsigned int uint; char ctb[512]="33733b2663236b763e7e362b6e2e667bd393db0643034b96de9ed60b4e0e4\ 69b57175f82c787cf125a1a528fca8ac21fd999d10049094190d898d001480840913d7d35246\ d2d65743c7c34256c2c6475dd9dd5044d0d4594dc9cd4054c0c449559195180c989c11058185\ 081c888c011d797df0247074f92da9ad20f4a0a429f53135b86c383cb165e1e568bce8ec61bb\ 3f3bba6e3a3ebf6befeb6abeeaee6fb37773f2267276f723a7a322f6a2a627fb9f9b1a0e9a9e\ 1f0b8f8b0a1e8a8e0f15d1d5584cd8dc5145c1c5485cc8cc415bdfdb5a4edade5f4bcfcb4a5e\ cace4f539793120692961703878302168286071b7f7bfa2e7a7eff2bafab2afeaaae2ff"; typedef unsigned char uchar;uint tb0[11]={5,0,1,2,3,4,0,1,2,3,4};uchar* F=NULL; uint lf0,lf1,out;void ReadKey(uchar* key){int i;char hst[3]; hst[2]=0;if(F==\ NULL){F=malloc(256);for(i=0;i21)^(lf1>>24))&1;lf0=(lf0

DeCSS: The shirt (and tie!)

DeCSS: The poemHow to decrypt aDVD: in haiku form.(Thanks, Prof. D. S. T.)------------------------

(I abandon myexclusive rights to make orperform copies of

this work, U. S. CodeTitle Seventeen, sectionOne Hundred and Six.)

Muse! When we learned tocount, little did we know allthe things we could do

some day by shufflingthose numbers: Pythagorassaid "All is number"

long before he sawcomputers and their effects,or what they could doTable Zero is:Five, zero, one, two, three, four,oh, one, two, three, four.

Table One is long:two to the eighth power bytes.Ready? Here they are:

Fifty one; then onehundred fifteen; fifty nine;thirty eight; ninety

nine; thirty five; onehundred seven; one hundredeighteen; sixty two;

one hundred twentysix; fifty four; forty three;one hundred ten; then

DeCSS: The numberThe worlds first illegal prime number:

485650789657397829309841894694286137707442087351357924019652073668698513401047237446968797439926117510973777701027447528049058831384037549709987909653955227011712157025974666993240226834596619606034851742497735846851885567457025712547499964821941846557100841190862597169479707991520048667099759235960613207259737979936188606316914473588300245336972781813914797955513399949394882899846917836100182597890103160196183503434489568705384520853804584241565482488933380474758711283395989685223254460840897111977127694120795862440547161321005006459820176961771809478113622002723448272249323259547234688002927776497906148129840428345720146348968547169082354737835661972186224969431622716663939055430241564732924855248991225739466548627140482117138124388217717602984125524464744505583462814488335631902725319590439283873764073916891257924055015620889787163375999107887084908159097548019285768451988596305323823490558092032999603234471140776019847163531161713078576084862236370283570104961259568184678596533310077017991614674472549272833486916000647585917462781212690073518309241530106302893295665843662000800476778967984382090797619859493646309380586336721469695975027968771205724996666980561453382074120315933770309949152746918356593762102220068126798273445760938020304479122774980917955938387121000588766689258448700470772552497060444652127130404321182610103591186476662963858495087448497373476861420880529443

DeCSS: The images

DeCSS: The recordingsAll this info from http://www-2.cs.cmu.edu/~dst/DeCSS/Gallery/

Or do a Google search for decss gallery

DeCSS: The movie

Other operatorsEquality operators == and !=

Operator ==Returns true if the operands have the same value; otherwise, returns falseThis is not the assignment operator!

Operator !=Returns true if the operands have different values; otherwise, returns false

The operators work with all types of values

Evaluating boolean expressionsSupposeboolean p = true;boolean q = false;boolean r = true;boolean s = false;

What is the value ofp p && s !s p == q q q != r p && r r == s q || s q != s

Evaluating boolean expressionsSupposeint i = 1;int j = 2;int k = 2;char c = '#';char d = '%';char e = '#';

What is the value ofj == k i != ki == j j != kc == e d != ec == d c != e

Floating point precissionWhat gets printed?

class FloatTest {public static void main (String args[]) {double y = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1;

System.out.println (y);}}

FloatTest.javaThere are 10 0.1s

Take care with floating-point valuesConsiderdouble a = 1;double b = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1double c = .9999999999999999;

Two true expressions! c == bb != a

Two false expressions!a == bb != c

Problem lies with the finite precision of the floating-point typesInstead with the ordering operators for closeness

How to solve thisDont compare floating-point values if you can help it!Both doubles and floats

Need to test if the two doubles are close in value

final double EPSILON = 0.000001;boolean foo = Math.abs (a-b) < EPSILON;

Ordering operatorsJava provides ordering operators for the primitive typesFour ordering operators, , =They correspond to mathematical operators of , , and

Together the equality and ordering operators are known as the relational operators

False is less than true

Evaluation boolean expressionsSupposeint i = 1;int j = 2;int k = 2;

What is the value ofi < jj < ki = ki >= k

Unicode valuesCharacter comparisons are based on their Unicode values

Characters 0, 1, 9 have expected orderCharacter 0 has the encoding 48Character 1 has the encoding 49, and so on.

Upper case Latin letters A, B, Z have expected orderCharacter A has the encoding 65, character B has the encoding 66, and so on.

Lower case Latin letters a, b, z have expected orderCharacter a has the encoding 97Character b has the encoding 98, and so on.

Evaluation boolean expressionsSupposechar c = '2';char d = '3';char e = '2';

What is the value ofc < dc < ec = ec >= e

Operator precedence revisitedHighest to lowestParenthesesUnary operatorsMultiplicative operatorsAdditive operatorsRelational orderingRelational equalityLogical andLogical orAssignment

Expressions vs. statementsA statement is a single command for Java to do, and always ends in a semi-colon (for now, at least)System.out.println (hello world);int x = 4;++x;

An expression returns a value, and does not have a semi-colon5circle.getRadius()x

Note the difference between the following:++iis an expression++i;is a statement

Quick surveyI felt I understood expressionsVery wellWith some review, Ill be goodNot reallyNot at all

A bit of humor

End of lecture on 28 February 2005

Conditional constructsProvideAbility to control whether a statement list is executed

Two constructs

If statementifif-elseif-else-if

Switch statement

Basic if statementSyntaxif (Expression) Action

If the Expression is true then execute Action

Action is either a single statement or a group of statements within braces

For us, it will always be a group of statements within braces

Exampleif (value < 0) { value = -value;}

Sorting two valuesSystem.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();

// rearrange numbers if necessaryif (value2 < value1) {// values are not in sorted orderint rememberValue1 = value1;value1 = value2;value2 = rememberValue1;}// display valuesSystem.out.println("The numbers in sorted order are "+ value1 + " and then " + value2);What happens if the user enters 11 and 28?

What happens if the user enters 11 and 4?

If semantics

What an if statement executesAn if statement executes the next block of code

A block is either:

A single statement without curly brackets:

if (a == b)System.out.println (a==b!!!);

A number of statements enclosed by curly brackets:

if (a == b) {System.out.print (a);System.out.print (==);System.out.print (b);System.out.println (!!!);}

Why we always use bracesWhat is the output?

int m = 5;int n = 10;

if (m < n) ++m;++n;

System.out.println(" m = " + m + " n = " n);

Quick surveyI felt I understood the if statementVery wellWith some review, Ill be goodNot reallyNot at all

Todays demotivators

The if-else statementSyntaxif (Expression) Action1 else Action2

If Expression is true then execute Action1 otherwise execute Action2 The actions are either a single statement or a list of statements within braces

Finding the maximum of two valuesSystem.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();

int maximum;if (value1 < value2) { // is value2 larger? maximum = value2; // yes: value2 is larger}else { // (value1 >= value2) maximum = value1; // no: value2 is not larger}System.out.println("The maximum of " + value1 + " and " + value2 + " is " + maximum);

But is it initialized?

Finding the maximum of two values

Why we use whitespaceWhat does the following do?

System.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();if (value2 < value1) {int rememberValue1 = value1;value1 = value2;value2 = rememberValue1;}System.out.println("The numbers in sorted order are "+ value1 + " and then " + value2);

How do you like your braces?if (a == b){//...} else {//...}if (a == b){//...}else {//...}if (a == b){//...} else {//...}if (a == b) {//...} else {//...}if (a == b) {//...} else {//...}

Quick surveyI felt I understood the if-else statementVery wellWith some review, Ill be goodNot reallyNot at all

If-then-else precedenceif (number != 0)if (number > 0) System.out.println("positive");else System.out.println("negative");Which if does this else refer to?

If-then-else precedence without whitespaceif (number != 0)if (number > 0)System.out.println("positive");elseSystem.out.println("negative");elseSystem.out.println("zero");

If-else-ifConsiderif (number == 0) {System.out.println("zero");}elseSystem.out.println("positive");}else { System.out.println("negative");}}{These braces arent neededif (number > 0) {Same results as previous segment but this segment better expresses the meaning of what is going onWe can change the whitespace of the code

Quick surveyI understand if-else-ifVery wellWith some review, Ill be goodNot reallyNot at all

Review of HW J4

Sorting three valuesFor sorting values n1, n2, and n3 there are six possible orderingsn1 n2 n3n1 n3 n2n2 n1 n3n2 n3 n1n3 n1 n2n3 n2 n1

Suppose s1, s2, s3 are to be a sorted version of n1, n2, and n3

Sorting three valuesif ((n1

Finding the minimum valueConsider:

// z is to hold the minimum of x and yif ( x < y )z = x;elsez = y;

Another way to do this:

z = (x

The ?: notationOnly works when both cases return a value!Example: z = (x

Switch statementSoftware engineers often confronted with programming tasks where required action depends on the values of integer expressionsThe if-else-if construct can be usedSeparately compare the desired expression to a particular valueIf the expression and value are equal, then perform the appropriate action

Because such programming tasks occur frequentlyJava includes a switch statementThe task is often more readable with the switch then with the if-else-if

A switch statement exampleif (a == 0)System.out.println (zero);else if (a == 1)System.out.println (one);else if (a == 2)System.out.println (two);else if (a == 3)System.out.println (three);else if (a == 4)System.out.println (four);elseSystem.out.println (five+);switch (a) {case 0:System.out.println (zero);break;case 1:System.out.println (one);break;case 2:System.out.println (two);break;case 3:System.out.println (three);break;case 4:System.out.println (four);break;default:System.out.println (five+);break;}

Switch statement

Testing for vowel-nessswitch (ch) {case 'a': case 'A':case 'e': case 'E':case 'i': case 'I':case 'o': case 'O':case 'u': case 'U': System.out.println("vowel);break;default: System.out.println("not a vowel);}The break causes an exiting of the switchHandles all of the other cases

A better way to format that switch statementswitch (ch) {case 'a': // FALL THRUcase 'A':// FALL THRUcase 'e': // FALL THRUcase 'E':// FALL THRUcase 'i': // FALL THRUcase 'I':// FALL THRU

...

Processing a requestSystem.out.print("Enter a number: ");int n1 = stdin.nextInt();

System.out.print("Enter another number: ");int n2 = stdin.nextInt();

System.out.print("Enter desired operator: ");char operator = stdin.nextLine().charAt(0);

switch (operator) {case '+' : System.out.println(n1 + n2); break;case '-' : System.out.println(n1 - n2); break;case '*' : System.out.println(n1 * n2); break;case '/' : System.out.println(n1 / n2); break;default: System.out.println(Illegal request);}

Quick surveyI understand the switch statementVery wellWith some review, Ill be goodNot reallyNot at all

Biggest software errorsAriane 5 rocket explosion (1996)Due to loss of precision converting 64-bit double to 16-bit intPentium division error (1994)Due to incomplete look-up table (like an array)Patriot-Scud missile error (1991)Rounding error on the timeThe missile did not intercept an incoming Scud missile, leaving 28 dead and 98 woundedMars Climate Orbiter (1999)Onboard used metric units; ground computer used English unitsAT&T long distance (1990)Wrong break statement in C codeTherac-25, X-ray (1975-1987)Badly designed software led to radiation overdose in chemotherapy patientsNE US power blackout (2003)Flaw in GE software contributed to it

References: http://www5.in.tum.de/~huckle/bugse.html, http://en.wikipedia.org/wiki/Computer_bug, http://www.cs.tau.ac.il/~nachumd/verify/horror.html

End of lecture on 2 March 2005

Testing objects for equalityConsider

System.out.print("Enter an integer number: ");int n1 = stdin.nextInt();System.out.print("Enter another integer number: ");int n2 = stdin.nextInt();

if (n1 == n2) {System.out.println("Same");}else {System.out.println(Different");}What is the output if the user enters 88 both times?

What is the output if the user enters 88 and 3?


String s1 = pastel;String s2 = s1;

if (s1 == s2) {System.out.println("Same");}else {System.out.println("Different");}

Testing objects for equality

Memory looks like

The comparison is between the references!

Thus, s1 and s2 are the same (they refer to the same object)


System.out.print("Enter a string: ");String s1 = stdin.nextLine();System.out.print("Enter another string: ");String s2 = stdin.nextLine();

if (s1 == s2) {System.out.println("Same");}else {System.out.println("Different");}What is the output if the user enters "pastel" both times?

Testing objects for equalityWhen it is executedSystem.out.print("Enter a string: ");String s1 = stdin.nextLine();System.out.print("Enter another string: ");String s2 = stdin.nextLine();

Memory looks like

As a result no matter what is entered s1 and s2 are not the sameThey refer to different objects

Comparing strings for equalityConsider:String s = "hello";String t = "hello";System.out.println (s == t);What gets printed?true

Consider:String u = new String("hello");String v = new String("hello");System.out.println (u == v);What gets printed?falseHuh?

Testing operators for equalityConsiderSystem.out.print("Enter a string: ");String s1 = stdin.nextLine();System.out.print("Enter another string: ");String s2 = stdin.nextLine();

if (s1.equals(s2)) {System.out.println("Same");}else {System.out.println("Different");}Tests whether s1 and s2 represent the same objectAll classes have a method equals(). Their implementation is class-specific. The String equals() method like many others tests for equivalence in representation

Assignment vs. comparison= is the assignment operatorIt copies the value on the right to the location on the leftConsider:int x;x = 5;The value 5 is copied to the spot x in memory

== is the comparison operatorReturns a boolean (true or false) if the two sides are equalConsider:int x = 5;System.out.println (x == 5);System.out.println (x == 6);Prints out true, false

Some handy Character class methodsisDigit()Tests whether character is numeric

isLetter()Tests whether character is alphabetic

isLowerCase()Tests whether character is lowercase alphabetic

isWhiteSpace()Tests whether character is one of the space, tab, formfeed, or newline characters

Some handy Character class methodsisUpperCase()Tests whether character is uppercase alphabetic

toLowerCase()If the character is alphabetic then the lowercase equivalent of the character is returned; otherwise, the character is returned

toUpperCase()If the character is alphabetic then the uppercase equivalent of the character is returned; otherwise, the character is returned

Quick surveyI understand comparing Strings and assignment vs. comparisonVery wellWith some review, Ill be goodNot reallyNot at all

Why speling is not so importantI cdnuolt blveieetaht I cluod aulaclty uesdnatnrd waht I was rdanieg. The phaonmneal pweor of thehmuan mind. Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht thefrist and lsat ltteer be in the rghit pclae. The rset can be a taotl mses andyou can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deosnot raed ervey lteter by istlef, but the wrod as a wlohe. Amzanig huh? yaeh and I awlyas thought slpeling was ipmorantt.

Short-circuit evaluationThe value of a logical expression can be known before all the operands have been consideredIf left operand of && is false, then the value must be falseIf right operand of || is true, then the value must be true

Java uses these properties to make logical operations efficientEvaluates left operand before it evaluates right operandIf the operator value is determined from the left operand, then the right operand is not evaluatedThe operation is short-circuited

Short-circuit evaluationShort-circuit evaluation is useful when some property must be true for some other expression to be evaluated

Suppose you are interested in knowing whether scoreSum divided by nbrScores is greater than valueThe condition can be evaluated only if nbrScores is nonzero

The following expression correctly represents the condition (nbrScores != 0) && ((scoreSum / nbrScores) > value)

class ShortCircuit { static boolean returnsFalse() {System.out.println ("returnsFalse() called");return false; } static boolean returnsTrue() {System.out.println ("returnsTrue() called");return true; }

public static void main (String args[]) {

if ( returnsFalse() && returnsTrue() ) {}

if ( returnsTrue() && returnsFalse() ) {}

if ( returnsFalse() || returnsTrue() ) {}

if ( returnsTrue() || returnsFalse() ) {}

}}returnsFalse() called

returnsTrue() calledreturnsFalse() called

returnsFalse() calledreturnsTrue() called

returnsTrue() calledOutput

A digression: PerlPerl uses short-circuit evaluation also:

open (LOGFILE, ">outputfile") || die (Cannot open outputfile!);

If the open command succeeds, then the die command is not evaluated

If the open command fails, then the die command is evaluated

Quick surveyI understand short circuit evaluationVery wellWith some review, Ill be goodNot reallyNot at all

All your base are belong to us

Flash animationReference: http://en.wikipedia.org/wiki/All_your_base_are_belong_to_us

A Circle classWe are going to develop a Circle classPerhaps for use in a graphics program

Why?Partly to review creating classesGo over some topics that were a bit fuzzyConstructors and creating objectsShow why one uses the get/set methods instead of directly modifying the instance variables

Circle class propertiesWhat properties does a circle have?RadiusPI = 3.141592653589793234Color (if plotting in a graphics program)(x,y) location

These properties will become instance variablesWe are only going to play with the first two (radius and PI) in this exampleThus, we are ignoring the color and location

Our Circle classpublic class Circle {double radius;double PI = 3.1415926536;}Note the fieldsare not staticNote the radius field is not initialized by usWere ignoring the public for now

Accessing our Circle objectAny variable or method in an object can be accessed by using a periodThe period means follow the reference

Example: System.in

Example: System.out.println (c.radius);

Example: c.PI = 4;

This is bad PI should have been declared final(this will be done later)

Whats the output?public class Circle {double radius;double PI = 3.1415926536;}

public class CircleTest {public static void main (String[] args) { int x;Circle c = new Circle();System.out.println (x);}}

When a variable is declared as part of a method, Java does not initialize it to a default valueJava will give a variable not initialized error

Whats the output now?public class Circle {double radius;double PI = 3.1415926536;}

public class CircleTest {public static void main (String[] args) { int x;Circle c = new Circle();System.out.println (c.radius);}}

When a variable is declared as part of a class, Java does initialize it to a default valueJava outputs 0.0!

Whats going on?A (method) variable needs to be initialized before it is usedUsually called a local variableA instance variable is automatically initialized by JavaAll numbers are initialized to 0, booleans to false, etc.This is a bit counter-intuitive

Circle class behaviorsWhat do we want to do with (and to) our Circle class?Create circlesModify circles (mutators)Find out about our circles properties (accessors)Find the area of the circlePlot it on the screen (or printer)A few others

These will be implemented as methods

Calling the Circle constructorTo create a Circle object:

Circle c1 = new Circle();

This does four things:Creates the c1 referenceCreates the Circle objectMakes the c1 reference point to the Circle objectCalls the constructor with no parameters (the default constructor)

The constructor is always the first method called when creating (or constructing) an object

Calling the Circle constructorTo create a Circle object:

Circle c1 = new Circle(2.0);

This does four things:Creates the c1 referenceCreates the Circle objectMakes the c1 reference point to the Circle objectCalls the constructor with 1 double parameters (the specific constructor)

The constructor is always the first method called when creating (or constructing) an object

ConstructorsRemember, the purpose of the constructor is to initialize the instance variablesPI is already set, so only radius needs setting

public Circle() {radius = 1.0;}

public Circle (double r) {radius = r;}

Note there is no return type for constructorsNote that the constructorname is the EXACT sameas the class nameNote that there are two methods with the same name!

Quick surveyHow are we doing with constructors?I totally got itIts making a lot more sense nowIm still a bit confusedI have no idea what you were talking about

Demotivators

What happens in memoryConsider: Circle c = new Circle();A double takes up 8 bytes in memoryThus, a Circle object takes up 16 bytes of memoryAs it contains two doublesShorthand representation

Consider the following codepublic class CircleTest {public static void main (String[] args) { Circle c1 = new Circle();Circle c2 = new Circle();Circle c3 = new Circle();Circle c4 = new Circle();}}

What happens in memoryThere are 4 Circle objects in memoryTaking up a total of 4*16 = 64 bytes of memory

Consider the following codepublic class CircleTest {public static void main (String[] args) { Circle c1 = new Circle();//...Circle c1000000 = new Circle();}}This program creates 1 million Circle objects!

What happens in memoryThere are 1 million Circle objects in memoryTaking up a total of 1,000,000*16 16 Mb of memoryNote that the final PI field is repeated 1 million times

The use of static for fieldsIf a variable is static, then there is only ONE of that variable for ALL the objectsThat variable is shared by all the objectsTotal memory usage: 8 Mb + 8 bytes (1,000,000+1=1,000,001 doubles)Total memory usage: 16 bytes (1+1=2 doubles)Total memory usage: 40 bytes (4+1=5 doubles)c1000000c4PI3.1415926536

More on static fieldsWhat does the following printNote that PI is not final

Circle c1 = new Circle();Circle c2 = new Circle();Circle c3 = new Circle();Circle c4 = new Circle();c1.PI = 4.3;System.out.println (c2.PI);

It prints 4.3Note you can refer to static fields byobject.variable

Even more on static fieldsThere is only one copy of a static field no matter how many objects are declared in memoryEven if there are zero objects declared!The one field is common to all the objects

Static variables are called class variablesAs there is one such variable for all the objects of the classWhereas non-static variables are called instance variables

Thus, you can refer to a static field by using the class name:Circle.PI

Even even more on static fieldsThis program also prints 4.3:

Circle c1 = new Circle();Circle c2 = new Circle();Circle c3 = new Circle();Circle c4 = new Circle();Circle.PI = 4.3;System.out.println (c2.PI);

Even even even more on static fieldsWeve seen static fields used with their class names:System.in(type: InputStream)System.out(type: OutputStream)Math.PI(type: double)Integer.MAX_VALUE(type: int)

Quick surveyHow are we doing with static fields?I totally got itMore or lessIm still a bit confusedI have no idea what you were talking about

How much will Christmas cost?

http://www.pncbank.com/12daysFlash animation (local)

Back to our Circle classpublic class Circle {double radius;final static double PI = 3.1415926536;

public Circle() {radius = 1.0;}


}

But it doesnt do much!Note that PI is now final and static

Adding a methodpublic class Circle {double radius;final static double PI = 3.1415926536;

// Constructors...

double computeArea () { return PI*radius*radius; }

}Note that a (non-static) method can use both instance and class variables

Using that methodpublic class CircleTest {public static void main (String[] args) {Circle c = new Circle();c.radius = 2.0;System.out.println (c.computeArea()); }}Prints 12.566370614356

End of lecture on 14 March 2005Actually ended 2 slides ahead, but I want to start up here

Review of our Circle classpublic class Circle {

double radius;final static double PI = 3.1415926536;

public Circle() {}



}Slight change from before

Adding another methoddouble oneOverRadius() { return 1.0/radius;}

I couldnt think of a good reason to divide something by the radius

What happens now?Code in class CircleTests main() method

Circle c = new Circle(); // c.radius is now 0.0System.out.println (c.oneOverRadius());

Java wont crash, but many other programming languages (C and C++, in particular) willSo well call this a crash for the sake of this lecture

Java prints InfinityNot what we wanted, though!

One way to fix thispublic class Circle {double radius = 1.0;final static double PI = 3.1415926536;

// Constructors...


double oneOverRadius() { return 1.0/radius;}

}Note that the radius variable is now initialized to 1.0

Back to our programThis code will now run properly:

Circle c = new Circle(); // c.radius = 1.0System.out.println (c.oneOverRadius());

But this code will crash:

Circle c = new Circle(); // c.radius = 1.0c.radius = 0.0;System.out.println (c.oneOverRadius());

Where the crash occurspublic class CircleTest {

public static void main (String[] args) {

Circle c = new Circle(); // c.radius = 1.0

c.radius = 0.0;

System.out.println (c.oneOverRadius());}}public class Circle {double radius = 1.0;final static double PI = 3.1415926536;



Here is the badly written codeHere is where the crash occurs

Motivation for private fieldsProblem: We do not want people using our Circle class to be able to modify the fields on their own

Solution: Dont allow other code to modify the radius fieldGive it private visibility

private means that only code within the class can modify the field

One way to fix thispublic class Circle {private double radius = 1.0;final static double PI = 3.1415926536;

// Constructors...



}Note that the radius variable is now private

Back to our programThis code will now not compile:

Circle c = new Circle(); // c.radius = 1.0c.radius = 0.0;System.out.println (c.oneOverRadius());

Java will give a compile-time error:radius has private access in Circle

Back to our programThis code will also not compile:

Circle c = new Circle(); // c.radius = 1.0System.out.println (c.radius);

Java will give the same compile-time error:radius has private access in Circle

The problem nowBut now you cant have a Circle with a radius other than 1.0!

Solution: Use a get/set methods in Circle:

A mutator method:

void setRadius (double r) { radius = r;}

An accessor method:

double getRadius () { return radius;}

Our Circle class so farpublic class Circle { private double radius = 1.0; final static double PI = 3.1415926536;

// Constructors...


double oneOverRadius() { return 1.0/radius; }

void setRadius (double r) { radius = r; } double getRadius () { return radius; }}

Using the get/set methodspublic class CircleTest {


Circle c = new Circle();

c.setRadius (1.0);

System.out.println (c.computeArea());

System.out.println (c.getRadius());

}}public class Circle { private double radius = 1.0; final static double PI = 3.1415926536;


double oneOverRadius() { return 1.0/radius; }

void setRadius (double r) { radius = r; } double getRadius () { return radius; }}Here a method is invokedHere the change to radius occurs

Wait! Another problem!public class CircleTest {


Circle c = new Circle();

c.setRadius (0.0);

System.out.println (c.oneOverRadius());

}}Here is the problem now

This problem is easily fixedChange the setRadius method to the following

void setRadius (double r) {if ( r > 0.0 ) radius = r;else radius = 1.0;}

Now there is (almost) no way for code outside the Circle class to change the radius to zero

Visibilities in JavaThere are four visibilities:private: Only code within the same class can access the field or methodNote: access means reading or writing the field, or invoking the method

public: Any code, anywhere, can access the field or method

protected: Used with inheritanceWe wont get to that this semester

default: Almost the same as publicThis is the default (duh!)Note that it cant be specified like the others

A few notes on visibilitiesYou can NOT specify visibilities for method variablesAny method variable can only be accessed within that methodThink of it as public within the method (after its defined) and private outside the method

You can also specify visibilities for methods and classesWe wont get to that in this course

Quick surveyI understand visibilities in JavaVery wellWith some review, Ill be goodNot reallyNot at all

Todays demotivators

Overriding methods (and constructors)Consider the following code:

Circle c1 = new Circle (); Circle c2 = new Circle (2.0);

Java knows which constructor to call by the list of parametersThis is called overloadingMeaning it means multiple things, depending on the context

Weve seen overloading before:3+4Performs integer addition3.0+4.0Performs floating-point addition3+4Performs string concatenation

The + operator is overloadedCreates a Circleof radius 1.0Creates a Circleof radius 2.0

Overriding methods (and constructors), take 2The following Circle constructors would not be allowed:We are assuming PI is not final for this example

public Circle() { radius = 1.0; }

public Circle (double r) { radius = r; }

public Circle (double p) { PI = p; }When Circle(1.0) is called, which one is meant?

Using mutators in the constructorOur second constructor has a problem:

public Circle (double r) { radius = r; }

Consider the following code:

Circle c = new Circle (0.0); System.out.println (c.oneOverRadius());The method is dividing by zero (again)

Using mutators in the constructorThis is easily fixed!

Our revised constructors:

public Circle() { setRadius (1.0); }

public Circle (double r) { setRadius (r);}

The mutator will properly set the radius (and wont set it to zero)

Why we always use the mutatorsConsider a modified version of our circle class:

class Circle {double radius;double diameter;String size;

// ...

Our mutator now looks like this:

Thats a lot of code to copy if you decide not to call the mutator!void setRadius (double r) { if ( radius

End of lecture on 16 March 2005Also did the first 20 (or so) slides of the slide set for chapter 6

Back to the static discussionRemember that there is one (and only one) static PI field, regardless of how many objects are declared

Consider the following method:

double getPI() { return PI;}

It doesnt read or modify the state of any objectIn this example, it doesnt read/write the radius

In fact, that particular method doesnt care anything about the objects declaredIts only accessing a static field

Make getPI() staticConsider the following:

static double getPI() { return PI; }

As the method is static, it can ONLY access static fields

A static method does not care about the state of an objectExamples: Math.sin(), Math.tan(), Math.cos()They dont care about the state of any Math objectThey only perform the computation

Invoking static methodsAs with static fields, they can be called using either an object or the class name:

Circle c = new Circle(); System.out.println (c.getPI()); System.out.println (Circle.getPI());

Static methods are also called class methods

static methods and non-static fieldsConsider the following (illegal) Circle method:

static double getRadius() { return radius; }

And the code to invoke it:

public static void main (String[] args) { Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); System.out.println (Circle.getRadius()); }

What happening in memoryThere are 4 Circle objects in memory

Which radius field does Circle.getRadius() want?There are 1 million Circle objects in memoryThere are no Circle objects in memoryc1000000c4PI3.1415926536

The main static lessonA static method cannot access or modify the state of the object it is a part of

If you remember nothing else about static methods, remember this!

static and non-static rulesNon-static fields and methods can ONLY be accessed by the object nameStatic fields and methods can be accessed by EITHER the class name or the object nameNon-static methods can refer to BOTH static and non-static fieldsStatic methods can ONLY access static fields of the class they are part of

Back to our main() method public static void main (String[] args)

Any code anywherecan call this methodIts a static method:Cant access non-static fields or methods directlyCan be called only by the class nameThe method does not return a valueWell learn about arrays in chapter 8

Quick surveyI felt I understood the material in this slide setVery wellWith some review, Ill be goodNot reallyNot at all

Quick surveyThe pace of the lecture for this slide set wasFastAbout rightA little slowToo slow

Quick surveyHow interesting was the material in this slide set? Be honest!Wow! That was SOOOOOOO cool!Somewhat interestingRather boringZzzzzzzzzzz

Sidewalk chalk guySource: http://www.gprime.net/images/sidewalkchalkguy/

decisions, decisions, decisions

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