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Review : abstract

• abstract method– a method without implementation

• abstract class– an “incomplete” class– force concrete subclasses to

implement abstract methods (if any)– cannot be instantiated

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class Student{ : public String getGrade() { return “ERROR”; }}

class PassFailStudent extends Student{ : public String getGrade() { return (average > 70) ? “pass” : “fail”; }}

class LetterGradeStudent extends Student{ : public String getGrade() { if (average > 90) return “A”; else if (average > 70) return “B”; else if (average > 40) return “C”; else return “D”; }}

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Student students[] = new Student[numOfStudents];

// create students:if (enrolledAsPassFail) students[i] = new PassFailStudent(..);else students[i] = new LetterGradeStudent(..);

// prints the name and final grade of all students

for (int i = 0; i < numOfStudents;i++) System.out.println( students[i].getName() + “\t” + students[i].getGrade());

PassFailStudent

LetterGradeStudent

PassFailStudent

students

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abstract class Student{ : public abstract String getGrade();}

class PassFailStudent extends Student{ : public String getGrade() { return (average > 70) ? “pass” : “fail”; }}

class LetterGradeStudent extends Student{ : public String getGrade() { if (average > 90) return “A”; else if (average > 70) return “B”; else if (average > 40) return “C”; else return “D”; }}

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Interface

• “Pure” abstract class• Must contain only constant

members and abstract methods• Members are public final static

by default• Methods are public abstract by

default

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Implements

• an interface does not contain any implementation

• a class provides implementations• We say : a class implements an

interface

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interface Hero { boolean NEVER_DIES = true; void swim(); void fight(); void jump();};

class Jedi implements Hero { public void fight() { // swing light saber } public void swim() { ... } public void jump() { ... }}

class WesternHero implements Hero { public void fight() { // punch } public void swim() { ... } public void jump() { ... }}

interface A

nd

implements

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interface NumericSet { void add(Number n); void remove(Number n); void union(NumericSet n); void intersect(NumbericSet n); boolean isEmpty(); boolean contains(Number n); int size();}

interface Printable{ void print();}

More interface E

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class ArraySet implements NumericSet, Printable { public void add(Number n) { ... } public void remove(Number n) { ... } public void union(NumericSet n) { ... } public void intersect(NumbericSet n) { ... } public boolean isEmpty() { ... } public boolean contains(Number n) { ... } public int size() { ... } public void print() { ... }}M

ore implements E

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Differences

• a class can implement more than one interfaces

• A implements B =“A has the behaviors of B”

• a class can only extend one other class

• A extends B = “A is a kind of B”

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Extending Interface

• We can extend an interface• interface A extends B : B is

called superinterface• We can override methods in

superinterface• We can overload methods in

superinterface

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interface FlyingHero extends Hero{ void fly();}

interface RoboticHero extends Hero{ void fight(Weapon w); void swim() throws RustException;}

interface SortedSet extends NumericSet{ boolean isSorted(); void sort(); Number first(); Number last();}

interface DoubleClickable extends Clickable{ void doubleClicked();}

Exte

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ing interface

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Try it out yourself 1

• can we declare a class without abstract methods as “abstract” ?

abstract class A { int f() { return 1; } }

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Try it out yourself 2

• must we implement all abstract methods from the interface ?

interface I { int f(); int g(); }

class A implements I { int f() { returns 1; } }

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Try it out yourself 3

• can a class implement two interfaces with methods of the same name?

interface I1 { int f(); }interface I2 { int f(); }interface I3 { double f(); }class A implements I1, I2 { int f() { return 1; }}class B implements I1, I3 { int f() { return 1; } double f() { return 1.0; }}

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Bugs

• Programming Errors• First bug

– A moth stuck in a Harvard Mark II mainframe in 1947.

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Bugs are bad

• 1990 – AT&T long distance service failed for 9 hours and was traced to a single faulty line of code

• 1991 – Scud missile killed 28 soldiers because a bug caused the Patriot defense system to be off by 0.34 seconds

• 2000 – Y2K

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Today’s Lecture

• How to prevent bugs ?– Understand the problem– Understand Java– Follow good programming practices

• How to find bugs ?– Testing

• How to kill bugs ?

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Program Development

Design

Implement

Testing

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Program Design : Classes

Studentdouble averageGradeint grades[6]double weight[6]calcAverageGrade()getAverageGrade()

LetterGradeStudentprintGrade()

PassFailStudentprintGrade()

Coursedouble averageGradedouble maxdouble mindouble sumint numOfStudentStudent students[]calcAverageGrade()getAverageGrade()

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Program Design : Pseudocode

Average grade for studentsfor ( i = 0 .. 6) average += grade[i]*weight[i]return average

Average Grade for coursefor each student sum += student’s weight averagereturn sum/number of students;

Get inputget number of studentsfor i = 1 .. number of students get name get enrollment status get all six grades if enroll as pass fail then create a PassFailStudent else create a LetterGradeStudent

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Program Design : Data Flow

create a student

name

pass/fail ?

6 grades

calc student’s average grade

Student objget final grade

average grade

final grade

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Program Design : Control Flow

i = 0

i == # of students ?

get nameget statusget grades

increment i

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Good Program vs. Bad Program

• Easy to Read– Good Comments– Meaningful Names– Properly Indented– Blank Lines

• Well-Designed– Cover all cases– Anticipate Changes– Facilitate Reuse

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Very

Bad

Pro

gra

m// A program that reads some inputs and outputs some

stuffsclass P2Q3 {public static void main(String X[]) {int temp = 0, temp2 = 0, s = 0, c=0, M = 0, X[];TokenReader tr = new TokenReader();System.out.println(“number:”);int N=tr.ReadInt();X = new int[N];N = tr.ReadInt();while (N!=0){s+=N;if (N>M)M=N;X[temp] = N;temp++;System.out.println(“number”);N=tr.ReadInt();temp2++;}System.out.println(s + “ “ + temp2 + “ “ + temp);temp2 = temp1 = 0;while (temp2 < X.length) System.out.println(X[temp1])temp2++; temp1++;}}

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class P2Q3 { public static void main(String X[]) { // sum – total of values entered so far // max – maximum of values entered so far // count – number of values entered int sum = 0, count = 0, max = Integer.MAX_VALUE;

TokenReader tr = new TokenReader(); System.out.println(“Enter number :”); int input = tr.ReadInt();

// keep reading until user enters 0 while (input != 0){ sum += input; // compare current input with maximum so far. update // maximum so far if input is larger. if ( input < max ) { max = input; count++; }

System.out.println(“Enter number :”); input = tr.ReadInt(); } }}

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Good Design

• Anticipate Changes• Reusable• Encapsulation• Think “LEGO”

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public static void main(String args[]){ TokenReader in = new TokenReader(); int histogram = new int[10]; for (int i = 0; i < 10; i++) { histogram[i] = 0; } int input = in.readInt(); while (input != 0) { if (input > 1 && input <= 10) { histogram[0]++; } else if (input > 10 && input <= 20) { histogram[1]++; } else if (... : :

// smarter implementation // int n = (input – 1)/10; // histogram[n]++;

Bad

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What if ..

• create histogram for data between 1 .. 200 ?

• tally data for smaller intervals ?

1 – 5 | ** 6 – 10 | ***** 11 – 15 | * : : 196 – 200 | ***

• draw a histogram for average grade ?

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class Histogram { int numOfBuckets, bucketSize, buckets[];

public Histogram(int numOfBuckets, int bucketSize) { this.numOfBuckets = numOfBuckets; this.bucketSize = bucketSize; buckets = new int[numOfBucket]; for (int j = 0; j < buckets.length; j++) buckets[j] = 0; }

public void addData(int data) { buckets[(data-1)/bucketSize]++; }

public void print() { for (int j = 0; j < numOfBucket; j++) { System.out.println((j*bucketSize+ 1)+

“-” + ((j+1)*bucketSize))); for (int k = 0; k < buckets[j]; k++)

System.out.print(“*”); Sysmte.out.println(); } }} G

ood

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Bug Preventions

• code reuse – fewer lines of code to write, fewer

bugs • anticipate changes

– fewer lines of code to change,fewer bugs

• encapsulation– bugs are confined to one place, easier

to detect and fix.

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Good Programs

• Easy to read– blank lines– indentation– comments– meaningful names

• Easy to read, easy to spot bugs !

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Finding Bugs

• Wrong attitude : “My program works ! I am done.”• Did you test it with all possible

inputs ?– negative numbers ?– zero ?

• Did you test all possible paths of execution ?

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Component Testing

• Another motivation for encapsulations !

• Test each component separately• Make sure they worked before

using them

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Debugging Techniques : Think “high-level”

1. scan from left to right2. swap two adjacent elements

if they are out of order3. repeat until everything is in

order

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Debugging Techniques :Printout

• Print out your code• Spread it on a large table• Walkthrough your code• Draw diagrams• Make notes

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Debugging Techniques : Explain it to Someone Else

• Old Chinese Proverb :

“Onlookers see most of the game; Players see very little”

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Debugging Techniques :System.err.println

• Let you inspect the intermediate value of variables

53 2147483647 034 2147483647 010 2147483647 0 82 2147483647 0 72 2147483647 0

• Can you guess what is wrong now ?

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class P2Q3 { public static void main(String X[]) { : : // keep reading until user enters 0 while (input != 0){ sum += input; // compare current input with maximum so // far. update maximum so far if input // is larger. if ( input < max ) { max = input; count++; } System.err.println(input + “ “ + max + “ “

+ count); System.out.println(“Enter number :”); input = tr.ReadInt(); } }}

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Debugging Techniques : assert

• a method to make sure that your invariants are true.

void assert(boolean condition, String errorMessage) {

if (!condition) throw new Error(errorMessage);

}

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class Histogram {

int numOfBuckets; int bucketSize; int buckets[]; : :

public void addData(int data) { assert(data >= 0 && data < numOfBuckets*bucketSize, “data” + data + “is out of range.”); buckets[(data-1)/bucketSize]++; } : :}

Usin

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Debugging Techniques : assert

• The Error exception will cause the stack trace to be printed.

java.lang.Error: data 364 is out of rangeat java.lang.Throwable.<init>at java.lang.Error.<init>at Histogram.assertat Histogram.addDataat P2Q4.createHistogramat P2Q4.main

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Debugging Techniques :Debugger

breakpoint pause execution

stepincremental execution

continue resume execution

watchstop if value of a variable changes

call stacklist currently active frames

variablesinspect value of variables

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Summary

• Bug Prevention– understand the problem and

language – design before sit in front of computer– design for change/reuse

• Bug Discovery– test all flow of controls– test small components separately

before using them

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Summary

• Bug Termination– re-think your algorithm from a higher-

level– manually trace through your program– explain your program to others– System.err.println– assert– use a debugger