1. Programming and Problem Solving With Java Copyright 1999, James M. Slack Chapter 7 Writing Your Own Classes Writing New Classes Parameters Example: A Money Class Writing Well-Designed Classes Class Methods and Variables Computer Networks

2. Programming and Problem Solving With Java 2 Writing New Classes  Package related group of methods  Example: Math class  Use methods with name of the class int x = Math.abs(y);  Make a new data type  Example: Turtle class  Make an object of the class Turtle myTurtle = new Turtle();  Use methods with objects myTurtle.move(100);

3. Programming and Problem Solving With Java 3 Writing New Classes  To model real-world object that doesn't correspond to existing class or type, either  Adjust view of real-world object: make it fit into existing class or type  Create a new type that fits the real-world object

4. Programming and Problem Solving With Java 4 Writing New Classes  Adjust view of real-world object: make it fit into existing class or type  Example: use double to store money  Probably not a perfect fit  Quick and simple for small programs  Resulting program not as easy to understand

5. Programming and Problem Solving With Java 5 Writing New Classes  Create a new type that fits the real-world object  More effort initially  Resulting program easier to understand  Usually the best approach for most programs

6. Programming and Problem Solving With Java 6 Writing New Classes  First step for class design: choose operations (methods)  Helps specify purpose of class  Example: suppose no Integer or int in Java  Operations for a new Integer class  add()  subtract()  multiply()  divide()  remainder()  set()  get()

7. Programming and Problem Solving With Java 7 Writing New Classes  Class operations provide access to object internals  Can't get to private parts of object directly  Benefits  Can test operations as we write them  Can substitute a new implementation  Class provides organization to program  Can reuse the class Object internals Operations Method call

8. Programming and Problem Solving With Java 8 Writing New Classes: Defining  Class is template for objects  Named group of methods and variables  Objects share methods  Each object has copy of variables class Counter { // Constructor: initializes the counter // to 0 public Counter() { value = 0; } // increment: Adds one to the counter's // value public void increment() { value++; } // getValue(): Returns the value of the // counter public int getValue() { return value; } // toString: Returns the value of the // counter as a string public String toString() { return String.valueOf(value); } private int value; }

9. Programming and Problem Solving With Java 9 Writing New Classes: Defining // Demonstrates use of the Counter class. import Counter; public class DemonstrateCounter { public static void main(String[] args) { // Make a new Counter object Counter aCounter = new Counter(); // Show the initial value of the object System.out.println("Initial value of counter is " + aCounter); // Loop ten times, incrementing the aCounter object // each time System.out.println("Starting loop..."); for (int i = 1; i <= 10; i++) { // Add one to the Counter object aCounter.increment(); System.out.println(" Counter is now " + aCounter); } System.out.println("Loop completed"); // Show the final value of the object int aCounterValue = aCounter.getValue(); System.out.println("Final value of counter is " + aCounterValue); }

10. Programming and Problem Solving With Java 10 Writing New Classes: Methods  Method: named group of executable statements  Belongs to a class  Can access other methods and variables of the class  Method signature  Access (public, private)  Return type (or void)  Method name  Parameters  No two methods with same signature in one class // increment: Adds one to the // counter's value public void increment() { value++; } Method signature

11. Programming and Problem Solving With Java 11 Writing New Classes: Methods  void Method  No return value  Specify with "void" in signature  Definition (no parameters) // Comment that describes the method public void nameOfMethod() { statements; }  Use  Use as executable statement object.nameOfMethod();  Example definition class Counter {... // increment: Adds one to the counter's value public void increment() { value++; }... }  Example use Counter aCounter = new Counter(); aCounter.increment();

12. Programming and Problem Solving With Java 12 Writing New Classes: Methods  Typed Method  Has return value  Specify return type in signature  Definition (no parameters) // Comment that describes the method public type nameOfMethod() { statements; return expression; }  Use  Use in an expression type x = object.nameOfMethod();  Example definition class Counter {... // getValue: Returns the value // of the counter public int getValue() { return value; }... }  Example use Counter aCounter = new Counter(); int aCounterValue = aCounter.getValue();

13. Programming and Problem Solving With Java 13 Writing New Classes: Methods  The toString() method  Java uses toString() to convert objects to String  Useful when displaying object values  Without toString() SomeClass anObject = new SomeClass(); System.out.println(anObject);  With toString() Counter aCounter = new Counter(); System.out.println(aCounter); No conversion to String: SomeClass has no toString() method SomeClass@1cc803 0 Automatic conversion to String: Counter has a toString() method

14. Programming and Problem Solving With Java 14 Writing New Classes: Constructor  Constructor  Special method that initializes an object's variables  Has same name as class, no return type  Computer executes on object when it creates the object  Example // Constructor: initializes the counter // to 0 public Counter() { value = 0; }  Use of constructor Counter aCounter = new Counter();  Default constructor : no parameters  Automatic constructor: default (does nothing) Use of constructor

15. Programming and Problem Solving With Java 15 Parameters  Can send information to a method System.out.println("Hello"); int x = Math.abs(y); myTurtle.turnRight(90);  Parameter  Like local variable, but gets initial value from caller  Actual parameter  Value passed to method  Also called argument  Formal parameter  Name used in method  Also called parameter

16. Programming and Problem Solving With Java 16 Parameters  Actual parameter DemonstrateParameter myObject = new DemonstrateParameter(); myObject.voidMethod(123);  Formal parameter class DemonstrateParameter { // voidMethod: Displays the value of aParameter public void voidMethod(int aParameter) { System.out.println("voidMethod argument is " + aParameter); } // typedMethod: Displays the value of aParameter, then // returns aParameter + 1 public int typedMethod(int aParameter) { System.out.println("typedMethod argument is " + aParameter); return aParameter + 1; } Value 123 passed to voidMethod()

17. Programming and Problem Solving With Java 17 Parameters: More than One  Method can have any number of parameters class DemonstrateTwoParameters { // displaySum: Displays the sum of its two arguments public void displaySum(int firstParameter, int secondParameter) { System.out.println("The sum of " + firstParameter + " and " + secondParameter + " is " + (firstParameter + secondParameter)); } public class TestDemonstrateTwoParameters { public static void main(String[] args) { DemonstrateTwoParameters myObject = new DemonstrateTwoParameters(); myObject.displaySum(100, 200); } Values passed in order to displaySum()

18. Programming and Problem Solving With Java 18 Parameters: More than One  Rules with parameters  Number of actual parameters = number of formal parameters  Type of each actual parameter = type of corresponding formal parameter  Examples  Signature public void displaySum(int firstParameter, int secondParameter)  Valid call myObject.displaySum(100, 200);  Invalid calls myObject.displaySum(200); // Wrong: Not enough actuals myObject.displaySum(100, 200, 300); // Wrong: Too many actuals myObject.displaySum(123.45, 67.1); // Wrong: Needs two integers

19. Programming and Problem Solving With Java 19 Constructors with Parameters  Constructor can have parameters public class Counter { // Constructor: initializes the counter to the given argument public Counter(int value) { this.value = value; }... }  Must use parameter when creating object import Counter; public class DemonstrateCounter { public static void main(String[] args) { Counter firstCounter = new Counter(0); // Start counter at 0 Counter secondCounter = new Counter(12); // Start counter at 12 firstCounter.increment(); secondCounter.increment();... }

20. Programming and Problem Solving With Java 20 Constructors with Parameters  Can make parameter optional  Provide two constructors: one with parameter, one without (default constructor) public class Counter { // Default Constructor: initializes the counter to 0 public Counter() { value = 0; } // Constructor: initializes the counter to the given argument public Counter(int value) { this.value = value; }... }

21. Programming and Problem Solving With Java 21 Constructors with Parameters  Better way to write two constructors  Have one do the work, the other calls it  Makes code more centralized -- easier to manage public class Counter { // Default Constructor: initializes the counter to 0 public Counter() { this(0); } // Constructor: initializes the counter to the given argument public Counter(int value) { this.value = value; }... } How to call one constructor from another

22. Programming and Problem Solving With Java 22 Constructors with Parameters  Java compiler knows which constructor to use by the arguments public class DemonstrateCounter { public static void main(String[] args) { Counter firstCounter = new Counter(); // Start counter at 0 Counter secondCounter = new Counter(12); // Start counter at 12 firstCounter.increment(); secondCounter.increment();... } Use default constructor Use constructor with argument

23. Programming and Problem Solving With Java 23 Example: Money Class  Floating-point types not appropriate for currency  Round-off error unacceptable  Design our own Money class with these operations  getDollars()Return dollars of this Money object  getCents()Return cents of this Money object  add() Return this Money object + another  subtract()Return this Money object - another  multiply()Return this Money object * integer  equal()Return true if this object == another  less()Return true if this object < another  toString()Return value as a String  setAmount()Change value of this Money object  Constructors

24. Programming and Problem Solving With Java 24 Example: Money Class  Want result to be like "Money calculator"  User uses operations, not internals

25. Programming and Problem Solving With Java 25 Example: Money Class  Example of Money class use // Demonstrates use of the Money class. The program // displays the sum of 12.50 and 3.75. import Money; public class DemonstrateMoney { public static void main(String[] args) { Money total; Money firstValue = new Money(12, 50); Money secondValue = new Money(3, 75); // Compute the sum of 12.50 and 3.75 total = firstValue.add(secondValue); System.out.println("12.50 + 3.75 is " + total); }  Shorter version Money total = new Money(12,50).add(new Money(3,75)); System.out.println("12.50 + 3.75 is " + total);

26. Programming and Problem Solving With Java 26 Example: Money Class (1) // The Money class. Each Money object stores dollars // and cents. public class Money { // Default Constructor: Initializes a new Money object to 0 public Money() { this(0, 0); } // Constructor: Initializes a new Money object to the given // amount public Money(int dollars, int cents) { setAmount(dollars, cents); } // setAmount: Assigns a new value to a Money object public void setAmount(int dollars, int cents) { this.dollars = dollars; this.cents = cents; normalize(); }

27. Programming and Problem Solving With Java 27 Example: Money Class (2) // getDollars: Returns the number of dollars in this object public int getDollars() { return dollars; } // getCents: Returns the number of cents in this object public int getCents() { return cents; } // toString: Converts the value in this object to string // format, for example: 123.45 // If the value is negative, displays the // negative sign after the number: 123.45- public String toString() { String result = Math.abs(dollars) + "."; if (Math.abs(cents) < 10) { result = result + "0"; } result = result + Math.abs(cents); if (dollars < 0 || cents < 0) { result = result + "-"; } return result; }

28. Programming and Problem Solving With Java 28 Example: Money Class (3) // add: Returns the sum of this Money object and another Money // object public Money add(Money otherMoneyObject) { return new Money(dollars + otherMoneyObject.getDollars(), (cents + otherMoneyObject.getCents())); } // subtract: Returns the difference of this Money object and // another Money object public Money subtract(Money otherMoneyObject) { return new Money(dollars - otherMoneyObject.getDollars(), (cents - otherMoneyObject.getCents())); } // multiply: Returns the product of this Money object and an // integer multiplier public Money multiply(int multiplier) { return new Money(dollars * multiplier, (cents * multiplier)); }

29. Programming and Problem Solving With Java 29 Example: Money Class (4) // equals: Returns true if this Money object is equal to // another, otherwise false public boolean equals(Money otherMoneyObject) { return dollars == otherMoneyObject.getDollars() && cents == otherMoneyObject.getCents(); } // less: Returns true if this Money object is less than another, // otherwise false public boolean less(Money otherMoneyObject) { return dollars < otherMoneyObject.dollars || (dollars == otherMoneyObject.getDollars() && cents < otherMoneyObject.getCents()); }

30. Programming and Problem Solving With Java 30 Example: Money Class (5) // normalize: Adjusts the dollars and cents so the cents is // between +0 and +99. private void normalize() { dollars = dollars + cents / 100; cents = cents % 100; if (dollars > 0 && cents < 0) { dollars--; cents = cents + 100; } else if (dollars 0) { dollars++; cents = cents - 100; } // Instance variables private int dollars; private int cents; }

31. Programming and Problem Solving With Java 31 Example: Money Class  Another example of use // Another demonstration of the Money class. Displays // the result of ((12.50 + 3.75) - 20.00) * 2. import Money; public class DemonstrateMoney2 { public static void main(String[] args) { Money firstValue = new Money(12,50); Money secondValue = new Money(3,75); Money thirdValue = new Money(20,0); // Add 12.50 and 3.75 Money sum = firstValue.add(secondValue); // Subtract 20.00 from the sum Money difference = sum.subtract(thirdValue); // Multiply the difference by 2 Money result = difference.multiply(2); // Display the result System.out.println("((12.50 + 3.75) - 20.00) * 2) is " + result); }

32. Programming and Problem Solving With Java 32 Example: Money Class  Same example, but without so many variables // Another demonstration of the Money class. Displays // the result of ((12.50 + 3.75) - 20.00) * 2. Doesn't use any // for literal values or intermediate results. import Money; public class DemonstrateMoney3 { public static void main(String[] args) { // Display ((12.50 + 3.75) - 20.00) * 2 Money result = new Money(12,50).add(new Money(3,75)).subtract(new Money(20,0)).multiply(2); System.out.println("((12.50 + 3.75) - 20.00) * 2) is " + result); }

33. Programming and Problem Solving With Java 33 Example: Money Class  Bank balance keeper // This program lets you deposit and withdraw // money from a bank account. It makes sure you don't withdraw // more than the amount of money currently in the bank. // This program demonstrates the use of the Money class. import Money; import Keyboard; public class BankBalance { public static void main(String[] args) throws java.io.IOException { Money balance = new Money(0, 0); char selection; int dollars; int cents; System.out.println("--- Bank Balance Keeper ---"); System.out.println(); selection = Keyboard.readChar("Deposit Withdraw Quit: ", "dwq");

34. Programming and Problem Solving With Java 34 Example: Money Class while (Character.toLowerCase(selection) != 'q') { switch (Character.toLowerCase(selection)) { case 'd': System.out.println("Enter amount to deposit"); dollars = Keyboard.readInt("Dollars: "); cents = Keyboard.readInt("Cents: "); balance = balance.add(new Money(dollars, cents)); break; case 'w': System.out.println("Enter amount to withdraw"); dollars = Keyboard.readInt("Dollars: "); cents = Keyboard.readInt("Cents: "); if (balance.less(new Money(dollars, cents))) { System.out.println("Error: Can't withdraw more than" + " balance"); } else { balance = balance.subtract(new Money(dollars, cents)); } break; default: System.out.println("Error in switch statement"); break; }

35. Programming and Problem Solving With Java 35 Example: Money Class System.out.println("Current balance is " + balance); System.out.println(); selection = Keyboard.readChar("Deposit Withdraw Quit: ", "dwq"); }  Example output --- Bank Balance Keeper --- Deposit Withdraw Quit: d Dollars: 75 Cents: 40 Current balance is 75.40 Deposit Withdraw Quit: d Dollars: 20 Cents: 15 Current balance is 95.55 Deposit Withdraw Quit: w Dollars: 100 Cents: 0 Error: Can't withdraw more than balance Current balance is 95.55 Deposit Withdraw Quit: q

36. Programming and Problem Solving With Java 36 Writing Well-designed Classes  Well-designed class should look like a type  Set of private values  Set of public operations on those values  Ideal class is  Complete : has all necessary operations  General : is useful in many applications

37. Programming and Problem Solving With Java 37 Writing Well-designed Classes  What not to do: this program requires poorly-designed Money class // This program lets you deposit and withdraw // money from a bank account. It makes sure you don't withdraw // more than the amount of money currently in the bank. // NOTE: This program demonstrates an inappropriate design of // an abstract data type. public class BankBalance { public static void main(String[] args) throws java.io.IOException { Money balance = new Money(0, 0); char selection; System.out.println("--- Bank Balance Keeper ---"); System.out.println(); selection = Keyboard.readChar("Deposit Withdraw Quit: ", "dwq");

38. Programming and Problem Solving With Java 38 Writing Well-designed Classes  What not to do (continued) while (Character.toLowerCase(selection) != 'q') { switch (Character.toLowerCase(selection)) { case 'd': balance.add(); break; case 'w': balance.subtract(); break; default: System.out.println("Error in switch statement"); break; } System.out.println("Current balance is " + balance); System.out.println(); selection = Keyboard.readChar("Deposit Withdraw Quit: ", "dwq"); }  Poor design here requires poor design of Money class Poor design

39. Programming and Problem Solving With Java 39 Writing Well-designed Classes  Base Money class on just this program  Program doesn't need getDollars(), getCents(), multiply(), less(), equals(): leave them out of Money class  add() method should ask user for input value, add that value to this object  subtract() method should ask user for input value, subtract that value from this object only if result >= 0  Problems  Incomplete  Specific to the application (not general)

40. Programming and Problem Solving With Java 40 Writing Well-designed Classes  Changes to the Money class add() method // add: Asks the user to enter an amount to deposit, then adds // what the user enters to the value of this object public void add() throws java.io.IOException { System.out.println("Enter amount to deposit"); int dollars = Keyboard.readInt("Dollars: "); int cents = Keyboard.readInt("Cents: "); setAmount(this.dollars + dollars, this.cents + cents); }  add() does more than add  Includes asking user for input  Not useful in other programs  Probably not useful in other parts of this program!

41. Programming and Problem Solving With Java 41 Writing Well-designed Classes  Changes to the Money class subtract() method // subtract: Asks the user to enter an amount to withdraw, then // subtracts what the user enters from the value of this // object. Won't let the user subtract more than the // current value of this object public void subtract() throws java.io.IOException { System.out.println("Enter amount to withdraw"); int dollars = Keyboard.readInt("Dollars: "); int cents = Keyboard.readInt("Cents: "); if (this.dollars < dollars || (this.dollars == dollars && cents < cents)) { System.out.println("Error: Can't withdraw more than" + " balance"); } else { setAmount(this.dollars - dollars, this.cents - cents); }  Not useful in other programs or other parts of this program!

42. Programming and Problem Solving With Java 42 Writing Well-designed Classes  Write a method specification before writing the method  Specification includes answers to  What will the method do?  What is the name of the method?  What assumptions does the method make?  What information does the method need?

43. Programming and Problem Solving With Java 43 Writing Well-designed Classes  Method specification example: Counter.increment()  What will the method do?  Our method will add one to the counter's value.  What is the name of the method?  Verb usually makes a good method name  "increment"  What assumptions does the method make?  None for this method  What information does the method need?  No parameters  Descriptive comment: // increment: Adds one to the counter's value

44. Programming and Problem Solving With Java 44 Class Methods and Variables  Class (static) vs. instance variables  Instance variable: each instance has its own copy  Class variable: the class has one copy for all instances  Example class Counter { public Counter() { value = 0; numCounters++; }... private int value; private static int numCounters = 0; }  Can use instance variables  In instance methods only  Can use class variables  In instance methods  In class methods

45. Programming and Problem Solving With Java 45 Class Methods and Variables  Class (static) vs. instance methods  Instance methods can access instance variables, class variables, instance methods, class methods  Class methods can access class variables and methods  Uses of class methods  Provide access to class variables without using an object class Counter {... public static int getNumCounters() { return numCounters; } }... System.out.println("Number of counters: " + Counter.getNumCounters());  Package a related group of methods (Math, Keyboard) without objects

46. Programming and Problem Solving With Java 46 The Keyboard Class  The Keyboard class is group of related methods  No object needed to use Keyboard class methods int name = Keyboard.readString("Enter your name: "); int age = Keyboard.readInt("Enter your age: ");  Keyboard class outline  Need BufferedReader and NumberFormat objects -- both are static public class Keyboard {... private static BufferedReader inputStream = new BufferedReader(new InputStreamReader(System.in)); private static NumberFormat aNumberFormatter = NumberFormat.getInstance(); }

47. Programming and Problem Solving With Java 47 The Keyboard Class  The readString() methods  readString() with a prompt // readString: Displays the given prompt, then waits for the // user to type a line, then returns all characters // the user typed as a string. public static String readString(String prompt) throws java.io.IOException { System.out.print(prompt); System.out.flush(); return inputStream.readLine(); }  readString() with no prompt (calls the other one) // readString: (Final version) Waits for the user to type a line, // then returns all characters the user typed as a // string. public static String readString() throws java.io.IOException { return readString(""); }

48. Programming and Problem Solving With Java 48 The Keyboard Class  The readLong() method // readLong: (First version) Displays the given prompt, then waits // for the user to type a number, then returns that // number as a long. public static long readLong(String prompt) throws java.io.IOException, ParseException { System.out.print(prompt); System.out.flush(); return aNumberFormatter.parse(inputStream.readLine()).longValue(); }  Doesn't handle non-numeric input  Throws ParseException instead (confusing to beginners)  Better to ask user to re-enter number Enter a number: 123x Please enter an integer Enter a number: _

49. Programming and Problem Solving With Java 49 The Keyboard Class  Catch ParseException error in readLong() method  Use try-catch block  try part: code that may throw an exception  catch part: code for handling the exception, if it occurs  Example NumberFormat formatter = NumberFormat.getInstance(); try { System.out.print("Enter a long value: "); System.out.flush(); value = formatter.parse(inputStream.readLine()).longValue(); } catch (ParseException e) { System.out.println("Not a long -- setting to 0"); value = 0; }

50. Programming and Problem Solving With Java 50 The Keyboard Class  The readLong() method, handles ParseExceptions // readLong: (Final version) Displays the given prompt, then waits // for the user to type a number, then returns that // number as a long. If the user enters an invalid // response, prompts the user to retype the response. public static long readLong(String prompt) throws java.io.IOException { long value = 0; boolean readAgain; do { try { readAgain = false; value = aNumberFormatter.parse(readString(prompt)).longValue(); } catch (ParseException e) { System.out.println("Please enter an integer"); readAgain = true; } } while (readAgain); return value; } Use readString()

51. Programming and Problem Solving With Java 51 The Keyboard Class  The readLong() method for ranges // readLong: Displays the given prompt, then waits for the user // to type a number, then returns that number as a long. // If the user enters an invalid response, prompts the // user to retype the response. If the user types a value // outside the range from lowValue to highValue, prompts // the user to retype the response. // NOTE: Assumes highValue >= lowValue public static long readLong(String prompt, int lowValue, int highValue) throws java.io.IOException { long value = readLong(prompt); while (value highValue) { System.out.println("Please enter an integer between " + lowValue + " and " + highValue); value = readLong(prompt); } return value; }  The readLong() method with no parameters // readLong: Waits for the user to type a number, then returns that // number as a long. If the user enters an invalid // response, prompts the user to retype the response. public static long readLong() throws java.io.IOException { return readLong(""); } Use the other readLong()

52. Programming and Problem Solving With Java 52 The Keyboard Class  Three versions of readLong  No parametersi = readLong();  Prompti = readLong("Value: ");  Prompt and rangei = readLong("Value: ", 0, 100);  Can use readLong() to write  readInt() (3 versions)  readShort() (3 versions)  readByte() (3 versions)  These are all integer types  long type has widest range - just narrow it for other types

53. Programming and Problem Solving With Java 53 The Keyboard Class // readInt: Waits for the user type a number, then returns that // number as an int. If the user enters an invalid // response, prompts the user to retype the response. public static int readInt() throws java.io.IOException { return (int) readLong("", Integer.MIN_VALUE, Integer.MAX_VALUE); } // readInt: Displays the given prompt, then waits for the user // to type a number, then returns that number as an int. // If the user enters an invalid response, prompts the // user to retype the response. public static int readInt(String prompt) throws java.io.IOException { return (int) readLong(prompt, Integer.MIN_VALUE, Integer.MAX_VALUE); } // readInt: Displays the given prompt, then waits for the user // to type a number, then returns that number as an int. // If the user enters an invalid response, prompts the // user to retype the response. If the user types a value // outside the range from lowValue to highValue, prompts // the user to retype the response. // NOTE: Assumes highValue >= lowValue public static int readInt(String prompt, int lowValue, int highValue) throws java.io.IOException { return (int) readLong(prompt, lowValue, highValue); }

54. Programming and Problem Solving With Java 54 Class Methods in App. Class  Class method can call other class methods without using an object  Example (no objects!) import Keyboard; class Test { public static void main(String[] args) throws java.io.IOException { int value = Keyboard.readInt("Enter value: "); System.out.println("You entered " + value); }  Can carry this further... All methods in Keyboard class are class methods main is a class method

55. Programming and Problem Solving With Java 55 Class Methods in App. Class  Can organize code (without objects) using class methods  Example public class DemonstrateClassMethodCalls { public static void firstMethod() { System.out.println("In firstMethod()"); } public static void secondMethod() { System.out.println("In secondMethod() " + "- now calling firstMethod()"); firstMethod(); System.out.println("Back in secondMethod()"); }  Output from secondMethod() In secondMethod() - now calling firstMethod() In firstMethod() Back in secondMethod()

56. Programming and Problem Solving With Java 56 Class Methods in App. Class  main() is also a class method  main() can call other class methods in the application class (the same class as main)  Can organize program with methods, but without objects  Divide work of methods in application class by purpose  Process called stepwise refinement or functional decomposition  The primary development technique in non-object-oriented programming languages

57. Programming and Problem Solving With Java 57 Stepwise Refinement

58. Programming and Problem Solving With Java 58 Stepwise Refinement  Example: Keep track of person's checking account  Divide into subproblems  Get new information (checks and deposits) from the user  Change existing information  Store the information in a file on the hard disk drive  Read information from a file  Balance the checkbook  Each subproblem may be too complex to solve without further planning  Keep dividing until each sub-sub-...-subproblem is trivial

59. Programming and Problem Solving With Java 59 Stepwise Refinement  Example: divide "Get new information (checks and deposits) from the user"  Read the check or deposit number from the user  Read the date from the user  Read the payee information from the user  Read the amount of the check or deposit  Read the income or expense category of the check or deposit  Store the information  Turn each step into a static method // getCheckOrDepositNumber: Returns the check or deposit number // entered by the user static int getCheckOrDepositNumber() throws java.io.IOException { return (Keyboard.readInt("Enter the check or deposit number: ")); }

60. Programming and Problem Solving With Java 60 Stepwise Refinement  Result of stepwise refinement: decomposition hierarchy  Shows how problem is divided into subproblems

61. Programming and Problem Solving With Java 61 Stepwise Refinement: Use in OO  Stepwise refinement different from object-oriented design  Stepwise refinement: emphasis on division of tasks  Object-oriented design: emphasis on objects and their relationships  Can use stepwise refinement in object-oriented design  Object-oriented design identifies classes and methods first  When methods identified, may want to divide the work of some methods with "helper" methods

62. Programming and Problem Solving With Java 62 Stepwise Refinement: Use in OO  Example: program to compute employee pay  Start with object-oriented design to get Employee class setName() setHours() setHourlyRate() setPay() getName() getHours() getHourlyRate() getPay()  Use stepwise refinement to divide main() by task readEmployeeInfo() computePay() displayPay()

63. Programming and Problem Solving With Java 63 Stepwise Refinement: Use in OO  Employee pay program without Employee class // Computes the pay for an employee by multiplying the // hours worked by the rate per hour. This program is an example of // using class methods in the application class to simplify main(). import Employee; import Keyboard; public class EmployeePay { // readEmployeeInfo: Read the employee's information from // the user static void readEmployeeInfo(Employee anEmployee) throws java.io.IOException { anEmployee.setName(Keyboard.readString("Employee name: ")); anEmployee.setHours(Keyboard.readDouble("Hours: ")); anEmployee.setHourlyRate(Keyboard.readDouble("Rate: ")); }

64. Programming and Problem Solving With Java 64 Stepwise Refinement: Use in OO // computePay: Compute the pay, based on the employee's // information static void computePay(Employee anEmployee) { anEmployee.setPay(anEmployee.getHourlyRate() * anEmployee.getHours()); } // displayPay: Display the pay on the screen static void displayPay(Employee anEmployee) { System.out.println("Name: " + anEmployee.getName() + " Pay: " + anEmployee.getPay()); } public static void main(String[] args) { Employee anEmployee = new Employee(); System.out.println("--- Compute Employee Pay Program ---"); readEmployeeInfo(anEmployee); computePay(anEmployee); displayPay(anEmployee); }

65. Programming and Problem Solving With Java 65 Changing Formal Parameters  If formal parameter is object  Changing object's value changes actual parameter's  If formal parameter is primitive  Changing parameter's value doesn't affect actual parameter  Can use this techique to return values from method  Especially useful when method needs to return more than one value  Common in non-object-oriented programs  (If returning just one value, use return statement)

66. Programming and Problem Solving With Java 66 Changing Formal Parameters // change: Changes the value of anIntFormal and // aMoneyFormal to 0 public static void change(int anIntFormal, Money aMoneyFormal) { anIntFormal = 0; aMoneyFormal.setAmount(0,0); } public static void main(String[] args) { int anIntActual = 123; Money aMoneyActual = new Money(123,45); System.out.println("--- Before method call ---"); System.out.println("anIntActual: " + anIntActual); System.out.println("aMoneyActual: " + aMoneyActual); change(anIntActual, aMoneyActual); System.out.println("--- After method call ---"); System.out.println("anIntActual: " + anIntActual); System.out.println("aMoneyActual: " + aMoneyActual); } --- Before method call --- anIntActual: 123 aMoneyActual: 123.45 --- After method call --- anIntActual: 123 aMoneyActual: 0.00

67. Programming and Problem Solving With Java 67 Quadratic Equation: ax 2 +bx+c=0 Class Methods: Quadratic Eq.

68. Programming and Problem Solving With Java 68 Class Methods: Quadratic Eq.  Quadratic equation: ax 2 + bx + c = 0  a, b, c are numbers  value of x that makes equation true is root of equation  Solution to quadratic equation  Example: graph of -2 x 2 + 3 x + 4 = 0

69. Programming and Problem Solving With Java 69 Class Methods: Quadratic Eq.  Program to find roots of quadratic equation // Finds roots of a quadratic equation entered by the user. import Keyboard; public class SolveQuadraticEquation { // plusRoot: Returns the "plus" root of a quadratic equation. // Assumes b*b - 4*a*c >= 0 and that a != 0 static double plusRoot(double a, double b, double c) { double numerator, denominator; numerator = b + Math.sqrt(b * b - 4 * a * c); denominator = 2 * a; return numerator / denominator; } // minusRoot: Returns the "minus" root of a quadratic equation. // Assumes b*b - 4*a*c >= 0 and that a != 0 static double minusRoot(double a, double b, double c) { double numerator, denominator; numerator = -b + Math.sqrt(b * b - 4 * a * c); denominator = 2 * a; return numerator / denominator; }

70. Programming and Problem Solving With Java 70 Class Methods: Quadratic Eq. public static void main(String[] args) { System.out.println("--- Solve quadratic equation " + "ax^2 + bx + c = 0 ---"); System.out.println(); double a = Keyboard.readDouble("Enter value of a: "); double b = Keyboard.readDouble("Enter value of b: "); double c = Keyboard.readDouble("Enter value of c: "); System.out.println(); System.out.println("Equation is " + a + "x^2 + " + b + "x + " + c + " = 0"); System.out.println("One root is " + plusRoot(a, b, c)); System.out.println("Another root is " + minusRoot(a, b, c)); } --- Solve quadratic equation ax^2 + bx + c = 0 --- Enter value of a: -2 Enter value of b: 3 Enter value of c: 4 Equation is -2.0x^2 + 3.0x + 4.0 = 0 One root is -0.8507810593582121 Another root is 2.350781059358212

71. Programming and Problem Solving With Java 71 Computer Networks  A computer network is  A set of computers connected for the purpose of sharing information  A computer network consists of  Communications media  Devices  Software

72. Programming and Problem Solving With Java 72 Computer Networks  Internet Protocol (IP)  Protocol: set of rules that computers follow to communicate with each other  Computers on the Internet use IP  Internet is packet-switched network  Sends information like post office

73. Programming and Problem Solving With Java 73 Computer Networks  Packet  Works like a postal letter  Contains up to about 65,000 bytes (characters) of information  Message often consists of many packets  Packet header  First several bytes of the packet  Contains the address of the packet's recipient  Computers examine packet header and forward packet to intended recipient  No dedicated physical connection between source of packet and its intended recipient

74. Programming and Problem Solving With Java 74 Computer Networks  Circuit-switching network  Used in some computer networks  Works like the telephone network -- dedicated physical connection between computers

75. Programming and Problem Solving With Java 75 Computer Networks  IP makes sure packets arrive correctly  But that’s all it does  Transmission Control Protocol (TCP)  Breaks messages into packets before sending  Reassembles packets into messages at receiver  Rearrange order of received packets, if necessary  Disposes of duplicate packets, when necessary  TCP/IP: the combination of TCP and IP

76. Programming and Problem Solving With Java 76 Computer Networks  Internet Addresses  Each computer on the Internet has a unique address  Address is a 32-bit number consisting of four 8-bit bytes (each between 0 and 255)  Example: ACM’s address is 199.222.69.10  Address is hierarchical  First byte: which of 256 large networks  Last byte: a particular computer on a network  Current scheme running out of numbers  Scheduled for replacement with 128-bit numbering

77. Programming and Problem Solving With Java 77 Computer Networks  Domain Name System (DNS)  Numbered addresses convenient for computers  But, inconvenient for people  DNS allows users to use mnemonic names: www.acm.org instead of 199.222.69.10  DNS translates the mnemonic names into numbered addresses  DNS is a distributed database  User uses mnemonic name  Network consults the closest DNS table on the Internet to translate the name to a number

78. Programming and Problem Solving With Java 78 Computer Networks  DNS names hierarchical  But from right to left  Right-most part of the name is the top-level domain  Common top-level domains

79. Programming and Problem Solving With Java 79 Computer Networks  The World Wide Web  Very large, distributed collection of linked documents accessible by a Web browser  Web documents use Hypertext Markup Language (HTML)  HTML  Includes regular text, formatting tags, and links to other documents.  Uses Uniform Resource Locators (URLs) to specify documents

80. Programming and Problem Solving With Java 80 Computer Networks  URLs  Use default port, specific directory protocol://hostname/location/  Use default port, specific file protocol://hostname/location  Use specific port, specific directory protocol://hostname:port/location/  Use specific port, specific file protocol://hostname:port/location  Example URL http://www.acm.org/index.html  Protocols  http (Hypertext Transmission Protocol)  ftp (File Transfer Protocol)  mailto -- for sending mail messages  telnet -- for using another computer remotely.

81. Programming and Problem Solving With Java 81 Networks: Java Programming  Network programming in Java is relatively simple  java.net package has lots of net functionality  java.net.URL class  For storing URLs, like a File object stores file information  Example URL acmURL = new URL("http://www.acm.org");  Methods

82. Programming and Problem Solving With Java 82 Networks: Java Programming  Example of URL class // Demonstrates the methods in the URL class import Keyboard; import java.net.*; class DemonstrateURL { public static void main(String[] args) throws java.net.MalformedURLException, java.io.IOException { System.out.println("--- URL Demonstration ---"); do { System.out.println(); // Get the URL name from the user, make URL object URL loc = new URL(Keyboard.readString("Enter URL: ")); // Display information about the URL System.out.println("Protocol is " + loc.getProtocol()); System.out.println("Port is " + loc.getPort()); System.out.println("Host is " + loc.getHost()); System.out.println("File is " + loc.getFile()); } while (Keyboard.readChar("Another (y/n): ", "yn") == 'y'); }

83. Programming and Problem Solving With Java 83 Networks: Java Programming  Sample run Enter URL: http://www.acm.org Protocol is http Port is -1 Host is www.acm.org File is / Another (y/n): y Enter URL: ftp://www.acm.org:9999/myfile.txt Protocol is ftp Port is 9999 Host is www.acm.org File is /myfile.txt Another (y/n): y Enter URL: http://www.bogus.bogus/bogus/bogus.txt Protocol is http Port is -1 Host is www.bogus.bogus File is /bogus/bogus.txt Another (y/n): n

84. Programming and Problem Solving With Java 84 Networks: Java Programming  Simple way to get content of URL  URL.openStream() gives InputStream object connected to content  Use just about like System.in  Wrap InputStream object in BufferedReader, then use BufferedReader.readline()

85. Programming and Problem Solving With Java 85 Networks: Java Programming  Example of URL.openStream() // This program displays the contents of a URL on the screen import Keyboard; import java.net.*; import java.io.*; // Need this for BufferedReader class DisplayURLFile { public static void main(String[] args) throws java.net.MalformedURLException, java.io.IOException { System.out.println("--- URL Content Retriever ---"); System.out.println(); // Get the URL name from the user, make URL object URL loc = new URL(Keyboard.readString("Enter URL: ")); // Make a BufferedReader object, connect to URL BufferedReader input = new BufferedReader(new InputStreamReader(loc.openStream())); System.out.println("--- Content Follows ---"); System.out.println(); // Read and display all lines from URL String line = input.readLine(); while (line != null) { System.out.println(line); line = input.readLine(); }

86. Programming and Problem Solving With Java 86 Networks: Java Programming  Sample run --- URL Content Retriever --- Enter URL: http://www.acm.org --- Content Follows --- ACM Brings You the World of Computing (Rest of HTML document appears here)

87. Programming and Problem Solving With Java 87 Networks: Java Programming  Sockets  Allow two-way communication between computers  (URL class allows commication in one direction only)  Socket: connection to a port on another computer  Ports  Port specifies communication line into a computer  First 1024 port numbers usually reserved for system use  Default port numbers between 0 to 1023 http server: port 80 ftp server: port 21  For two-way communication, each computer should create socket connected to same port number

88. Programming and Problem Solving With Java 88 Networks: Java Programming  Socket programming  Server: computer that listens for connections  Client: requests connection from the server  How to write a server program  Make server listen ServerSocket myServerSocket = new ServerSocket(PORT_NUMBER);  It hears request Socket mySocket = myServerSocket.accept();  Create input or output streams (or both) to client BufferedReader input = new BufferedReader(new InputStreamReader( mySocket.getInputStream()));  Can read from client just like any other stream  Close everything when done input.close(); mySocket.close(); myServerSocket.close();

89. Programming and Problem Solving With Java 89 Networks: Java Programming  Example server program // A short demonstration of how to set up a server // process on a socket, then send listen for information sent // through the socket to this server process import java.net.*; import java.io.*; public class Server { static final int PORT_NUMBER = 9999; public static void main(String[] args) throws java.io.IOException { System.out.println("--- Server Process ---"); System.out.println(); ServerSocket myServerSocket = new ServerSocket(PORT_NUMBER); System.out.println("Ready for client connection..."); // Wait for the client to start communication Socket mySocket = myServerSocket.accept(); System.out.println("Client connected..."); System.out.println();

90. Programming and Problem Solving With Java 90 Networks: Java Programming  Example server program (continued) // Make a BufferedReader object, connect to socket BufferedReader input = new BufferedReader(new InputStreamReader( mySocket.getInputStream())); // Read and display all lines from socket System.out.println("Messages from client:"); String line = input.readLine(); while (line != null) { System.out.println(">> " + line); line = input.readLine(); } input.close(); mySocket.close(); myServerSocket.close(); }

91. Programming and Problem Solving With Java 91 Networks: Java Programming  How to write a client program  Request connection from server Socket mySocket = new Socket("www.myHost.edu", 1234);  Create input or output stream (or both) to server PrintWriter output = new PrintWriter(new OutputStreamWriter( mySocket.getOutputStream()), true);  Send information to server with print() or println()  Close everything when done output.close(); mySocket.close(); tells object to flush stream after every println()

92. Programming and Problem Solving With Java 92 Networks: Java Programming  Example client program // A short demonstration of how to connect a client // socket, then send information through the socket to the // server import Keyboard; import java.net.*; import java.io.*; public class Client { static final int PORT_NUMBER = 9999; public static void main(String[] args) throws java.io.IOException { // Display program title System.out.println("--- Client Process ---"); System.out.println(); // Get host address from user, connect socket to that // host at PORT_NUMBER Socket mySocket = new Socket(Keyboard.readString("Host: "), PORT_NUMBER); Note: same port number as server!

93. Programming and Problem Solving With Java 93 Networks: Java Programming  Example client program (continued) // Make an output stream, connect to the socket PrintWriter output = new PrintWriter(new OutputStreamWriter( mySocket.getOutputStream()), true); System.out.println("Connected to server..."); // Send some information to server String message = Keyboard.readString("Message to send: "); while (message != "") { output.println(message); message = Keyboard.readString("Message to send: "); } // Close the output stream and the socket output.close(); mySocket.close(); }

94. Programming and Problem Solving With Java 94 Networks: Java Programming  Sample run of server and client programs  Server (Unix machine) % java Server --- Server Process --- Ready for client connection... Client connected... Messages from client: >> I am typing this message on the client >> machine, and it will show up on the >> server. %  Client (Windows 95 machine) C:\> java Client --- Client Process --- Host: myserver Connected to server... Message to send: I am typing this message on the client Message to send: machine, and it will show up on the Message to send: server. Message to send: C:\>

95. Programming and Problem Solving With Java 95 Communications Media  Twisted pair wiring  Similar to phone wire  May be shielded -- protects from electromagnetic interference  Inexpensive, but slow  Coaxial cable (coax)  Similar to cable TV wire  Consists of wire surrounded by dielectric (insulation)  Medium expense and speed  Fiber optic cable  Consists of glass strands  Transmits light, not electricity  No electromagnetic interference  Most expensive, fastest

96. Programming and Problem Solving With Java 96 Communications Media  Can transmit signals via radio  Microwaves are a popular approach  Line-of-signt transmission

97. Programming and Problem Solving With Java 97 Communications Media  Satellites can transmit much farther than microwaves

98. Programming and Problem Solving With Java 98 Communications Media  Other wireless transmission  Conventional radio  Cellular telephones

99. Programming and Problem Solving With Java 99 Network Topology  Topology  Layout or organization of network connections  Most common topologies  Ring  Star  Bus

100. Programming and Problem Solving With Java 100 Network Topology  Ring topology (token ring)  Computers linked in a circle  Token constantly traveling around the ring  Token travels in only one direction  Computer must wait until it reads the token before it can send a message to another computer  When a computer reads the token, can append message to end of messages following the token

101. Programming and Problem Solving With Java 101 Network Topology  Star topology  Requires a central computer (server)  Central computer acts as the hub for all network communication  To send a message, computer sends packet to server, which routes message to destination  To read a message, computer waits for packets from the server

102. Programming and Problem Solving With Java 102 Network Topology  Bus topology  Most common topology today  Each computer connected to the bus (a cable)  To send a message, computer broadcasts packet on the bus  To read a message from the network, computer monitors all broadcast messages for any packets with its identifier