1. 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. 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);
Programming and Problem Solving With Java

3. Writing New Classes Existing New Class Class
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
Existing
Class
New
Class
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
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
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()
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
Method call
Operations
Object
internals
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;
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);
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
Method signature
// increment: Adds one to the
// counter's value
public void increment() {
value++; }
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();
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();
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
Automatic conversion to String:
Counter has a
toString() method
Programming and Problem Solving With Java

14. Writing New Classes: 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
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15. Parameters Can send information to a method Parameter Actual parameter
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
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16. Parameters Actual parameter Formal parameter Value 123 passed
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()
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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()
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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
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();
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;
...
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
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
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
Programming and Problem Solving With Java

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

25. Example: Money Class $$$ Example of Money class use Shorter version
// Demonstrates use of the Money class. The program
// displays the sum of and 3.75.
import Money;
public class DemonstrateMoney {
public static void main(String[] args)
Money total;
Money firstValue = new Money(12, 50);
Money secondValue = new Money(3, 75);
// Compute the sum of and 3.75
total = firstValue.add(secondValue);
System.out.println(" is " + total); }
Shorter version
Money total = new Money(12,50).add(new Money(3,75));
$$$
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();
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:
// If the value is negative, displays the
// negative sign after the number:
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;
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));
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()); $
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 && cents > 0)
dollars++;
cents = cents - 100;
// Instance variables
private int dollars;
private int cents; $$
Programming and Problem Solving With Java

31. Example: Money Class Another example of use
// Another demonstration of the Money class. Displays
// the result of (( ) ) * 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 and 3.75
Money sum = firstValue.add(secondValue);
// Subtract from the sum
Money difference = sum.subtract(thirdValue);
// Multiply the difference by 2
Money result = difference.multiply(2);
// Display the result
System.out.println("(( ) ) * 2) is "
+ result); }
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 (( ) ) * 2. Doesn't use any
// for literal values or intermediate results.
import Money;
public class DemonstrateMoney3 {
public static void main(String[] args)
// Display (( ) ) * 2
Money result = new Money(12,50).add(new Money(3,75))
.subtract(new Money(20,0)).multiply(2);
System.out.println("(( ) ) * 2) is "
+ result); }
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");
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");
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));
default:
System.out.println("Error in switch statement");
Programming and Problem Solving With Java

35. Example: Money Class Example output
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
Dollars: 20
Cents: 15
Current balance is 95.55
Deposit Withdraw Quit: w
Dollars: 100
Cents: 0
Error: Can't withdraw more than balance
Deposit Withdraw Quit: q
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
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");
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();
default:
System.out.println("Error in switch statement"); }
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
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)
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!
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!
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?
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
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
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
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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(); }
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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()
return readString("");
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: _
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;
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()
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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 < lowValue || 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()
return readLong("");
Use the other
readLong()
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52. The Keyboard Class Three versions of readLong
No parameters i = readLong();
Prompt i = readLong("Value: ");
Prompt and range i = 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
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)
return (int) readLong(prompt, Integer.MIN_VALUE, Integer.MAX_VALUE);
// 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)
return (int) readLong(prompt, lowValue, highValue);
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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
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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()
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
Programming and Problem Solving With Java

57. Stepwise Refinement
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
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: ")); }
Programming and Problem Solving With Java

60. Stepwise Refinement
Result of stepwise refinement: decomposition hierarchy
Shows how problem is divided into subproblems
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
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()
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: ")); }
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);
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)
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 ---");
--- Before method call ---
anIntActual: 123
aMoneyActual:
--- After method call ---
aMoneyActual: 0.00
Programming and Problem Solving With Java

67. Class Methods: Quadratic Eq.
Quadratic Equation:
ax2+bx+c=0
Programming and Problem Solving With Java

68. Class Methods: Quadratic Eq.
Quadratic equation: ax2 + 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 -2x2 + 3x + 4 = 0
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)
numerator = -b + Math.sqrt(b * b - 4 * a * c);
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("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^ x = 0
One root is
Another root is
Programming and Problem Solving With Java

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

73. Computer Networks Packet Packet header
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
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
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
Programming and Problem Solving With Java

76. Computer Networks Internet Addresses Address is hierarchical
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
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
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: instead of
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
Programming and Problem Solving With Java

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

79. Computer Networks The World Wide Web HTML
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
Programming and Problem Solving With Java

80. Computer Networks URLs Example URL Protocols
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
Protocols
http (Hypertext Transmission Protocol)
ftp (File Transfer Protocol)
mailto -- for sending mail messages
telnet -- for using another computer remotely.
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("
Methods
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'); }
Programming and Problem Solving With Java

83. Networks: Java Programming
Sample run
Enter URL:
Protocol is http
Port is -1
Host is
File is /
Another (y/n): y
Enter URL: ftp://
Protocol is ftp
Port is 9999
File is /myfile.txt
Enter URL:
Host is
File is /bogus/bogus.txt
Another (y/n): n
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()
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 ---");
// Read and display all lines from URL
String line = input.readLine();
while (line != null)
System.out.println(line);
line = input.readLine(); }
Programming and Problem Solving With Java

86. Networks: Java Programming
Sample run
--- URL Content Retriever ---
Enter URL:
--- Content Follows ---
<HTML>
<HEAD>
<TITLE>ACM Brings You the World of Computing</TITLE>
(Rest of HTML document appears here)
<!-- end bottom spacer and copyright line -->
</BODY>
</HTML>
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
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();
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...");
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();
Programming and Problem Solving With Java

91. Networks: Java Programming
How to write a client program
Request connection from server
Socket mySocket = new Socket(" 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()
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!
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();
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:\>
Programming and Problem Solving With Java

95. Communications Media Twisted pair wiring Coaxial cable (coax)
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
Programming and Problem Solving With Java

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

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

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

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