School of Engineering and Applied Science Department of Computer Science CSci 53 -- Introduction to Software Development  http://www.seas.gwu.edu/~csci53/fall03 Prof. Michael B. Feldman mfeldman@gwu.edu

Thursday, Nov. 13, 2003

More on Classes and Methods

• An object has state (a set of data values, sometimes called fields or attributes) and behavior (a set of methods). The methods act on the data values, which changes the overall state.
• The data values are usually declared to be private, so that a calling program must call methods to modify them and can't modify them directly.
• The methods are usually declared to be public, so a calling program can use them. Sometimes there are also private methods; why would we want these?

Common kinds of methods

• Constructor: creates and returns object from a class. Usually has a list of parameters, and uses these to set the object's data values. The constructor's name is always that of the class; it has no return statement.
• "Setter": usually has parameters, which it uses to modify one or more of an existing object's data values.
• "Getter": usually has no parameters; it just returns one of the object's data values.
• input: reads an objects data values from the keyboard or from a file.
• toString: collects (some of) an object's data values together into a string, suitable for displaying or writing to a file.

Aggregation

Tuesday, Nov. 11, 2003

Writing Your Own Classes and Methods

• Sets of reusable classes which provide generally usable methods are the basis for all modern software development, regardless of language or operating system.

 
• There are a number of kinds of methods:

 
• A public method is available to any program that imports the class; a private method is available only to other methods within the class. At this time we'll consider only public ones.

 
• A static method is associated with the class; a non-static method is associated with each object. At this time we'll consider only static ones.

 
• A value-returning method is one that returns a value to the calling program; a void method is one that does not return a value. At this time we'll consider both value-returning and void methods.

Decision (IF) Constructs

• Monday: work at home
  
• Tuesday: CSci 53 11 AM -12:15 PM
  CSci 133 2 - 3:15 PM
  Office hours 4 - 5:30 PM
  
• Wednesday: Office hours 2 - 5:30 PM
  
• Thursday: CSci 53 11 AM -12:15 PM
  CSci 133 2 - 3:15 PM
  Office hours 4 - 5:30 PM
  
• Friday: work at home
  

Basic Algorithm

1. Prompt user to enter a day of the week.
   
2. Display the schedule items for that day
   

Combining conditions

if ((day == 1) || (day == 5)) // Monday, Friday
{
  System.out.println ("Work at home today");
}

if ((day == 2) || (day == 4)) // Tuesday, Thursday
{
  System.out.println ("CSci 53, 11 AM - 12:15 PM");
  System.out.println ("CSci 133, 2 - 3:15 PM");
  System.out.println ("Office hours, 4 - 5:30 PM");
}

if (day == 3) // Wednesday
{
  System.out.println ("Office hours, 2 - 5:30 PM");
}

if ((day == 6) || (day == 7)) // Saturday, Sunday
{
  System.out.println ("Have a nice weekend");
}

Nested if Statements

if ((day == 1) || (day = 5)) // Monday, Friday
{
  System.out.println ("Work at home today");
}
else
{
  if ((day == 2) || (day == 4)) // Tuesday, Thursday
  {
    System.out.println ("CSci 53, 11 AM - 12:15 PM");
    System.out.println ("CSci 133, 2 - 3:15 PM");
    System.out.println ("Office hours, 4 - 5:30 PM");
  }
  else
  {
    if (day == 3) // Wednesday
    {
      System.out.println ("Office hours, 2 - 5:30 PM");
    }
    else
    {
      if ((day == 6) || (day == 7)) // Saturday, Sunday
      {
        System.out.println ("Have a nice weekend");
      }
      else // none of the above
      {
        System.out.println ("Out of range input; try again")
      }
    }
  }
}

Simplifying Nested if Statements
a Different Formatting Style

if ((day == 1) || (day == 5)) // Monday, Friday
{
  System.out.println ("Work at home today");
}
else if ((day == 2) || (day == 4)) // Tuesday, Thursday
{
  System.out.println ("CSci 53, 11 AM - 12:15 PM");
  System.out.println ("CSci 133, 2 - 3:15 PM");
  System.out.println ("Office hours, 4 - 5:30 PM");
}
else if (day == 3) // Wednesday
{
  System.out.println ("Office hours, 2 - 5:30 PM");
}
else if ((day == 6) || (day == 7)) // Saturday, Sunday
{
  System.out.println ("Have a nice weekend");
}
else // none of the above
{
  System.out.println ("Out of range input; try again")
}

Switch Statements

switch(day)
{
  case 1:
  case 5: // Monday, Friday
    System.out.println ("Work at home today");
    break; // jump to bottom of switch statement

  case 2:
  case 4: // Tuesday, Thursday
    System.out.println ("CSci 53, 11 AM - 12:15 PM");
    System.out.println ("CSci 133, 2 - 3:15 PM");
    System.out.println ("Office hours, 4 - 5:30 PM");
    break;

  case 3: // Wednesday
    System.out.println ("Office hours, 2 - 5:30 PM");
    break;

  case 6:
  case 7: // Saturday, Sunday
    System.out.println ("Have a nice weekend");
    break;

  default: // anything else
    System.out.println("Bad input");
}

Use Constants, Not "Magic Numbers"

final int MON = 1, TUE = 2, WED = 3, THU = 4;
final int FRI = 5, SAT = 6, SUN = 7;
...
switch(day)
{
case MON:
case FRI: // Monday, Friday
System.out.println ("Work at home today");
break; // jump to bottom of switch statement

case TUE:
case THU: // Tuesday, Thursday
System.out.println ("CSci 53, 11 AM - 12:15 PM");
System.out.println ("CSci 133, 2 - 3:15 PM");
System.out.println ("Office hours, 4 - 5:30 PM");
break;

case WED: // Wednesday
System.out.println ("Office hours, 2 - 5:30 PM");
break;

case SAT:
case SUN: // Saturday, Sunday
System.out.println ("Have a nice weekend");
break;

default: // anything else
System.out.println("Bad input");
}

Sept. 9-11, 2003

Example: Average Age of Professors

Problem Specification
You're a very curious person. One thing you're curious about is the ages of three of your professors. But how can you find out how old they are? Some people are offended if you ask their age.
Luckily, the GW Undergraduate Bulletin has a list of (most of) the professors in the back of the book. Each professor's listing gives his/her degrees, showing the college or university and year of each degree.
If we assume the typical student is 21 when (s)he earns a bachelors degree, we can estimate a professor's age just by knowing the current calendar year and that professor's graduation year.
You're also curious about the mean (average) age of the profs.

1. Prompt the user and read in each professor's graduation year, then calculate and display that professor's estimated age.
2. Prompt the user and read in the current year. 
3. Compute the mean age.
4. Display the mean age.
   

Algorithm Refinements
Step 2 can be refined:

Improved Algorithm

We can improve the algorithm a bit by realizing that we don't need 3 separate age variables. We can use one variable over and over, as long as we accumulate the sum of the ages as we go along. We need a sum variable; let's call it ageSum.
This gives a new refinement of Step 2:

2.1 Prompt the user and read in 1st professor's graduation year, then calculate and display that professor's estimated age, and add it into the running age sum.
2.2 Prompt the user and read in 2nd professor's graduation year, then calculate and display that professor's estimated age, and add it into the running age sum.
2.3 Prompt the user and read in 3rd professor's graduation year, then calculate and display that professor's estimated age, and add it into the running age sum.

See MeanAge2.java in the program distribution.

Sept. 4, 2003

Example: Average Speed on Trip

Problem Specification

Next week, you're driving to Long Island to visit your family. When you drive, you'd like to know what your average end-to-end speed, in miles per hour, was for the trip.

You're not very good at arithmetic. Luckily, you'll have your trusty laptop with you, so you can develop a little program that will compute your average speed from the distance you've driven and the number of hours the trip took.
Unfortunately, your laptop is a bit lame-brained, so it's able to caculate only with "whole numbers" (integers).

Analysis

To solve this problem, we must be given the number of miles (to the nearest mile) and the number of hours (to the nearest hour). The average speed (to the nearest MPH) will be the quotient of the number of miles divided by the number of hours.

Data Requirements and Formulas

Problem Inputs:

miles  - the number of miles, an integer
hours - the number of hours, an integer

Problem Outputs:

mph - the average speed, to the nearest integer value
Relevant Formulas
mph = miles ÷ hours

Initial Algorithm

1. Prompt the user and read in the number of miles.
2. Prompt the user and read in the number of hours.
3. Compute the average speed.
4. Display the average speed.
   

Algorithm Refinements

This is a very simple algorithm and needs no refinements.

Test Plan

Let's develop one right here! (Here is what we developed in class.)

Implementation

Start with the template - it's unnecessary to start with a blank page

mfeldman@hobbes.seas.gwu.edu:15:
mfeldman@hobbes.seas.gwu.edu:16: cat ProjectTemplate.java

//--------------------------------------------------------------
//| ProjectTemplate.java <-- change to your project filename
//| Use this as a basis for your own programs
//| Author: <your name> The George Washington University
//| Last Modified: <date>
//--------------------------------------------------------------
import cs1.Keyboard;
public class ProjectTemplate <-- change to your project name
{
  public static void main (String[] args)
  {
    // put your code here
  }
}

Declare the variables and copy the main algorithm steps as comments

//--------------------------------------------------------------
//| AverageSpeed.java
//| Finds average speed on a trip.java
//| Author: M.B. Feldman The George Washington University
//| Last Modified: September 5 2002
//--------------------------------------------------------------
import cs1.Keyboard;
public class AverageSpeed
{
  public static void main (String[] args)
  {

    int miles;           // input  - number of miles driven
    int hours;           // input  - number of hours driven
    int mph;             // output - miles per hour

    // Prompt the user and read in the number of miles
    // Prompt the user and read in the number of hours
    // Compute the average speed
    // Display the average speed
  }
}

Compile the framework; fix any errors. Then fill in the rest of the program statements.

//--------------------------------------------------------------
//| AverageSpeed.java
//| Finds average speed on a trip.java
//| Author: M.B. Feldman The George Washington University
//| Last Modified: September 5 2002
//--------------------------------------------------------------
import cs1.Keyboard;
public class AverageSpeed
{
  public static void main (String[] args)
  {

    int miles;           // input  - number of miles driven
    int hours;           // input  - number of hours driven
    int mph;             // output - miles per hour


    // Prompt the user and read in the number of miles
    System.out.print
      ("How many miles did you drive (to the nearest mile)? ");
    miles = Keyboard.readInt();

    // Prompt the user and read in the number of hours
    System.out.print
      ("How many hours did you drive (to the nearest hour)? ");
    hours = Keyboard.readInt();

    // Compute the average speed
    mph = miles / hours;

    // Display the average speed
    System.out.print   ("Your average speed was ");
    System.out.println (mph + " miles per hour.");

  }
}

(end of document)