---

Nov. 22, 2005

Grouping Data Elements Together

• We have learned that an arbitrarily sophisticated algorithm can be implemented by grouping together very simple structures (assignment, loop, decision, method call).
• Yet, in looking at data elements, we have dealt mostly with individual variables that contain scalar values (integer, float) or simple objects (Student, Address).
• We need to round out our experience by looking at how to group simple data elements together to form arbitrarily sophisticated data structures.
• Just as a subprogram (method) collects program statements into a single named entity, a composite data structure collects data elements into a single named entity.

Arrays
A particular value in an array is referenced using the array name followed by the index in brackets
For example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
That expression represents a place to store a single integer and can be used wherever an integer variable can be used

Arrays
For example, an array element can be assigned a value, printed, or used in a calculation:   
  scores[2] = 89;
  scores[first] = scores[first] + 2;
  mean = (scores[0] + scores[1])/2;
  System.out.println ("Top = " + scores[5]);

Arrays

• The values held in an array are called array elements
• An array stores multiple values of the same type (the element type)
• The element type can be a primitive type or an object reference
• Therefore, we can create an array of integers, or an array of characters, or an array of String objects, etc.
• In Java, the array itself is an object
• Therefore the name of the array is an object reference variable, and the array itself must be instantiated

Declaring Arrays

• The scores array could be declared as follows:
•     int[] scores = new int[10];
• The type of the variable scores is int[] (an array of integers)
• Note that the type of the array does not specify its size, but each object of that type has a specific size
• The reference variable scores is set to a new array object that can hold 10 integers
• See BasicArray.java (page 322)

---

Nov. 15-17

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.

More on Classes and Methods

Let's look at classes which are instantiated to produce objects. What are some of the common characteristics of such classes and objects?

• 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

An aggregate object is an object that contains references to other objects
For example, an Account object contains a reference to a String object (the owner's name)
An aggregate object represents a has-a relationship
A bank account has a name
Likewise, a student may have one or more addresses

---

Sept. 15-20, 2005

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.

---

Analysis
To solve this problem, we must be given the current year, and the graduation years of the three profs. The estimated age (to the nearest year) of a prof can be computed from these values. The mean age will be the sum of the ages divided by the number of profs (3).
Data Requirements and Formulas
Problem Inputs:
thisYear - the current calendar year, an integer
year - the graduation year of each prof in turn, an integer
Problem Outputs:
age1, age2, age3 - the ages of the three professors (integers)
meanAge - the mean age of the professors
Relevant Formulas
Estimated age = current year - graduation year + 21

---

Initial Algorithm

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:

---

Test Plan
Let's develop one right here!

---

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

mfeldman@hobbes.seas.gwu.edu:15:
cp programs53/ProjectTemplate.java .
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: framework file.

//--------------------------------------------------------------
//| MeanAge.java
//| Finds mean estimated age of three professors
//| Author: M.B. Feldman, The George Washington University
//| Last Modified: September 9, 2002
//--------------------------------------------------------------
import cs1.Keyboard;
public class MeanAge
{
  public static void main (String[] args)
  {

    int thisYear; // input - the current calendar year
    int year; // input - year of a prof's bachelors deg.
    int age1; // output - 1st prof's estimated age
    int age2; // output - 1st prof's estimated age
    int age3; // output - 1st prof's estimated age
    int meanAge; // output - mean of 3 ages

    // Prompt the user and read in the current calendar year

    // Prompt the user and read in 1st prof's degree year
    // and compute and display that prof's estimated age

    // Prompt the user and read in 2nd prof's degree year
    // and compute and display that prof's estimated age

    // Prompt the user and read in 3rd prof's degree year
    // and compute and display that prof's estimated age

    // Compute the mean age

    // Display the mean age

  }
}

---

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

See MeanAge.java in the program distribution.

---

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.

---

Notes from Thursday, Sept. 8, 2005

What Does It Take to Be a
Good Software Developer?

• a “logical mind”
• a willingness to approach problems methodically
• break problems up into sub-problems
• don’t panic at the size of a problem;
• it may seem overwhelming but it is
• within your grasp to solve it if
• you approach it methodically

---

The Software Development Method

1.    Problem specification
2.    Analysis
3.    Design
4.    Test plan
5.    Implementation or coding
6.    Testing

---

The Software Development Method

1.    Problem specification: State the problem and gain a clear understanding of what is required for its solution. This sounds easy, but it can be the most critical part of problem solving. A good problem solver must be able to recognize and define the problem precisely. If the problem is not totally defined, you must study the problem carefully, eliminating the aspects that are unimportant and zeroing in on the root problem.

2.    Analysis: Identify problem inputs, desired outputs, and any additional requirements of or constraints on the solution. Identify what information is supplied as problem data and what results should be computed and displayed. Also, determine the required form and units in which the results should be displayed (for example, as a table with specific column headings).

3.    Design: Develop a list of steps (called an algorithm) to solve the problem and verify that the algorithm solves the problem as intended. Writing the algorithm is often the most difficult part of the problem-solving process. Once you have the algorithm, you should verify that it is correct before proceeding further.

4.    Test plan: Develop a strategy for proving to yourself and to others that your algorithm will get the proper results. It is highly advisable to write a plan for testing the program you will write, even before you have written it. Which test cases will you choose? What are the special cases that must be tested? Pretend you are a potential purchaser of the program and ask, “Which tests would I require to be convinced that this program behaves as advertised?”

5.    Implementation or coding: Implement the algorithm as a program. This requires knowledge of a particular programming language. Each algorithm step must be converted into a statement in that programming language.

6.    Testing: Run the completed program, testing it with the test cases specified in the test plan.

---

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!

---

Implementation

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

mfeldman@hobbes.seas.gwu.edu:15:
 cp programs53/ProjectTemplate.java .
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. This is called a framework file.

//--------------------------------------------------------------
//| 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.");

  }
}