Chapter 1:

The basic concepts and terms of solving problems using a computer are covered, including use of the BlueJ application and object oriented modeling of problems.

• Class - A class defines common fields (attributes) and methods (actions) to objects.

  All objects classified as desks have height, color, location, etc. fields. The desk class can be defined by its fields as:

  class Desk      height     color     location

• Object - An object is one specific instance of a class where the fields have a value.

A room may have 20 objects classified as desks, each desk has color, height, location, etc. fields. Two desk objects, different instances of desk classification, can then have field values of:

height   30 inches color      black location row 3, position 2

height   30 inches color      tan location row 4, position 1

 

Exercise 1 Name three classes of objects in your immediate possession at this moment. Nothing risqué please! Write each of the three class names and four obvious fields for each class. Pick one of the classes for which you possess several objects and define values for each of the four fields.

Create object - To create a new instance of a real desk requires lumber, nails, etc. but since we are modeling a desk on a computer two new desk instances are created in Java by:

new Desk()

new Desk()

 

Name object - Naming each desk allows us to distinguish one desk from another by:

Desk desk1 = new Desk()

Desk desk2 = new Desk()

 

Exercise 2 From Exercise 1, Question 3; Use Java to create and name two of the objects.

The following assumes that textbook projects has been installed, if not, see Getting Started.

Exercise 3 created two circles that included an isVisible field that when false the circle is not visible and when true the circle is visible.

circle_1		circle_2
diameter    30 isVisible     false		diameter    30 isVisible     false

 

Method makeVisible() sets the isVisible field to true.

 

Exercise 4 - Calling methods in BlueJ Right-click on circle_1 icon . Choose makeVisible(). Right-click on circle_1 icon. Choose moveUp(). Repeat several times. Make circle_1 invisible and visible several times.

 

Two circles have been created with field values of:

circle_1		circle_2
diameter    30 isVisible     false		diameter    30 isVisible     false

The method call changeSize(50) specifies 50 as the parameter to the method.

Exercise 5 - Calling methods with parameters in BlueJ                                                                  Right-click on circle_1 icon in BlueJ. Choose makeVisible(). Click on the taskbar icon . Right-click on circle_1 icon in BlueJ. Choose changeSize(int). Make circle_1 diameter 150 as at right. Figure out how to call slowMoveVertical method.

Describing real problems as a computer model requires defining the problem data.

The following fields describe circle_1's diameter, visibility, and color.

circle_1
int          diameter   30 boolean isVisible     false String    color           "blue"

 

• Data type - Specifies the type of data value of a field or parameter.

  • int - Integer (whole number). Examples: 34, -25, 9999999, 0.
  • double - real number. Examples: 14.76, 3963.4, 0.5.
  • boolean - Either true or false.
  • String - Anything enclosed in double quotes. Examples: "blue", "12", "Hi there".

The following field values completely define circle_1's state. If the state does not change, the object does not change. Changing field diameter to 90 changes the state of the object (and triples the diameter).

circle_1
int          diameter   30 int          xPosition   70 int          yPosition   60 boolean isVisible     false String    color           "blue"

 

Exercise 8 - Inspecting Object State                                                                Click on the taskbar icon . Inspect the state of circle_1 by right-clicking on . Choose Inspect. Call changeColor method using a different color parameter. Note the color field value. Inspect two objects of the same class (you should have two Circle objects). Are all fields the same? Are all values the same? Inspect a Triangle and Circle object simultaneously. What fields are the same? What fields are different?

All objects of a class have identical fields; the fields of one object can hold different values from other objects.

Circle class		circle_1 object		circle_2 object
int          diameter       int          xPosition int          yPosition boolean isVisible String    color		int          diameter   30 int          xPosition   70 int          yPosition   60 boolean isVisible     false String    color           "blue"		int          diameter   30 int          xPosition   140 int          yPosition   72 boolean isVisible     true String    color           "red"

All objects of a class have identical methods; methods are called on one, designated object.

Circle methods
void changeColor(String) void changeSize(int) void makeInvisible() void makeVisible() void modeDown()

 

Exercise 9 - Using Objects       Create a house and sun similar to the picture at right using the shapes project classes Circle, Triangle and Square. Write down what tasks you did; list the objects created and methods called. For Example: a. Circle circle_1 = new Circle() b. circle_1.makeVisible()        .        . Could the methods be called in any order?

The tasks of Exercise 9 would normally be written as Java instructions that could be performed by the computer rather than entering by hand each time a house is to be drawn. In Exercise 10, the tasks have been listed in the picture project.

Exercise 10 - Object Interaction       Open the picture project. Click Project | Open Project Type File Name: J:\C201\Projects\chapter01 Click on: chapter01 | picture Create an instance of class Picture. Call the draw method.

Tasks to create objects and call methods can be written in Java, saved to a file, and reused over and over. The list of Java statements define a new class, the written text of the statements are the source code for the class.

Exercise 11 - Source Code      Open the picture project. Right-click on icon . Click on Open Editor to see the source code text. What is the name of the class? Find the statement that defines the name of the class. Find the fields for the sun and parts of the house. What are the field names? Find the public void draw method. What are the wall tasks? Does the order in which the tasks are performed matter?

Java statements are written and read by humans but must be compiled (translated from Java to computer code) before performed by computer.

This is a simple example example of a student database that tracks students in a laboratory class and prints class lists.

Exercise 16 - Student Database       Open project lab-classes. Create a Student object. Fill in the name and id fields as String (enclose String's in "). Inspect the object.

Any object can be a method parameter, as long as the object class matches the method signature.

Exercise 18 - LabClass Continue Exercise 17. Create a LabClass object with maximum of 5 students The LabClass constructor signature indicates an int parameter which must be passed the maximum number of students. Call the numberOfStudents method. What is the result?

The enrolStudent method is passed a Student parameter.

Exercise 19 - Objects as Parameters Continue Exercise 18. Create a Student object with a maximum of 5 students. Call enrolStudent method with the name of one Student object as the parameter. The enrolStudent signature indicates a Student parameter must be passed. Call the numberOfStudents method. What is the result? Call LabClass method enrolStudent with the same Student object as before. Call the numberOfStudents method again. What is the result? What does the method do?

The printList method prints the fields of a LabClass object to a terminal window.

Exercise 20 - Print LabClass object Continue Exercise 19. Call the printList method for the LabClass object. What does printList the method do?