Week 2 An Introduction to Objects and Classes

CS 1063 Week 2: An Introduction to Objects and Classes

Use and Construct Objects

Objectives

Describe the concepts of classes and objects
Describe the difference between objects and object references
Construct and use Objects
Use the Rectangle class located in the java.awt package
Become familiar with WebCT - This should be done on Thursday or Friday of week 2

How to log in to your account
How to send an email
Take your first quiz. You will need at least 45 minutes in case there are any account problems.

MWF classes need to do this on Wednesday of this week
TR classes need to do this on Thursday

Assignments

Reading Week 2
Begin Lab 1
Take Quiz 1 (Thursday and Friday)

Activity 1 More `println` examples

Part 1: Load an existing program

Open the HelloWorld file you created last week.

Part 2: Add the following statements to the HelloWorld class

You can also print numerical values. The following code prints the number 7.
```
System.out.println(3 + 4);
```
The print method can be used to print an item without starting a new line afterward. The following code outputs 007 in a single line.
```
System.out.print("00");
System.out.println(3 + 4);
```
Add the highlighted statements in part 2 to your HelloWorld class file and verify that your code works as intended.

Part 3: Escape Sequences: Add the highlighted lines to your HelloWorld class and run it.

When printing some characters the compiler will report an error. In an attempt to print out the sequence Hello, "World", the following statement would generate such an error. Why?
```
System.out.println("Hello, "World"!");
```
If you precede each quotation mark inside the string with a backslash character (\) the " will be printed. The backslash character is used as an escape character, and the character sequence \" is called an escape sequence. The backslash does not denote itself; instead, it is used to encode other characters that would otherwise be difficult to include in a string. For example, the correct way to print Hello, "World" is:
```
System.out.println("Hello, \"World\"!");
```
The backslash character can be printed out using the escape sequence \\. For example, to print out C:\courses\cs1063, use the statement:
```
System.out.println("C:\\courses\\cs1063");
```
The escape sequence for a newline is \n. Each time it appears between quotes the output will go to the next line. For example, you can print the following lines:
```
*
**
***
```
using a single statement:
```
System.out.print("*\n**\n***\n");
```
The escape sequence for a tab is \t. Each time it appears between quotes the output will space over before printing the next part of the string. For example, to print the following line:
```
*	**	***
```
use the statement:
```
System.out.println("*\t**\t***");
```
Add the highlighted statements in part 3 to your HelloWorld class and verify that your code works as intended.

Activity 2 Use the `println` method to draw text pictures

Setup:

Create a new class called DrawMain by typing the following in the Definitions pane.
```
public class DrawMain {
  public static void main(String[] args) {

  }
}
```
Save the class.
Write, compile, and run a program to do the tasks below.

Write a program that displays your name inside a box on the console screen, like this:

+----------+
| Bylander |
+----------+

Add code to print this staircase:

    +---+
    |   |
+---+---+
|   |   |
+---+---+

Activity 3 Lab 1 The `FaceMain` class

What is Lab 1? Where can it be found?
Zip a folder (aka directory) to upload to WebCT using the Face folder.

The `Rectangle` Class in the `java.awt` package

A public interface is a list of methods you can use. The implementation or code is hidden. The public interface for the Rectangle class is given below. Some methods are missing.

Constructor:

Rectangle(int x, int y, int width, int height)
This constructs a rectangle with given top left corner and size.
Parameters:
     x,y - The upper left corner
     width - The width
     height - The height

Methods:

void setLocation(int x, int y)
This method moves this rectangle to a new location.
Parameters:
     x,y - The new top left corner

void setSize(int width, int height)
This method changes the size of this rectangle.
Parameters:
     width - The new width
     height - The new height

void translate(int dx, int dy)
This method moves the rectangle.
Parameters:
     dx - The distance to move along the x-axis (horizontal)
     dy - The distance to move along the y-axis (vertical)

Activity 4: Constructing Rectangle Objects

Setup:

Create a new class called RectangleMain by typing the following in the Definitions pane.

import java.awt.Rectangle;

public class RectangleMain {
  public static void main(String[] args) {

  }
}

Save the class.
Write, compile, and run a program to do the tasks below.

Constructing Objects

Objects are entities in your program that you manipulate by invoking methods. To construct any object, you do the following:

Use the new operator.
Give the name of the class.
Supply construction parameters(if any) inside parentheses.

Construct a Rectangle object with top left corner at (5,10), width 20, and height 30. Add the following line to your main and run the program. You should see java.awt.Rectangle[x=5,y=10,width=20,height=30]

System.out.println( new Rectangle(5, 10, 20, 30) );
System.out.println();  // Prints a blank line

Variable Names

Usually you want to do something more to an object than just create it, print it and forget it. To remember an object, you need to hold it in an object variable. A variable is an item of information in memory whose location is identified by a symbolic name. You can choose any variable names you like, provided you follow a few simple rules.

Names can be made up of letters, digits, and the underscore (_) character. They cannot start with a digit.
You cannot use other symbols such as ? and % in variable names.
Spaces are not permitted inside names, either.
You cannot use reserved words such as public as names; these words are reserved exclusively for their special Java meanings.
Variable names are also case-sensitive; that is, room (below) is NOT the same thing as Room.

An object variable is a container that stores the location of an object. All object variables must be initialized before you access them. Construct a Rectangle object named room whose top left corner is at (10,20), width 30 and height 40. Add the following code to your program and run.

// Declare a variable named room
Rectangle room;
// Initialize room
room = new Rectangle(10,20,30,40);
// Print room and a double space
System.out.println("room = " + room + "\n");

This is a picture of what is happening in memory after the assignment statement is executed. Locate the object reference and the object.

     Object      Object 
    Variable    Reference         Object

   +---------+               +----------------+
   |         |               |   Rectangle    |
   | room = ---------------> |                |
   |         |               |       x = 10   |
   +---------+               |       y = 20   |
                             |   width = 30   |
                             |  height = 40   |
                             |                |
                             +----------------+

Vocabulary

Rectangle is the class. Note that a class name begins with a capital letter.
room is the object variable. It stores an object reference. Note that a variable name begins with a lower case letter.
Why do you get a double space after the System.out.println above?
An object reference describes the location of an object.
An object variable (such as room) must be initialized before you access it.

Test all of the other methods listed in the public interface.

Move the rectangle to coordinates (15,25). Add the code to your project and run.

Your program uses objects in the following way:
1. Construct an object with the new operator. We did this above.
2. Store the object reference in an object variable. We did this above.
3. Call methods on the object variable using the dot operator. See below.
```
// Set x = 15 and y = 25 using the setLocation method
room.setLocation(15,25);
// Print room
System.out.println("room = " + room + "\n");
```

Set the room dimensions at width = 35 and height = 45 using the setSize method. Add the code to your project and run.

// Set width = 35 and height = 45 using the setSize method
room.setSize(35,45);
// Print room
System.out.println("room = " + room + "\n");

Move the rectangle coordinates by adding 15 to the x coordinate and 15 to the y coordinate by using the translate method. Your new coordinates should be (30, 40). Why? Add the code to your project and run.
```
// Set coordinates to (30,40) using the translate method
room.translate(15,15);
// Print room
System.out.println("room = " + room + "\n");
```

Use WebCT to take your first Quiz

All 10 quizzes will be available on Thursday, Friday, and Saturday of each week. (Always check your calendar.) That means you will have 3 days to complete the quiz. The first quiz will be taken together in class but from then on the student is responsible for taking the quiz on their own.

WebCt can be entered using the following link: http://webct.utsa.edu

Your WebCT ID (or user login name) is the last 8 digits of your Banner ID.

UTSA ID Number	Banner ID	WebCT ID
000087211	@00087211	00087211
000693300	@00693300	00693300
000097470	@00097470	00097470

Your initial password is set to your birth date entered as MMDDYYYY where MM is the 2-digit month, DD is the 2-digit day of month, and YYYY is the 4-digit year.

For example, if your birth date is January 3, 1972, then your password is 01031972.

Note: If you had a WebCT account in a previous semester, your username and password have not changed. If you forgot your password or have other problems logging in, see the WebCT FAQ.

Become familiar with the different links available and their uses.

Send an email to the student sitting beside you.
Communication Area -> Mail -> compose message -> Send to (Browse) -> Select one or more names -> Type in your message -> send

Begin quiz 1