Chapter Goals

• To learn about variables
• To understand the concepts of classes and objects
• To be able to call methods
• To learn about parameters and return values
• To be able to browse the API documentation
• To implement test programs
• To understand the difference between objects and object references
• To write programs that display simple shapes

Types

• A type defines a set of values and the operations that can be carried out on the values
• Examples:
  • 13 has type int
  • "Hello, World" has type String
  • System.out has type PrintStream
• Java has separate types for integers and floating-point numbers
  • The double type denotes floating-point numbers
• A value such as 13 or 1.3 that occurs in a Java program is called a number literal

Number Literals

Number Types

• Number types are primitive types
  • Numbers are not objects
• Numbers can be combined by arithmetic operators such as +, -, and *

Self Check 2.1

What is the type of the values 0 and "0"?

• Answer: int and String.

Self Check 2.2

Which number type would you use for storing the area of a circle?

• Answer: double.

Self Check 2.3

Why is the expression 13.println() an error?

• Answer: An int is not an object, and you cannot call a method on it.

Self Check 2.4

Write an expression to compute the average of the values x and y.

• Answer: (x + y) * 0.5

Variables

• Use a variable to store a value that you want to use at a later time
• A variable has a type, a name, and a value:

  String greeting = "Hello, World!";
  PrintStream printer = System.out;
  int width = 13;

• Variables can be used in place of the values that they store:

  printer.println(greeting); // Same as System.out.println("Hello, World!") 
  printer.println(width); // Same as System.out.println(20)

• It is an error to store a value whose type does not match the type of the variable:

  String greeting = 20; // ERROR: Types don’t match

Variable Declarations

Identifiers

• Identifier: name of a variable, method, or class
• Rules for identifiers in Java:
  • Can be made up of letters, digits, and the underscore (_) and dollar sign ($) characters
  • Cannot start with a digit
  • Cannot use other symbols such as ? or %
  • Spaces are not permitted inside identifiers
  • You cannot use reserved words such as public
  • They are case sensitive
• By convention, variable names start with a lowercase letter
  • Camel case: Capitalize the first letter of a word in a compound word such as farewellMessage
• By convention, class names start with an uppercase letter
• Do not use the $ symbol in names — it is intended for names that are automatically generated by tools

Syntax 2.1 Variable Declaration

Variable Names

Self Check 2.5

Which of the following are legal identifiers?

Greeting1
g
void
101dalmatians
Hello, World
<greeting>

• Answer: Only the first two are legal identifiers.

Self Check 2.6

Declare a variable to hold your name. Use camel case in the variable name.

• Answer:

  String myName = "John Q. Public";

The Assignment Operator

• Assignment operator: =
• Used to change the value of a variable

The Assignment Operator

int width = 10;

The Assignment Operator

int width = 10;
width = 20;

Uninitialized Variables

• It is an error to use a variable that has never had a value assigned to it:

  int height; 
  width = height;   // ERROR—uninitialized variable height

  
• Remedy: assign a value to the variable before you use it:

  int height = 30; 
  width = height; // OK

• Even better, initialize the variable when you declare it:

  int height = 30; 
  int width = height; // OK

Syntax 2.2 Assignment

Assignment

• The right-hand side of the = symbol can be a mathematical expression:

  width = height + 10;

• Means compute the value of width + 10 and store that value in the variable width

Animation 2.1: Variable Initialization and Assignment

Link to Flash animation

Self Check 2.7

Is 12 = 12 a valid expression in the Java language?

• Answer: No, the left-hand side of the = operator must be a variable.

Self Check 2.8

How do you change the value of the greeting variable to "Hello, Nina!"?

• Answer:

  greeting = "Hello, Nina!";

  Note that

  String greeting = "Hello, Nina!";

  is not the right answer – that statement defines a new variable.

Objects and Classes

• Object: entity that you can manipulate in your programs by calling methods
• Each object belongs to a class. For example, System.out belongs to the class PrintStream
  

Methods

• Method: sequence of instructions that accesses the data of an object
• You manipulate objects by calling its methods
• Class: declares the methods that you can apply to its objects
• Class determines legal methods:

  String greeting = "Hello, World!";
  int n = greeting.length();
  System.out.length(); // This method call is an error

• Public interface: specifies what you can do with the objects of a class
• Overloaded method: when a class declares two methods with the same name, but different parameters
  • Example: the PrintStream class declares a second method, also called println, as

    public void println(int output)

A Representation of Two String Objects

String Methods

• length: counts the number of characters in a string:

  String greeting = "Hello, World!";
  int n = greeting.length(); // sets n to 13

• toUpperCase: creates another String object that contains the characters of the original string, with lowercase letters converted to uppercase:

  String river = "Mississippi";
  String bigRiver = river.toUpperCase(); // sets bigRiver to "MISSISSIPPI"

• When applying a method to an object, make sure method is defined in the appropriate class:

  System.out.length(); // This method call is an error

Self Check 2.9

How can you compute the length of the string "Mississippi"?

• Answer: river.length() or "Mississippi".length()

Self Check 2.10

How can you print out the uppercase version of "Hello, World!"?

• Answer: System.out.println(greeting.toUpperCase());

Self Check 2.11

Is it legal to call river.println()? Why or why not?

• Answer: It is not legal. The variable river has type String. The println method is not a method of the String class.

Parameters

• Parameter: an input to a method
• Implicit parameter: the object on which a method is invoked:

  System.out.println(greeting)

• Explicit parameters: all parameters except the implicit parameter:

  System.out.println(greeting)

• Not all methods have explicit parameters:

  greeting.length() // has no explicit parameter

  

Return Values

• Return value: a result that the method has computed for use by the code that called it:

  int n = greeting.length(); // return value stored in n

  

Passing Return Values

• You can use the return value as a parameter of another method:

  System.out.println(greeting.length());

  
• Not all methods return values. Example: println

A More Complex Call

• String method replace carries out a search-and-replace operation:

  river.replace("issipp", "our") // constructs a new string ("Missouri")

  
• This method call has
  • one implicit parameter: the string "Mississippi"
  • two explicit parameters: the strings "issipp" and "our"
  • a return value: the string "Missouri"

Animation 2.2: Parameter Passing

Link to Flash animation

Self Check 2.12

What are the implicit parameters, explicit parameters, and return values in the method call river.length()?

• Answer: The implicit parameter is river. There is no explicit parameter. The return value is 11.

Self Check 2.13

What is the result of the call river.replace("p", "s")?

• Answer: "Missississi".

Self Check 2.14

What is the result of the call greeting.replace("World", "Dave").length()?

• Answer: 12.

Self Check 2.15

How is the toUpperCase method defined in the String class?

• Answer: As public String toUpperCase(), with no explicit parameter and return type String.

Rectangular Shapes and Rectangle Objects

• Objects of type Rectangle describe rectangular shapes:
  

Rectangular Shapes and Rectangle Objects

• A Rectangle object isn't a rectangular shape — it is an object that contains a set of numbers that describe the rectangle:
  

Constructing Objects

new Rectangle(5, 10, 20, 30)

• Detail:
  1. The new operator makes a Rectangle object
  2. It uses the parameters (in this case, 5, 10, 20, and 30) to initialize the data of the object
  3. It returns the object
• Usually the output of the new operator is stored in a variable:

  Rectangle box = new Rectangle(5, 10, 20, 30);

Constructing Objects

• Construction: the process of creating a new object
• The four values 5, 10, 20, and 30 are called the construction parameters
• Some classes let you construct objects in multiple ways:

  new Rectangle()
  // constructs a rectangle with its top-left corner
  // at the origin (0, 0), width 0, and height 0

Syntax 2.3 Object Construction

Self Check 2.16

How do you construct a square with center (100, 100) and side length 20?

• Answer:

  new Rectangle(90, 90, 20, 20)

Self Check 2.17

The getWidth method returns the width of a Rectangle object. What does the following statement print?

System.out.println(new Rectangle().getWidth());

• Answer:

  0

Accessor and Mutator Methods

• Accessor method: does not change the state of its implicit parameter:

  double width = box.getWidth();

• Mutator method: changes the state of its implicit parameter:

  box.translate(15, 25);

  

Self Check 2.18

Is the toUpperCase method of the String class an accessor or a mutator?

• Answer: An accessor — it doesn't modify the original string but returns a new string with uppercase letters.

Self Check 2.19

Which call to translate is needed to move the box rectangle so that its top-left corner is the origin (0, 0)?

• Answer: box.translate(-5, -10), provided the method is called immediately after storing the new rectangle into box.

The API Documentation

• API: Application Programming Interface
• API documentation: lists classes and methods in the Java library
• http://java.sun.com/javase/7/docs/api/index.html

The API Documentation of the Standard Java Library

The API Documentation for the Rectangle Class

Method Summary

Detailed Method Description

The detailed description of a method shows:

• The action that the method carries out
• The parameters that the method receives
• The value that it returns (or the reserved word void if the method doesn’t return any value)

Packages

• Package: a collection of classes with a related purpose
• Import library classes by specifying the package and class name:

  import java.awt.Rectangle;

• You don't need to import classes in the java.lang package such as String and System

Syntax 2.4 Importing a Class from a Package

Self Check 2.20

Look at the API documentation of the String class. Which method would you use to obtain the string "hello, world!" from the string "Hello, World!"?

• Answer: toLowerCase

Self Check 2.21

In the API documentation of the String class, look at the description of the trim method. What is the result of applying trim to the string " Hello, Space ! "? (Note the spaces in the string.)

• Answer: "Hello, Space !" – only the leading and trailing spaces are trimmed.

Self Check 2.22

The Random class is defined in the java.util package. What do you need to do in order to use that class in your program?

• Answer: Add the statement

  import java.util.Random;

  at the top of your program.

Implementing a Test Program

1. Provide a tester class.
2. Supply a main method.
3. Inside the main method, construct one or more objects.
4. Apply methods to the objects.
5. Display the results of the method calls.
6. Display the values that you expect to get.

ch02/rectangle/MoveTester.java

Program Run:

• x: 20
  Expected: 20
  y: 35
  Expected: 35

Self Check 2.23

Suppose we had called box.translate(25, 15) instead of box.translate(15, 25). What are the expected outputs?

• Answer:

  x: 30, y: 25

Self Check 2.24

Why doesn't the MoveTester program print the width and height of the rectangle?

• Answer: Because the translate method doesn't modify the shape of the rectangle.

Testing Classes in an Interactive Environment

Object References

• Object reference: describes the location of an object
• The new operator returns a reference to a new object:

  Rectangle box = new Rectangle();

• Multiple object variables can refer to the same object:

  Rectangle box = new Rectangle(5, 10, 20, 30);
  Rectangle box2 = box;
  box2.translate(15, 25);

• Primitive type variables ≠ object variables

Object Variables and Number Variables

 
 

Copying Numbers

int luckyNumber = 13;

Copying Numbers

int luckyNumber = 13;
int luckyNumber2 = luckyNumber;

Copying Numbers

int luckyNumber = 13;
int luckyNumber2 = luckyNumber;
luckyNumber2 = 12;

Copying Object References

Rectangle box = new Rectangle(5, 10, 20, 30);
  
  

Copying Object References

Rectangle box = new Rectangle(5, 10, 20, 30);
Rectangle box2 = box;

Copying Object References

Rectangle box = new Rectangle(5, 10, 20, 30);
Rectangle box2 = box;
box2.translate(15, 25);

Self Check 2.25

What is the effect of the assignment greeting2 = greeting?

• Answer: Now greeting and greeting2 both refer to the same String object.

Self Check 2.26

After calling greeting2.toUpperCase(), what are the contents of greeting and greeting2?

• Answer: Both variables still refer to the same string, and the string has not been modified. Recall that the toUpperCase method constructs a new string that contains uppercase characters, leaving the original string unchanged.

Mainframes – When Dinosaurs Ruled the Earth

Graphical Applications and Frame Windows

To show a frame:

1. Construct an object of the JFrame class:

   JFrame frame = new JFrame();

2. Set the size of the frame:

   frame.setSize(300, 400);

3. If you'd like, set the title of the frame:

   frame.setTitle("An Empty Frame");

4. Set the "default close operation":

   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

5. Make the frame visible:

   frame.setVisible(true);

A Frame Window

ch02/emptyframe/EmptyFrameViewer.java

Self Check 2.27

How do you display a square frame with a title bar that reads "Hello, World!"?

• Answer: Modify the EmptyFrameViewer program as follows:

  frame.setSize(300, 300);
  frame.setTitle("Hello, World!");

Self Check 2.28

How can a program display two frames at once?

• Answer: Construct two JFrame objects, set each of their sizes, and call setVisible(true) on each of them.

Drawing on a Component

• In order to display a drawing in a frame, define a class that extends the JComponent class
• Place drawing instructions inside the paintComponent method. That method is called whenever the component needs to be repainted:

  public class RectangleComponent extends JComponent
  {
     public void paintComponent(Graphics g)
     {
        Drawing instructions go here
     }
  }

Classes Graphics and Graphics2D

• Graphics class lets you manipulate the graphics state (such as current color)
• Graphics2D class has methods to draw shape objects
• Use a cast to recover the Graphics2D object from the Graphics parameter:

  public class RectangleComponent extends JComponent
  {
     public void paintComponent(Graphics g)
     {
        // Recover Graphics2D
        Graphics2D g2 = (Graphics2D) g;
        . . .
     }
  }

• Call method draw of the Graphics2D class to draw shapes, such as rectangles, ellipses, line segments, polygons, and arcs:

  public class RectangleComponent extends JComponent
     {
     public void paintComponent(Graphics g)
     {
        . . .
        Rectangle box = new Rectangle(5, 10, 20, 30);
        g2.draw(box);
        . . .
     }
  }

Drawing Rectangles

ch02/rectangles/RectangleComponent.java

Using a Component

1. Construct a frame.
2. Construct an object of your component class:

   RectangleComponent component = new RectangleComponent();

3. Add the component to the frame:

   frame.add(component);

4. Make the frame visible.

ch02/rectangles/RectangleViewer.java

Self Check 2.29

How do you modify the program to draw two squares?

• Answer: Rectangle box = new Rectangle(5, 10, 20, 20);

Self Check 2.30

How do you modify the program to draw one rectangle and one square?

• Answer: Replace the call to box.translate(15, 25) with

  box = new Rectangle(20, 35, 20, 20);

Self Check 2.31

What happens if you call g.draw(box) instead of g2.draw(box)?

• Answer: The compiler complains that g doesn't have a draw method.

Applets

• Applet: program that runs inside a web browser
• To implement an applet, use this code outline:

  public class MyApplet extends JApplet
  {
     public void paint(Graphics g)
     {
        // Recover Graphics2D
        Graphics2D g2 = (Graphics2D) g;
        
        // Drawing instructions go here
        . . .
     }
  }

Applets

• This is almost the same outline as for a component, with two minor differences:
  1. You extend JApplet, not JComponent
  2. You place the drawing code inside the paint method, not inside paintComponent
• To run an applet, you need an HTML file with the applet tag
• An HTML file can have multiple applets; add a separate applet tag for each applet
• You view applets with the applet viewer or a Java enabled browser:

  appletviewer RectangleApplet.html

ch02/applet/RectangleApplet.java

ch02/applet/RectangleApplet.html

ch02/applet/RectangleAppletExplained.html

Applets

Ellipses and Circles

• Ellipse2D.Double describes an ellipse
• This class is an inner class — doesn't matter to us except for the import statement:

  import java.awt.geom.Ellipse2D; // no .Double

• Must construct and draw the shape:

  Ellipse2D.Double ellipse = new Ellipse2D.Double(x, y, width, height);
  g2.draw(ellipse);

An Ellipse

Lines

• To draw a line:

  Line2D.Double segment = new Line2D.Double(x1, y1, x2, y2);
  g2.draw(segment);

  or,

  Point2D.Double from = new Point2D.Double(x1, y1);
  Point2D.Double to = new Point2D.Double(x2, y2);
  Line2D.Double segment = new Line2D.Double(from, to);
  g2.draw(segment);

Drawing Text

g2.drawString("Message", 50, 100);

Colors

• Standard colors Color.BLUE, Color.RED, Color.PINK etc.
• Specify red, green, blue between 0 and 255:

  Color magenta = new Color(255, 0, 255);

• Set color in graphics context:

  g2.setColor(magenta);

• Color is used when drawing and filling shapes:

  g2.fill(rectangle); // filled with current color

Predefined Colors and Their RGB Values

Color	RGB Value
Color.BLACK	0, 0, 0
Color.BLUE	0, 0, 255
Color.CYAN	0, 255, 255
Color.GRAY	128, 128, 128
Color.DARKGRAY	64, 64, 64
Color.LIGHTGRAY	192, 192, 192
Color.GREEN	0, 255, 0
Color.MAGENTA	255, 0, 255
Color.ORANGE	255, 200, 0
Color.PINK	255, 175, 175
Color.RED	255, 0, 0
Color.WHITE	255, 255, 255
Color.YELLOW	255, 255, 0

Alien Face

ch02/face/FaceComponent.java

ch02/face/FaceViewer.java

Self Check 2.32

Give instructions to draw a circle with center (100, 100) and radius 25.

• Answer:

  g2.draw(new Ellipse2D.Double(75, 75, 50, 50));

Self Check 2.33

Give instructions to draw a letter "V" by drawing two line segments.

• Answer:

  Line2D.Double segment1 = new Line2D.Double(0, 0, 10, 30);
  g2.draw(segment1);
  Line2D.Double segment2 = new Line2D.Double(10, 30, 20, 0);
  g2.draw(segment2);

Self Check 2.34

Give instructions to draw a string consisting of the letter "V".

• Answer:

  g2.drawString("V", 0, 30);

Self Check 2.35

What are the RGB color values of Color.BLUE?

• Answer: 0, 0, and 255

Self Check 2.36

How do you draw a yellow square on a red background?

• Answer: First fill a big red square, then fill a small yellow square inside:

  g2.setColor(Color.RED);
  g2.fill(new Rectangle(0, 0, 200, 200));
  g2.setColor(Color.YELLOW);
  g2.fill(new Rectangle(50, 50, 100, 100));