Horstmann Chapter 2

Big Java book cover

Chapter 2 – Using Objects

Chapter Goals

goals

  • To learn about variables
  • To understand the concepts of classes and objects
  • To be able to call methods
  • To learn about arguments 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

Objects and Classes

""

 

Each part a home builder uses, such as a furnace or a water heater, fulfills a particular function. Similarly, you build programs from objects, each of which has a particular behavior.

  • In Java, you build programs for objects.
  • Each object has certain behaviors.
  • You can manipulate the object to get certain effects.

Using Objects

  • Object: an entity in your program that you can manipulate by calling one or more of its methods.
  • Method: consists of a sequence of instructions that can access the data of an object.
    • You do not know what the instructions are
    • You do know that the behavior is well defined
  • System.out has a println method
    • You do not know how it works
    • What is important is that it does the work you request of it
  • System.out object

    Figure 1 Representation of the System.out Object

Using Objects

soloar panel

You can think of a water heater as an object that can carry out the "get hot water" method. When you call that method to enjoy a hot shower, you don't care whether the water heater uses gas or solar power.

Classes

  • A class describes a set of objects with the same behavior.
  • Some string objects
      "Hello World"
      "Goodbye"
      "Mississippi"
  • You can invoke the same methods on all strings.
  • System.out is a member of the PrintStream class that writes to the console window.
  • You can construct other objects of PrintStream class that write to different destinations.
  • All PrintStream objects have methods println and print.

Classes

  • Objects of the PrintStream class have a completely different behavior than the objects of the String class.
  • Different classes have different responsibilities
    • A string knows about the letters that it contains
    • A string doesn't know how to send itself to a console window or file.
  • house with windows
    All objects of the Window class share the same behavior.

Self Check 2.1

In Java, objects are grouped into classes according to their behavior. Would a window object and a water heater object belong to the same class or to different classes? Why?
  • Answer: Objects with the same behavior belong to the same class. A window lets in light while protecting a room from the outside wind and heat or cold. A water heater has completely different behavior. It heats water. They belong to different classes.

Self Check 2.2

Some light bulbs use a glowing filament, others use a fluorescent gas. If you consider a light bulb a Java object with an "illuminate" method, would you need to know which kind of bulb it is?
  • Answer: When one calls a method, one is not concerned with how it does its job. As long as a light bulb illuminates a room, it doesn't matter to the occupant how the photons are produced.

Variables

  • Use a variable to store a value that you want to use later
  • To declare a variable named width
    int width = 20;
  • Like a variable in a computer program, a parking space has an identifier and a contents.
    parking analogy

Syntax 2.1 Variable Declaration

Variable Declaration

Variables

  • A variable is a storage location
    • Has a name and holds a value
  • When declaring a variable, you usually specify an initial value.
  • When declaring a variable, you also specify the type of its values.
  • Variable declaration: int width = 20:
    • width is the name
    • int is the type
    • 20 is the initial value
  • Each parking space is suitable for a particular type of vehicle, just as each variable holds a value of a particular type.
    bicycle parking

Variable Declarations

table of variable declarations

 

Types

  • Use the int type for numbers that cannot have a fractional part.
      int width = 20;
  • Use the double type for floating point numbers.
      double milesPerGallon = 22.5;
  • Numbers can be combined by arithmetic operators such as +, -, and *
  • Another type is String
      String greeting = "Hello";
  • A type specifies the operations that can be carried out with its values.
    • You can multiply the value width holds by a number
    • You can not multiply greetings by a number.

Names

  • Pick a name for a variable that describes its purpose.
  • Rules for the names of variables, methods, and classes:
    • Must start with a letter or the underscore (_) character, and the remaining characters must be letters, numbers, or underscores.
    • Cannot use other symbols such as ? or % or a space
      • camelUse uppercase letters to denote word boundaries, as in milesPerGallon. (Called camel case)
    • Names are case sensitive
    • You cannot use reserved words such as double or class
  • By Java convention:
    • variable names start with a lowercase letter.
    • class names start with an uppercase letter.

Variable Names in Java

table 2 variable names

 

Comments

  • Use comments to add explanations for humans who read your code.
    double milesPerGallon = 33.8; // The average fuel efficiency of new U.S. cars in 2011
  • The compiler does not process comments
    • It ignores everything from a // delimiter to the end of the line.
  • For longer comment, enclose it between /* and */ delimiters.
    • The compiler ignores these delimiters and everything in between.
  • Example of longer comments
    /*
      In most countries, fuel efficiency is measured in liters per hundred
      kilometer. Perhaps that is more useful—it tells you how much gas you need
      to purchase to drive a given distance. Here is the conversion formula.
    */
    double fuelEfficiency = 235.214583 / milesPerGallonS

Assignment

  • Use the assignment operator (=) to change the value of a variable.
    • You have the following variable declaration
    int width = 10;
  • To change the value of the variable, assign the new value
width = 20;
assignment

Figure 2 Assigning a New Value to a Variable

Assignment

  • It is an error to use a variable that has never had a value assigned to it:
    int height;
    int width = height; // ERROR - uninitialized variable height
    • The compiler will complain about an "uninitialized variable"
    • uninitialed variable

      Figure 3 An Uninitialized Variable

  • Remedy: assign a value to the variable before you use it.
    int height = 20;
    int width = height; // OK
  • All variables must be initialized before you access them.

Assignment

  • The right-hand side of the = symbol can be a mathematical expression:
    width = height + 10;
    • This means
      1. compute the value of height + 10
      2. store that value in the variable width
width = width + 10
  • The assignment operator = does not denote mathematical equality.
  • assignment illustration

    Figure 4 Executing the Statement width = width + 10

    Syntax 2.2 Assignment

    Variable Declaration

    Self Check 2.3

    What is wrong with the following variable declaration?
    int miles per gallon = 39.4
    • Answer: There are three errors:
      1. You cannot have spaces in variable names.
      2. The variable type should be double because it holds a fractional value.
      3. There is a semicolon missing at the end of the statement.

    Self Check 2.4

    Declare and initialize two variables, unitPrice and quantity, to contain the unit price of a single item and the number of items purchased. Use reasonable initial values.
    • Answer:
      double unitPrice = 1.95;
      int quantity = 2;

    Self Check 2.5

    Use the variables declared in Self Check 4 to display the total purchase price.
    • Answer:
      System.out.print("Total price: ");
      System.out.println(unitPrice * quantity);

    Self Check 2.6

    What are the types of the values 0 and "0"?
    • Answer: int and String

    Self Check 2.7

    Which number type would you use for storing the area of a circle?
    • Answer: double

    Self Check 2.8

    Which of the following are legal identifiers?

      Greeting1
      g
      void
      101dalmatians
      Hello, World
      <greeting>
    • Answer: Only the first two are legal identifiers.

    Self Check 2.9

    Declare a variable to hold your name. Use camel case in the variable name.
    • Answer: String myName = "John Q. Public";

    Self Check 2.10

    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 2.11

    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 declares a new variable

    Self Check 2.12

    How would you explain assignment using the parking space analogy?
    • Answer: Assignment would occur when one car is replaced by another in the parking space.

    Calling Methods

    • You use an object by calling its methods.
    • All objects of a given class share a common set of methods.
    • The PrintStream class provides methods for its objects such as:
      • println
      • print
    • The String class provides methods that you can apply to all String objects.
    • Example: length
      String greeting = “Hello, World!”;
      int numberOfCharacters = greeting.length();
    • Example: toUpperCase
      String river = “Mississippi”;
      String bigRiver = river.toUpperCase();

    The Public Interface of a Class

    public interface of a car

    The controls of a car form its public interface. The private implementation is under the hood.

    • The String class declares many other methods besides the length and toUpperCase methods.
    • Collectively, the methods form the public interface of the class.
    • The public interface of a class specifies what you can do with its objects.
    • The hidden implementation describes how these actions are carried out.

    A Representation of Two String Objects

    • Each String object stores its own data.
    • Both objects support the same set of methods.
      • Those methods form the public interface
      • Public interface is specified by the String class.

    Method Arguments

    • Most methods require values that give details about the work that the method needs to do.
    • You must supply the string that should be printed when you call the println method.
    • The technical term for method inputs: arguments.
    • The string greeting is an argument of this method call:
      System.out.println(greeting);
    • argument passed to a method

      Figure 6 Passing an Argument to the println Method


    Method Arguments

    sew method

     

    At this tailor shop, the customer's measurements and the fabric are the arguments of the sew method. The return value is the finished garment.

    Method Arguments

    • Some methods require multiple arguments.
    • Other methods don't require any arguments at all.
      • Example: the length method of the String class
      • All the information that the length method requires to do its job is stored in the object
      • no argument method

        Figure 7 Invoking the length Method on a String Object


    Return Values

    • Some methods carry out an action for you.
      • Example: println method
    • Other methods compute and return a value.
      • Example: the String length method
        • returns a value: the number of characters in the string.
      • You can store the return value in a variable:
      int numberOfCharacters = greeting.length()
  • The return value of a method is a result that the method has computed.
  • Return Values

    • You can also use the return value of one method as an argument of another method:
      System.out.println(greeting.length());
      • The method call greeting.length() returns a value - the integer 13.
      • The return value becomes an argument of the println method.
    • return value passed

      Figure 8 Passing the Result of a Method Call to Another Method

    Return Values - replace method

    Example: assume String river = "Mississippi";

    • Then the statement
      river = river.replace("issipp", "our");
      • Constructs a new String by
        • Replacing all occurrences of "issipp" in"Mississippi" with "our"
      • Returns the constructed String object "Missouri"
      • And saves the return value in the same variable
    • You could pass the return value to another method:
      System.out.println(river.replace("issipp", "our"))

    Return Value - replace method - Continued

    • The method call
      river.replace("issipp", "our"))
      • Is invoked on a String object: "Mississippi"
      • Has two arguments: the strings "issipp" and "our"
      • Returns a value: the string "Missouri"
    • method call

      Figure 9 Calling the replace Method

    Method Arguments and Return Values

    table3 method arguments and return values

    Method Declaration

    • To declare a method in a class, specify
      • The types of the arguments
      • The return value
    • Example: public int length()
      • There are no arguments
      • The return value has the type int

    Method Declaration - continued

    • Example: public String replace(String target, String replacement)
      • Has two arguments, target and replacement
      • Both arguments have type String
      • The returned value is another string
    • Example: public void println(String output)
      • Has an argument of type String
      • No return value
      • Uses the keyword void

    Method Declaration

    • A class can declare two methods with the same name and different argument types.
    • The PrintStream class declares another println method
      public void println(int output)
      • Used to print an integer value
    • The println name is overloaded because it refers to more than one method.

    Self Check 2.13

    How can you compute the length of the string "Mississippi"?
    • Answer: river.length() or "Mississippi".length()

    Self Check 2.14

    How can you print out the uppercase version of "Hello, World!"?
    • Answer:
      System.out.println(greeting.toUpperCase());
      Or
      System.out.println("Hello, World!".toUpperCase());

    Self Check 2.15

    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.

    Self Check 2.16

    What are the arguments in the method call river.replace("p", "s")?
    • Answer: The arguments are the strings "p" and "s".

    Self Check 2.17

    What is the result of the call river.replace("p", "s"), where river is "Mississippi"?
    • Answer: "Missississi"

    Self Check 2.18

    What is the result of the call greeting.replace("World", "Dave").length(), where greeting is "Hello, World!"?
    • Answer: 12

    Self Check 2.19

    How is the toUpperCase method declared in the String class?
    • Answer: As public String toUpperCase(), with no argument and return type String.

    Constructing Objects

    Objects of the Rectangle class describe rectangular shapes.
    rectangular shapes

    Constructing Objects

    • The Rectangle object is not a rectangular shape.
    • It is an object that contains a set of numbers.
      • The numbers describe the rectangle
    • Each rectangle is described by:
      • The x- and y-coordinates of its top-left corner
      • Its width
      • And its height.

    Constructing Objects

    • In the computer, a Rectangle object is a block of memory that holds four numbers.

    Constructing Objects

    • Use the new operator, followed by a class name and arguments, to construct new 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
    • The process of creating a new object is called construction.
    • The four values 5, 10, 20, and 30 are called the construction arguments.
    • Usually the output of the new operator is stored in a variable:
      Rectangle box = new Rectangle(5, 10, 20, 30);
      
    • Additional constructor
      new Rectangle() 

    Syntax 2.3 Object Construction

    construction syntax

    Self Check 2.20

    How do you construct a square with center (100, 100) and side length 20?
    • Answer:
      new Rectangle(90, 90, 20, 20)

    Self Check 2.21

    Initialize the variables box and box2 with two rectangles that touch each other.
    • Answer:
      Rectangle box = new Rectangle(5, 10, 20, 30);
      Rectangle box2 = new Rectangle(25, 10, 20, 30);

    Self Check 2.22

    The getWidth method returns the width of a Rectangle object. What does the following statement print?
    System.out.println(new Rectangle().getWidth());
    • Answer:
      0

    Self Check 2.23

    The PrintStream class has a constructor whose argument is the name of a file. How do you construct a PrintStream object with the construction argument "output.txt"?
    • Answer: new PrintStream("output.txt");

    Self Check 2.24

    Write a statement to save the object that you constructed in Self Check 23 in a variable.
    • Answer: PrintStream out = new PrintStream("output.txt");

    Accessor and Mutator Methods

    • Accessor method: does not change the internal data of the object on which it is invoked.
      • Returns information about the object
      • Example: length method of the String class
      • Example: double width = box.getWidth();
    • Mutator method: changes the data of the object
      box.translate(15, 25);
      • The top-left corner is now at (20, 35).

    Self Check 2.25

    What does this sequence of statements print?
    Rectangle box = new Rectangle(5, 10, 20, 30);
    System.out.println("Before: " + box.getX());
    box.translate(25, 40);
    System.out.println("After: " + box.getX());
    • Answer:
      Before: 5
      After: 30

    Self Check 2.26

    What does this sequence of statements print?
    Rectangle box = new Rectangle(5, 10, 20, 30);
    System.out.println("Before: " + box.getWidth());
    box.translate(25, 40);
    System.out.println("After: " + box.getWidth());
    • Answer:

      Before: 20
      After: 20
      Moving the rectangle does not affect its width or height. You can change the width and height with the setSize method.

    Self Check 2.27

    What does this sequence of statements print?
    String greeting = "Hello";
    System.out.println(greeting.toUpperCase());
    System.out.println(greeting);
    • Answer:
      HELLO
      hello

    • Note that calling toUpperCase doesn't modify the string.

    Self Check 2.28

    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 2.29

    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 of the Java library
    • Application programmer: A programmer who uses the Java classes to put together a computer program (or application)
    • Systems Programmer: A programmer who designs and implements library classes such as PrintStream and Rectangle
    • http://docs.oracle.com/javase/7/docs/api/index.html

    Browsing the API Documentation

    API screenshot

    Figure 13 The API Documentation of the Standard Java Library

    • Locate Rectangle link in the left pane
    • Click on the link
      • The right pane shows all the features of the Rectangle class

    Browsing the API Documentation - Method Summary

    method summary in API

    Figure 14 The Method Summary for the Rectangle Class

    • The API documentation for each class has
      • A section that describes the purpose of the class
      • Summary tables for the constructors and methods
      • Clicking on a method's link leads to a detailed description of the method

    Browsing the API Documentation

    The detailed description of a method shows:
    • The action that the method carries out number one
    • The types and names of the parameter variables that receive the arguments when the method is called number 2
    • The value that it returns (or the reserved word void if the method doesn't return any value).

    Packages

    • Java classes are grouped into packages.
    • The Rectangle class belongs to the package java.awt.
    • To use the Rectangle class you must import the package:
      import java.awt.Rectangle;
    • Put the line at the top of your program.
    • 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

    package syntax

    Self Check 2.30

    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 2.31

    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 2.32

    Look into the API documentation of the Rectangle class. What is the difference between the methods void translate(int x, int y) and void setLocation(int x, int y)?
    • Answer: The arguments of the translate method tell how far to move the rectangle in the x- and y-directions. The arguments of the setLocation method indicate the new x- and y-values for the top-left corner. For example, box.translate(1, 1) moves the box one pixel down and to the right. box.setLocation( 1, 1) moves box to the top-left corner of the screen.

    Self Check 2.33

    The Random class is declared 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.

    Self Check 2.34

    In which package is the BigInteger class located? Look it up in the API documentation.
    • Answer: In the java.math package

    Implementing a Test Program

    • A test program verifies that methods behave as expected.
    • Steps in writing a tester class
      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.

    Implementing a Test Program

    • Code to test the behavior of the translate method
      Rectangle box = new Rectangle(5, 10, 20, 30);
          
      // Move the rectangle
      box.translate(15, 25);
      
      // Print information about the moved rectangle
      System.out.print("x: ");
      System.out.println(box.getX());
      System.out.println("Expected: 20");
    • Place the code inside the main method of the MoveTester class
    • Determining the expected result in advance is an important part of testing.

    section_7/MoveTester.java

    Your browser does not support this feature Program Run:
    x: 20
    Expected: 20
    y: 35
    Expected: 35

    Self Check 2.35

    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 2.36

    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.

    Object References

    • An object variable is a variable whose type is a class
      • Does not actually hold an object.
      • Holds the memory location of an object

    • variable in menory

      Figure 15 An Object Variable Containing an Object Reference

    Object References

    • Object reference: describes the location of an object
    • After this statement:
      Rectangle box = new Rectangle(5, 10, 20, 30);
      • Variable box refers to the Rectangle object returned by the new operator
      • The box variable does not contain the object. It refers to the object.

    Object References

    Multiple object variables can refer to the same object:

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

    Figure 16 Two Object Variables Referring to the Same Object

    Numbers

    • Numbers are not objects.
    • Number variables actually store numbers.

    • number variable containing 13

      Figure 17 A Number Variable Stores a Number

    Copying Numbers

    • When you copy a number
      • the original and the copy of the number are independent values.
    int luckyNumber = 13;
    int luckyNumber2 = luckyNumber;
    luckyNumber2 = 12;

    Figure 18 Copying Numbers

    Copying Object References

    • When you copy an object reference
      • both the original and the copy are references to the same object
      Rectangle box = new Rectangle(5, 10, 20, 30);
      Rectangle box2 = box;
      box2.translate(15, 25);
    copying references

    Figure 19 Copying Object References

    Self Check 2.37

    What is the effect of the assignment greeting2 = greeting?
    • Answer: Now greeting and greeting2 both refer to the same String object.

    Self Check 2.38

    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.

    Mainframe Computer


    Mainframe Computer

    Graphical Applications: Frame Windows

    butterflies in a frame

    A graphical application shows information inside a frame.

    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);

    section_9_1/EmptyFrameViewer.java

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    a frame

    Figure 20 A Frame Window

    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 stores 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;
            . . .
         }
      }

    Classes Graphics and Graphics2D

    • The draw method of the Graphics2D class draws 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);
            . . .
         }
      }

    Coordinate System of a Component

    • The origin (0, 0) is at the upper-left corner of the component.
    • The y-coordinate grows downward.
    • coordinate system

    Drawing Rectangles

    We want to create an application to display two rectangles.

    two rectangles

    Figure 2 Drawing Rectangles

    section_9_2/RectangleComponent.java

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    Displaying a Component in a Frame

    • In a graphical application you need:
      • A frame to show the application
      • A component for the drawing.
    • The steps for combining the two:
      1. Construct a frame object and configure it.
      2. Construct an object of your component class:
      3. RectangleComponent component = new RectangleComponent();
      4. Add the component to the frame:
      5. frame.add(component);
      6. Make the frame visible.

    section_9_3/RectangleViewer.java

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    Self Check 2.39

    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 2.40

    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.

    Self Check 2.41

    How do you modify the program to draw two squares?
    • Answer: Rectangle box = new Rectangle(5, 10, 20, 20);

    Self Check 2.42

    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 2.43

    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.

    Ellipses

    • To construct an ellipse, you specify its bounding box.
    • ellipse

      Figure 22 An Ellipse and Its Bounding Box

    • To construct an Ellipse:

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

    Ellipses

    • Ellipse2D.Double is an inner class — doesn't matter to us except for the import statement:
      import java.awt.geom.Ellipse2D; // No .Double
    • To draw the shape:
      g2.draw(ellipse);

    Circles

    • To draw a circle, set the width and height to the same values:
      Ellipse2D.Double circle = new Ellipse2D.Double(x, y, diameter, diameter);
      g2.draw(circle);
    • (x, y) is the top-left corner of the bounding box, not the center of the circle.

    Lines

    • To draw a line, specify its end points:
      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

    • To draw text, use the drawString method
      • Specify the string and the x- and y-coordinates of the basepoint of the first character
      g2.drawString("Message", 50, 100);

    drawstring method

    Figure 23 Basepoint and Baseline

    Colors

    • To draw in color, you need to supply a Color object.
    • Specify the amount of red, green, blue as values between 0 and 255:
      Color magenta = new Color(255, 0, 255);
    • The Color class declares a variety of colors:
        Color.BLUE, Color.RED, Color.PINK etc.
    • To draw a shape in a different color
      • First set color in graphics context:
      g2.setColor(Color.Red);
      g2.draw(circle); // Draws the shape in red

    Colors - continued

    • To color the inside of the shape, use the fill method:
      g2.fill(circle); // Fills with current color
    • When you set a new color in the graphics context, it is used for subsequent drawing operations.

    Predefined Colors

    Color RGB Value
    Color.BLACK 0, 0, 0     
    Color.BLUE 0, 0, 255     
    Color.CYAN 0, 255, 255     
    Color.GRAY 128, 128, 128     
    Color.DARK_GRAY 64, 64, 64     
    Color.LIGHT_GRAY 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

    drawing of alien face

    Figure 24 An Alien Face

    The code is on following slides:

    section_10/FaceComponent.java

    Your browser does not support this feature

     

    section_10/FaceViewer.java

    Your browser does not support this feature

    Self Check 2.44

    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 2.45

    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 2.46

    Give instructions to draw a string consisting of the letter "V".
    • Answer:
      g2.drawString("V", 0, 30);

    Self Check 2.47

    What are the RGB color values of Color.BLUE?
    • Answer: 0, 0, and 255

    Self Check 2.48

    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));