Java Review

Structure of a graphics program

Computer Graphics and User Interfaces

Java is Object-Oriented

• A program uses objects to model the solution to the problem being solved• Classes are used to define new data types needed by the program

– Java comes with a large collection of classes that have already been written

•Inheritance can be used to build on existing classes

Computer Graphics and User Interfaces

Java Programs

• A java program consists of one or more classes• Each class is defined in a file whose name is the name of the class followed by the extension .java

– The class named Demo2D is defined in the file Demo2D.java

• One class must contain a main method•The program executes by creating objects and calling their methods

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Structure of a Java Program

public class <classname> {// body of class goes here// Data members// Constructors// Methods}

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The main method

public static void main( String [] args) {

// code to make the program do// what it needs to

}

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Constructors

• All instantiable classes have at least one constructor– the constructor is used to create an

object– the name of the constructor is the

same as the name of the class– a constructor does not have a return

type

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Creating Objects

• You declare an object <classname> <objectname>;JFrame frame;

• Create an object using new <objectname> = new <classname>();frame = new JFrame();

• Some constructors have parameters– provide appropriate arguments when you call

themframe = newFrame( "frame title");

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Calling methods

• Call a method by giving the object name and the method name followed by a list of arguments if needed<objectname>.<methodname>(<args>);

frame.setTitle( "My Picture");

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Graphical Programs

• Graphical programs need a window– Frame and JFrame are used for

stand-alone graphical applications– Applet and JApplet are used for

programs that run in a browser

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Java Graphics Libraries

• AWT classes (e.g. Frame, Applet) were provided with original version of Java– import from java.awt package

• Swing classes (e.g. JFrame, JApplet) were added later to provide better cross-platform behavior– import from javax.swing package

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Using JFrame

• Create a class that extends JFrame • A JFrame has a content pane that

serves as – a container for other GUI components – a place to draw graphics

• Create a panel to use as the content pane– use setContentPane( panel)

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Defining a Frame class

public class <classname> extends JFrame {// constructor used to create a Frame object

public <classname>{// create a panelsetContentPane( panel); }

}

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Creating a panel

• Define a class that extends JPanel• Override the paintComponent method which

controls what is drawn in the panel• Create a panel using new

PanelClass panel

= new PanelClass()• Set the content pane of the frame to the panel

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Defining a Panel class

public class <classname> extends JPanel {// constructor if needed

public void paintComponent( Graphics g) {

// code for drawing here }}

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The main method

public static void main( String [] args) { FrameClass frame = new FrameClass(); frame.setVisible( true); frame.setDefaultClosingOperation(

Jframe.EXIT_ON_CLOSE);}

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Graphics Class

• Graphics class creates an object that can be used as a context for drawing

• The state of a Graphics object includes such properties as foreground color and font.

• Positions within the Graphics context are measured from the upper left corner and they have units of pixels.

Computer Graphics and User Interfaces

Graphics Methods

• Get and set methods are provided for properties like color, font, stroke, …

• There are methods for drawing lines, rectangles, ovals, …

• There is a method for drawing text• There is a method for displaying images

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Graphics2D extends Graphics

• It has more methods than Graphics• You can cast a Graphics to a

Graphics2D to use these methods

Graphics2D g2 = (Graphics2D)g• There is a method for drawing Shape

objects

Computer Graphics and User Interfaces

Shapes

• Shape is an interface used as a supertype for various graphical shapes– Rectangle– Ellipse2D– Polygon

• Drawing a shape on Graphics2D g2

g2.draw( shapeObject);

Displaying a String

• The drawString method can be used to

display text in a Graphics context.

void drawString( String text,

int x, int y;

* The position of the text relative to x and y

is shown in the figure

Computer Graphics and User Interfaces

Color Class

• The java.awt.Color class allows us to create a color

object.

• Color class has public constants for common colors:

– Gray scale: Color.black

– Primary colors: Color.red Color.green Color.blue

– Secondary colors: Color.yellow Color.cyan Color.magenta

– Others: Color.orange

Computer Graphics and User Interfaces

Custom Colors

• Create custom colors by specifying three

values ranging from 0 to 255 for red,

green, and blue. Color pinkColor

= new Color(255,175,175);