M C6java5

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Simple Control Structures

booleans, the if statementswitch-case

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What are control structures? You can’t do very much if your program

consists of just a list of commands to be done in order

– The program cannot choose whether or not to perform a command

– The program cannot perform the same command more than once

– Such programs are extremely limited!

Control structures allow a program to base its behavior on the values of variables

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boolean boolean is one of the eight primitive types

– booleans are used to make yes/no decisions– All control structures use booleans

There are exactly two boolean values, true (“yes”) and false (“no”)– boolean, true, and false are all lowercase

booleans are named after George Boole, the founder of Boolean logic

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Declaring boolean variables boolean variables are declared like any other

kind of variable: boolean hungry; boolean passingGrade; boolean taskCompleted = false;

boolean values can be assigned to boolean variables: taskCompleted = true;

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Numeric comparisons The following numeric comparisons

each give a boolean result: x < y // is x less than y? x <= y// is x less than or equal to y? x == y// is x equal to y? (do not use =) x != y // is x unequal to y? x >= y// is x greater than or equal to y? x > y // is x greater than y?

Reminder: Don’t use == or != for floating-point numbers

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The if statement The if statement has the form:

if (boolean-expression) statement

Examples: if (passingGrade) System.out.println("Whew!"); if (x > largest) largest = x; if (citBook.price < 40.00) citBook.purchase();

The if statement controls one other statement– Often this isn’t enough; we want to control a group of

statements

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Compound statements We can use braces to group together several

statements into one “compound” statement: { statement; statement; ...; statement; }

Braces can group any number of statements: { } // OK--this is an “empty” statement { x = 0; } // OK--braces don’t hurt { temp = x;

x = y; y = temp; } //typical use

The compound statement is the only kind of statement that does not end with a semicolon

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The if statement again The if statement controls one other statement, but it can

be a compound statement Example:

if (cost < amountInPocket) { System.out.println("Spending $" + cost); amountInPocket = amountInPocket - cost;}

It’s good style to use braces even if the if statement controls only a single statement:

if (cost > amountInPocket) { System.out.println("You can't afford it!");}

• I personally make an exception to this style rule when the controlled statement fits easily on the same line with the if:

if (x < 0) x = -x; // use absolute value of x

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Flowchart for the if statement

condition? statementtrue

false

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The if-else statement The if-else statement chooses which of two

statements to execute The if-else statement has the form:

if (condition) statement-to-execute-if-true ;else statement-to-execute-if-false ;

Either statement (or both) may be a compound statement

Notice the semicolon after each controlled statement

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Example if-else statements if (x >= 0) absX = x;

else absX = -x;

if (itemCost <= bankBalance) { writeCheck(itemCost); bankBalance = bankBalance - itemCost;}else { callHome(); askForMoreMoney(2 * itemCost);}

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Flowchart for the if-else statement

condition?true

statement-1 statement-2

false

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Aside: the “mod” operator The modulo, or “mod,” operator returns the

remainder of an integer division The symbol for this operation is % Examples:

57 % 10 gives 7 20 % 6 gives 2

Useful rule: x is divisible by y if x % y == 0 If the left operand is negative, the result is

negative (or zero)– Examples: -20 % 3 = -2, 20 % -3 = 2, -20 % -3 = -2

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Nesting if (or if-then) statements

A year is a leap year if it is divisible by 4 but not by 100, unless it is also divisible by 400

if (year % 4 == 0) { if (year % 100 == 0) { if (year % 400 == 0) leapYear = true; else leapYear = false; } else leapYear = true;}else leapYear = false;

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Operations on booleans Assume p and q are booleans There are four basic operations on

booleans:– Negation (“not”):

!p is true if p is false (and false otherwise)

– Conjunction (“and”): p && q is true if both p and q are true

– Disjunction (“or”): p || q is true if either of p and q is true

– Exclusive or (“xor”): p ^ q is true if just one of p and q is true

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Simpler tests A simpler leap-year test:

if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) leapYear = true;else leapYear = false;

An even simpler leap-year test:

leapYear = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);

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The if-else statement The if-else statement chooses which of two

statements to execute The if-else statement has the form:

if (condition) statement-to-execute-if-true ;else statement-to-execute-if-false ;

Either statement (or both) may be a compound statement

Notice the semicolon after each statement The else part is optional

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Dangling Else

Compiler cannot determine which “if” an “else” belongs to if there are no braces

String password = Keyboard.readString();if (password.equals (realPassword)) if (name.equals (“admin”))

loggedIn = superPrivileges = true; else System.out.println (“Error”);

Java matches else with last unfinished if Moral: Use shortcuts at your own risk – or don’t !

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Multiway selection

Multiple conditions, each of which causes a different block of statements to execute

Can be used where there are more than 2 options

if (condition1){ statements …}else{ if (condition2) { statements … } else …}

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“if” ladder

Just a nicer way to write multiway selection

if (operation == ‘a’){ answer = first + second;}else if (operation == ‘s’){ answer = first – second;}else if (operation == ‘m’){ answer = first * second;}

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The “switch” statement

Selects among different statements based on a single integer or character expression

Each set of statements starts in “case” and ends in “break” because switch does not use {}s

– break passes control to statement immediately after switch “default” applies if none of the cases match

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Sample switch statement

switch (SouperSandwichOrder){ case 1 : cheese = 1; break; case 2 : cheese = 1; tomato = 1; break; case 3 : cheese = 1; tomato = 1; chukka = 1; break; default : cheese = 1; break;}

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“break” optimisation

If break is omitted, control continues to next statement in the switch

switch (SouperSandwichOrder){ case 3 : chukka = 1; case 2 : tomato = 1; case 1 : default : cheese = 1;}

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Characters in “switch”

char Operation = Keyboard.readChar (“What to do?”);switch (Operation){ case ‘a’ : answer = a + b; break; case ‘s’ : answer = a – b; break; case ‘m’ : answer = a * b; break; case ‘d’ : if (b != 0) { answer = a / b; break; } default : answer = 0; System.out.println (“Error”); break;}

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Boolean operators

Boolean Algebra

Java Meaning

AND && true if both parameters are true

OR || true if at least one parameter is true

NOT ! true if parameter is false;false if parameter is true;

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Operator precedence Now that we have seen how operators can be mixed, we

need precedence rules for all operators– () (highest precedence – performed first)– !– * / %– + -– < <= > >=– == !=– &&– ||– = (lowest precedence – performed last)

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Reversing expressions

Use ! operator to reverse meaning of boolean expression, e.g.,if (mark >= 0){ // do nothing}else System.out.println (“Error”);

Instead, invert the conditionif (! (mark >= 0)) System.out.println (“Error”);

Can we do better ?

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Afvinkopdracht 5: Tic-tac-toe

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Graphic programming

The GUI revisited

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Java, unlike C & C++, has standard packages for graphics

2 related packages and sub-packages support graphics in Java

– java.awt (Abstract Windows Toolkit)

– javax.swing

AWT is ‘peer-based’– Depends on graphical elements native local platform’s graphics

system

– Unix/Windows graphical programs written using AWT will have a different ‘look and feel’

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Swing is much more platform independent – Graphical components are pre-built and are simply painted onto

windows

– Relies less on the underlying runtime environment

– Usually slower than AWT-based programs

In practice graphical programs are a mixture of Swing and AWT classes

– AWT takes care of all of the event handling for GUI’s (see later)

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Frames A frame is a top level window which is a

container for graphical components (canvas, buttons, menus etc)

The AWT has a Frame class and Swing has a JFrame class

The following program displays an empty frame

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import javax.swing.*;

class MyFrame extends JFrame{

public MyFrame() { setTitle("My first graphics program"); setSize(400,300); }}

public class FrameTest{ public static void main(String[] args) { JFrame frame=new MyFrame(); frame.show(); }}

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A class MyFrame is defined which is a sub-class of JFrame

– A title is added

– The frame is sized to 400x300 (by default, a frame is 0x0)

The frame is created by a call to the constructor

The frame is displayed by a call to JFrame.show()– This creates a separate thread which runs until the program is

terminated – the main thread terminates

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Swing inheritance hierarchy The JFrame class inherits attributes from

higher level container classes– Typically for resizing and positioning frames

Class names beginning with ‘J’ are Swing classes – everything else is part of AWT

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Component

Frame

Window

Container

JFrame

JComponent

JPanel…..

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Most swing components (for example JPanel) are derived from the JComponent class

JFrame, being a top level window, is derived from the Window class

Other top level windows include JApplet and JDialog

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Displaying graphics in frames – panels

Frames are containers – they can contain other user interface/graphical components

A frame contains a content pane into which components can be added

The following code is typical

Container contentPane=frame.getContentPane();

Component c= ….; // UI or graphical component

contentPane.add (c); // Add to the frame

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Content pane

FrameJPanel

SomeTextJField

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Panels Panels (JPanel class) are added to the content pane Panels are themselves containers

– The can contain other UI components– They also have a surface onto which graphics can be drawn

Text Basic shapes (lines, boxes etc) Images

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Drawing on panels The paintComponent() method in JComponent

(a superclass of JPanel) must be overridden paintComponent() is called automatically when

the window has to be drawn or redrawn – for example when it is moved by the user. It is also called when the repaint() method of a panel is called

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The following code creates a class MyPanel into which graphics can be drawn

class MyPanel extends JPanel{

public void paintComponent(Graphics g){

super.paintComponent(g);

// Code placed here to draw graphics}

}

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The Graphics object defines the graphics context (fonts, line styles, colours etc)

A call to super.paintComponent() calls the paintComponent() method in JComponent (the base class of JPanel)

– This call sets up the graphics context and performs other complex tasks

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Displaying text in graphics windows

Text can be drawn onto panels using the Graphics.drawString() method

The text font and size can be optionally set/reset

The following program draws a string onto a panel

– The panel is then added to a frame which is then displayed using JFrame.show()

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import javax.swing.*;import java.awt.*;

public class MyPanel extends JPanel{

public void paintComponent(Graphics g) { super.paintComponent(g); g.drawString("Hello there!",150,125); }}

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import java.awt.event.*;import javax.swing.*;import java.awt.*;

public class HelloFrame extends JFrame{ public HelloFrame() { setTitle("Drawing a string example"); setSize(400,300); addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } });

Container contentPane=getContentPane(); contentPane.add(new MyPanel()); }}

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public class FrameTest{

public static void main(String[] args){

JFrame frame=new HelloFrame(); frame.show();

}}

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Text fonts can be set/reset– The existing font applies until it is reset

The following code sets a bold Helvetica font with a larger font size

public class MyPanel extends JPanel{

public void paintComponent(Graphics g) { super.paintComponent(g);

Font f=new Font(“Helvetica”,Font.BOLD,25); g.setFont(f);

g.drawString("Hello there!",150,125); }}

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Drawing simple graphics Class java.awt.Graphics contains methods which allow

simple graphics to be drawn in different colours Graphics.setcolor() sets the drawing colour

– Colour is represented by the class java.awt.Color(int red, int blue, int green) defining the RGB components

– Preset constants exist (defined as static constants in Color) Color.red Color.orange Color.pink etc

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Examples of different shapes– Graphics.drawLine(int x1, int y1, int x2, int y2) draws a straight line

from (x1,y1) to (x2,y2)

– Graphics.drawRect(int x, int y, int w, int h) draws a rectangle from upper left hand corner (x,y) with width w and height h

– Graphics.drawOval(int x, int y, int w, int h) draws an outline of an ellipse with a ‘bounding rectangle’ as above

– Graphics.drawPolygon(int[] xc, int[] yc, int n) draws a polygon with n vertices with the co-ordinates being stored in arrays xc and yc

– Graphics.fillOval (int x, int y, int w, int h) fills the oval with the current draw colour

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class DrawPanel extends JPanel{ public void paintComponent(Graphics g) { super.paintComponent(g); g.setColor(Color.red); g.drawRect(20,30,50,50);

g.setColor(Color.green); g.drawOval(100,30,90,60); g.fillOval(100,30,90,60);

g.setColor(Color.yellow); int[] xcoords={180,200,250,275,225}; int[] ycoords={170,130,130,150,200}; g.drawPolygon(xcoords,ycoords,5); g.fillPolygon(xcoords,ycoords,5); }}

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Displaying images We can read images stored in GIF and JPEG formats and

draw the image onto a graphics panel using Graphics.drawImage()

When an image is read from file, a new thread of execution is started in parallel

– Usually, the program needs to wait until the image is loaded

– Loaded images need to be ‘tracked’ and the program informed when the loading is complete

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Normal program thread

Load image from fileCreate new thread

Image loading thread

Program waits to be informed when image loaded

Image loading complete – send signal

Normal program thread resumes

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Image read from file by a Toolkit object– getDefaultToolkit() returns the default toolkit– getDefaultToolkit().getImage(filename) reads the jpg or gif file

containing the image

An image is added to a tracker object which sends a signal back to the panel when the loading is complete

The try/catch statements are for exception handling – causes the program to wait for the image to be loaded (see later)

Following program draws an image into a panel

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import java.awt.*;import java.awt.event.*;import javax.swing.*;

class ImagePanel extends JPanel{

public ImagePanel(){ image = Toolkit.getDefaultToolkit().getImage(“Pisa.jpg”); MediaTracker tracker=new MediaTracker(this); tracker.addImage(image,0); try {tracker.waitForID(0);} catch (InterruptedException e){}}

public void paintComponent(Graphics g) {

super.paintComponent(g); g.drawImage(image,0,0,this);}

private Image image;}

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And finally …. Swing/AWT are massive and complex

– We have only scratched the surface Typically Java API’s have been built on top of Swing

– Java2D– Java3D

In practice, you would use these to do real work for example involving image processing or 3D rendering

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“640K ought to be enough for anybody.”

--Bill Gates, 1981

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