Fcps java packets answer key

Unit Three – User Interface

July 2011

Developed by Shane Torbert

edited by Marion Billington

under the direction of Gerry Berry

Thomas Jefferson High School for Science and Technology

Fairfax County Public Schools

Fairfax, Virginia

FCPS

Java

Packets

http://www.tjhsst.edu/

Three-1

Java

Unit3

Contributing Authors

The author is grateful for additional contributions from Michael Auerbach, Marion Billington, Charles Brewer, Margie

Cross, Cathy Eagen, Anne Little, John Mitchell, John Myers, Steve Rose, Ankur Shah, John Totten, and Greg W.

Price.

The students' supporting web site can be found at http://academics.tjhsst.edu/compsci/

The teacher's (free) FCPS Computer Science CD is available from Stephen Rose (srose@fcps.edu)

License Information

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Three-2

Java Unit3

Java Instruction Plan—Unit Three

Section One – Event Driven Programming Page

Lab00: Hello Button . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-3 to 5

Lab01: Hello Text Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-6 to 8

Lab02: Multiple Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-9 to 11

Lab03: Hailstone Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-12 to 14

Section Two – What’s Inside the Black Box?

Lab04: Odometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-15 to 16

Lab05: Press Your Luck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-17 to 19

Lab06: Luck of the Roll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-20 to 21

Lab07: GCD and LCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-22 to 24

Section Three – GUI Formatting

Lab08: Sum of a Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-25 to 26

Lab09: Subway Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-27 to 29

Lab10: High - Low . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-29 to 30

Lab11: Last Stone Wins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-31

Section Four – I/O Window Output

Lab12: Asterisks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-32 to 34

Lab13: The Necklace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-35

Lab14: Bowling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-36

Section Five – Additional GUI Topics

Lab15: More GUI Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-37

Lab16: The jGrasp Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-39

Lab17: Mixing Graphics and GUIs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-40

Three-3

Java

Unit3

Discussion Panels, GUI Components, and Listeners

A Graphical User Interface (GUI) allows users to point and click on on-screen "buttons." Almost all

computers today are operated through GUIs, but once upon a time people used keyboards, punchcards, or even

dials and toggle switches. A GUI is quite hard to program, unless you are in an object-oriented language, in

which case it is fairly easy. That's because an object oriented language takes advantage of abstraction,

inheritance, and encapsulation. For example, each object in a GUI, such as the buttons, textfields, labels,

panels, and frame, "knows" its own attributes and methods—that is encapsulation. Like the Polkadot and Ball

classes in Unit 2, each GUI object even knows how to draw itself. Each object inherits some of its methods

from higher up the hierarchy, overrides some methods, and adds new methods. The hierarchy proceeds from

abstract to concrete, with the programmers assigning powers at the appropriate level. For example, abstractly

considered, all JComponent objects can be assigned ActionListeners. You, the programmer, work with the

concrete objects when you assign a specific button to a specific listener. (You did the same thing in Unit 2,

when you assigned a timer to a specific listener.) You also, as the last programmer in the chain, get to say

exactly what happens when each button is clicked.

The structure of the applications in

this unit is exactly the same as in Unit

2. The driver creates a frame, puts the

panel on the frame, and displays the

frame. The panel has the buttons,

labels, text boxes, and even other

panels. Visually, a GUI is not a whole

lot different from straight graphics.

However, in a GUI the program reacts

to something the user does. Unlike

programs in Units 1 and 2, our GUI

will wait patiently until a button is

clicked, then it springs forth and does

its thing. Best of all, we don't have to

know the details of how all that

works. Someone else programmed

buttons and listeners for us.

Just for fun, here is the hierarchy, including interfaces, for a JButton:

java.lang.Object

java.awt.Component

java.awt.Container

javax.swing.JComponent

javax.swing.AbstractButton

javax.swing.JButton

All Implemented Interfaces:

Accessible, ImageObserver, ItemSelectable, MenuContainer, Serializable, SwingConstants

listener

component

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Component.html

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Container.html

http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/JComponent.html

http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/AbstractButton.html

http://java.sun.com/j2se/1.4.2/docs/api/javax/accessibility/Accessible.html

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/image/ImageObserver.html

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/ItemSelectable.html

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/MenuContainer.html

http://java.sun.com/j2se/1.4.2/docs/api/java/io/Serializable.html

http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/SwingConstants.html

Three-4

Java Unit3

Lab00 Hello Button

Objective

Understand frames, panels, and GUI components.

Background

The drivers in Unit 3 are exactly the same as those in Unit 2. The panel

will have GUI components like this:

Specification

Filenames Unit3\Lab00\Driver00.java and Unit3\Lab00\Panel00.java. Enter the source code for the driver.

Enter the source code for the resource class (the panel) as shown below, then compile and run the program.

Lines 4-6: import the swing,

awt, and event classes.

Line 9: The panel's JLabel is

a private field. This is

essential if we want to access

the label in the listener.

Lines 10-22: The constructor.

Line 12: set the panel‘s layout

Lines 14-17: Instantiate a

label, set its font and color,

then add it to the panel.

Lines 19-21: Instantiate a

button, register its listener,

then add it to the panel.

Line 23: Listener implements

the ActionListener interface,

meaning that actionPerformed

must have code.

Line 28: (" " + x) converts a

number to a string, because

setText requires a string.

The scope of a variable is the region of the program in which it is valid to use that variable. The global

private JLabel label in Line 9 has a wider scope than the local JButton button on Line 19 and the

local double x in Line 27. The blue lines down the side in jGrasp show the scope of the different variables.

We need to declare the panel‘s private Jlabel label on Line 9 because we want the listener on Line

28 to use it. In contrast, the listener does not need to use the button. Therefore, we declared JButton

button on Line 19, restricting its scope entirely inside the constructor. Similarly, the double x in Line 27

has a scope that entirely inside the actionPerformed method, which is where we want to use it.

Test Data Go to www.tjhsst.edu/compsci

JLabel

JButton

4 import javax.swing.*;

5 import java.awt.*;

6 import java.awt.event.*;

7 public class Panel00 extends JPanel

8 {

9 private JLabel label; //a private field

10 public Panel00()

11 {

12 setLayout(new FlowLayout());

13

14 label = new JLabel("0.0000000000000000");

15 label.setFont(new Font("Serif", Font.BOLD, 20));

16 label.setForeground(Color.blue);

17 add(label);

18

19 JButton button = new JButton("Random"); //local

20 button.addActionListener(new Listener());

21 add(button);

22 }

23 private class Listener implements ActionListener

24 {

25 public void actionPerformed(ActionEvent e)

26 {

27 double x = Math.random(); //local variable

28 label.setText("" + x);

29 }

30 }

31 }

http://www.tjhsst.edu/compsci

Three-5

Java

Unit3

Exercises Lab00

1) What three steps do you have to do to create buttons?

2) add(label) adds a label to the _______________.

3) There are at least four GUI components showing below. Label them all.

4) A fifth GUI component is hidden, but you know it is there. What is it?

5) What do you have to do to program the action of a button?

6) What interface does every button‘s listener implement? ____________________________

7) What method is in that interface? _________________________________

8) ―To implement an interface‖ means to write concrete methods for all the ___________________ methods in the interface.

9) Give the range of values that the following will generate:

a. Math.random(); ____________________

b. Math.random() * 10; _____________________

c. Math.random() * 100; _______________________

d. Math.random() * 6; ______________________

Three-6

Java Unit3

Discussion GUI components, strings, numbers

The initial text that appears in any label, button, or text box can be specified through the object's constructor.

private JLabel label;

private JTextField box;

public Panel01()

{

label = new JLabel("0.0");

JButton button = new JButton("SQRT");

box = new JTextField("0.0", 10);

In the last command, the text field named "box" will display 0.0, right justified, and it will be 10 characters in

total width. There is no way to set the font or color for all components with a single command. You must call

the set methods on each component individually. Look at the API for all the relevant formatting commands.

To a Java compiler, "6" is different from 6, which is different from 6.0. "6" is a string, 6 is an integer, and 6.0

is a double, and each representation of 6 is stored in a completely different way. We humans want to convert

each kind of 6 into the other kinds. In Lab01, we will want to retrieve a number from the text field. We use

the method getText, which returns a string. Then we turn that string (the ―6‖) into a double (the 6.0) using

the Double.parseDouble method.

double d = Double.parseDouble(box.getText());

After we do the arithmetic with d, often saving the result in a new double variable, then we need to display

the result on the screen. In other words, we have to change the double into a string. We can do so with

label.setText("" + d);

In Java, the quote-quote-plus returns the string representation of the double. Then setText accepts the

string and displays it on the label.

There is a difference between a declaration and an instantiation. In a declaration, such as,

private JLabel label;

you create a reference that eventually will point to an object. In a declaration you don‘t create the object itself.

A call to the class‘s constructor, using the keyword new, actually instantiates the object. Only then does the

JLabel reference point to the JLabel object.

label = new JLabel("");

label OBJECT

We declare and instantiate in separate steps whenever we need to access the private reference in several

methods, i.e. when the scope of the field must be global. We saw this in Lab00 at lines 9, 14, and 28.

label

Three-7

Java

Unit3

Lab01 Hello Text Box

Objective

Fields vs. local variables

Background

Lines 9-10: These three

declarations create private

fields because the fields need

to be accessed later in the

listener.

Line 13: FlowLayout is the

simplest GUI layout. The first

component added to the panel

is placed at the center top.

Additional components are

placed to the right, or on the

next line. Be sure to drag the

corner of the frame and watch

how the flow works.

Line 15-31. Notice the

different components are

instantiated, formatted, and

added to the panel.

Components appear in the

order in which they are added.

Line 19: This button is

declared local because it does

not need to be accessed in the

listener, unlike the label and

the textField.

Specification

Filename Unit3\Lab01\Driver01.java. Create an appropriate driver. Make sure to add a panel object of type

Panel01.

Filename Unit3\Lab01\Panel01.java. Type in the resource class above. Implement the method

actionPerformed to find square roots. What happens if you type a negative number into the text box?

Extension

Edit your program to handle negative numbers correctly, i.e., returns 5i. You will need an if-

statement.

Warning! A common error is to write Line 15 as JTextField box = new JTextField("0.0", 10);

Write Line 15 with this mistake. Compile and run. Explain the error message.

Test Data

Go to www.tjhsst.edu/compsci.

25

4 import javax.swing.*;

5 import java.awt.*;

6 import java.awt.event.*;

7 public class Panel01 extends JPanel

8 {

9 private JLabel label1; //field

10 private JTextField box; //field

11 public Panel01()

12 {

13 setLayout(new FlowLayout());

14

15 box = new JTextField("0.0", 10);

16 box.setHorizontalAlignment(SwingConstants.RIGHT);

17 add(box);

18

19 JButton button = new JButton("SQRT"); //local

20 button.addActionListener(new Listener());

21 add(button);

22

23 label1 = new JLabel("0.0");

24 label1.setFont(new Font("Serif", Font.BOLD, 20));

25 label1.setForeground(Color.blue);

26 add(label1);

27 }

28 private class Listener implements ActionListener

29 {

30 public void actionPerformed(ActionEvent e)

31 {

32 //get the number

33 //take the square root

34 //display it

35 }

36 }

37 }

http://www.tjhsst.edu/compsci

Three-8

Java Unit3

Exercises Lab01

1) There are at least five GUI components showing below. Label them all.

2) Describe a ―string‖ in Computer Science.

3) Describe a ―double‖ in Computer Science.

4) How do you get a string that is in a textField?

5) How do you change that string into a double?

6) How do you change a double into a string, as when you setText a numerical answer into a label?

7) Define and/or explain every underlined term.: double d = Double.parseDouble( textField.getText() );

Complete this scavenger hunt using Sun’s on-line Java API.

8) What is the lowest common ancestor of JButton, JLabel, and JTextField? _______________________

9) How many methods does the ActionListener interface specify? _________________________

10) What package is ActionEvent in? _________________________

11) How many classes extend JButton? __________________

12) How many constructors does JButton define? _________________________

13) How many methods does JButton define? _________________________

14) JButton has a setText method. When would you ever want to use a button's setText in the middle of

a program?

15) How many constructors does JTextField define? _________________________

16) Write the code to instantiate a JTextField with "Enter name" in the field, and the field is 20 spaces wide.

Three-9

Java

Unit3

import

public class Panel02 extends JPanel

{

//fields--for objects that must be accessed later

public Panel02() //constructor

{

format the panel,

instantiate and format the GUI components,

register the listeners,

add the GUI components.

}

private class Listener1 implements ActionListener

{

public void actionPerformed(ActionEvent e)

{

. . .

}

}

//another listener

//another listener

}

Discussion Multiple Buttons

The event model we have seen thus far can be applied to any number of buttons and listeners.

listener1 listener2

component1 component2

listener3

component3

Simply declare a separate listener class for each different action that has to be performed. Register listener

objects with buttons appropriately.

button1.addActionListener(new Listener1());

button2.addActionListener(new Listener2());

button3.addActionListener(new Listener3());

The code above will work, but the names are not informative. You should use informative names, such as:

randomNumber.addActionListener(new RandomListener());

reciprocalButton.addActionListener(new ReciprocalListener());

quitButton.addActionListener(new QuitListener());

As the code below says, each private listener class implements the ActionListener interface. That interface

specifies exactly one abstract method, actionPerformed. That means that each Listener code must define

its own actionPerformed method to specify what happens on each button-click.

Be careful! The placement of

braces is critical to ensure the

proper scope of the fields and

listeners. The listener classes

must be able to access the

fields. In other words, the

scope of the private fields must

include the listener classes.

Also, each of the listener classes

must be visible outside the

constructor. Do not define one

listener class inside another or

the one inside will not be visible

from the constructor.

Sadly, a common error is to put

the definition of one listener

class entirely inside the

definition of another.

Three-10

Java Unit3

Lab02 Multiple Buttons

Objective

GUIs and listeners.

Background

When working with numbers and text, remember that the argument passed to the setText method must be a

string.

Similarly, remember that the return type passed out of the setText method is always a string. If you want

to do arithmetic on the return type, you'll need to parse it, either to an int or to a double.

Some static (class) methods in the Math class that you should know:

Math function Example command

Absolute value x = Math.abs(y);

Square root x = Math.sqrt(y);

Square x = y * y;

Cube root x = Math.pow(y, 1.0/3.0);

Set x equal to 1 x = Math.pow(y, 1/3); //due to integer division, 1/3  0

Raise to a power x = Math.pow(2, 10); //Recall that 2 10

= 1024

Sine, y in degrees x = Math.sin(y * Math.PI / 180.0);

Cosine, y in degrees x = Math.cos(Math.toRadians(y));

Specification

Filename Unit3\Lab02\Driver02.java. Create an appropriate driver. Make sure to add a panel object of type

Panel02. Size recommendations for the frame are included as part of the on-line demo.

Filename Unit3\Lab02\Panel02.java. Reverse engineer the panel based on the on-line demo. Notice that there

is no text box, but that there is only one label. Since you have four buttons, you will have to have four

listeners. To make the Quit button work, check the System class in Sun‘s Java API for some hints.

Test Data

Go to www.tjhsst.edu/compsci.

http://www.tjhsst.edu/compsci

Three-11

Java

Unit3

Exercises Lab02

1) label.setText(number)gives an error message. What is the error, and how do you fix it?

2) How do you change a string into a double?

3) Define and/or explain every underlined item.

private class Handler implements ActionListener

{

public void actionPerformed(ActionEvent e)

{

int num = Integer.parseInt( text.getText() );

int result = num * num;

label2.setText ("The square of the number is " + result);

}

}

Vocabulary practice:

3a) In the code above, ActionListener is a(n) __________________

3b) In the code above, actionPerformed is a(n) __________________

3c) In the code above, Integer is a(n) __________________

3d) In the code above, parseInt is a(n) __________________

3e) In the code above, text is a(n) __________________

3f) In the code above, getText is a(n) __________________

3g) In the code above, int is a(n) __________________

3h) In the code above, result is a(n) __________________

3i) In the code above, label2 is a(n) __________________

3j) In the code above, setText is a(n) __________________

Three-12

Java Unit3

Discussion Primitive Data Types

A data type defines a specific kind of data, e.g., numbers, Strings, or Robots, and how a computer internally

stores and manipulates that data. A class, either imported or user-defined, defines some specific data type.

Java also provides several primitive data types including int (for integers) and double (for decimal

numbers). Primitive data types do not contain any methods. Useful integer methods are instead defined in a

wrapper class. The integer wrapper class is called Integer and the decimal wrapper class is called Double.

Thus, we have Integer.parseInt and Double.parseDouble. Other useful methods are in the API.

Internally, objects are accessed by references, but primitives store their data directly. The convention in Java

is that names of classes start with a capital letter and names of primitives are lower case, reserved words.

Robot karel = new Robot(); Turtle smidge = new Turtle(); int x = 5; double d = 5.00;

karel smidge x 5 d 500 2

Robot is a class, Turtle is a class, primitive type: int primitive type: double

karel is a pointer smidge is a pointer x stores 5 d stores 5.00

One tricky part about ints and doubles is casting, which means changing the data type from one to the

other. Sometimes Java casts automatically and sometimes you must cast explicitly. Suppose your code is:

int x = 4; //creates room for an integer, calls it "x", and stores 4 in it.

double y = 0; //creates room for a double, calls it "y", and stores 0.0 in it.

...

y = (double)x; //it works: the code casts 4 explicitly, which becomes 4.0 and is stored in y.

y = x; //it works: the integer 4 is promoted to 4.0 automatically.

However,

x = y; //illegal, due to loss of precision. The 0.0 does not become 0 automatically.

x = (int)y; //it works: you must explicitly tell the computer to truncate the decimal.

Presumably, the Java creators decided that 4 can become 4.0 without risk, but that decimals shouldn't lose their

decimal parts without the programmer explicitly saying that it is okay.

If you copy and paste code too much you may accidentally apply parseInt or parseDouble when you

need the other one. Always look at the type of the variable and choose appropriately.

int x = Integer.parseInt(“5”);

double y = Double.parseDouble(“5.0”);

Don‘t forget how to change numbers into strings: label.setText(“” + number);

Java is a strongly typed language, meaning that Java is very particular about requiring the correct data type,

either int, double, String, or some other object. A large part of the compiler‘s job is to guarantee

type-safe code. Other, uncompiled, languages do not care so much about type-safe code.

Three-13

Java

Unit3

Lab03 Hailstone Numbers

Objective

Integer division and modulus.

Background Here‘s the Hailstone algorithm: start with a random integer from 1 to 100. If the number is even, divide it by

two to generate the next number. If the number is odd, multiply it by three and add one to generate the next

number. Keep generating in that way. What happens? The answer is the hailstone numbers, which you will

see in action once your Lab03 is working.

This is new for you: in Java, when you divide two integers you will always get an integer quotient! The

remainder, if any, is ignored and thrown away. For example:

int x = (10 * 6 + 7 – 30) / 8;

Sometimes, finding the remainder is quite useful. The remainder is calculated by the modulus operator,

whose symbol in Java is %. For instance:

int y = (10 * 6 + 7 – 30) % 8;

To understand integer division and modulus, think back to long division. For example, 37 8

Notice that the / does not round up. Any remainder is truncated. Truncated means discarded. So even if you

took 9,999/10,000 it would equal zero.

The Hailstone algorithm requires you to determine if an integer is even. A good way to do this is to check to

see if the remainder (%) obtained from dividing the integer by two equals zero. For example: number % 2 == 0

Specification Filename Unit3\Lab03\Driver03.java. Create an appropriate

driver. Make sure to add a panel object of type Panel03. Size

recommendations for the frame are included as part of the on-

line demo.

Filename Unit3\Lab03\Panel03.java (load). Complete the

definition of each listener‘s actionPerformed to reverse

engineer the on-line demo.

Test Data

Be sure to play with the on-line demo that is at

www.tjhsst.edu/compsci.

Extension for math geniuses: Every sequence of Hailstone numbers seems to end in 4, 2, 1, but no one has

proved it to be so. If you come up with a proof, your name will be honored among mathematicians.

x 4

y 5

4

8 37

32

5__

37 / 8  4

37 % 8  5

http://www.tjhsst.edu/compsci

Three-14

Java Unit3

Exercises Lab03

1) Practice integer division, modulus division, and decimal division. Remember the order of operations.

14 / 5 

14 % 5  14.0 / 5.0  3 + 19 % 6 – 4 

15 / 5 

15 % 5  15.0 / 5  1 + 8 % 2 

16 / 5 

16 % 5  16 / 5.0  2 / 3 * 3 

17 / 5 

17 % 5  17 / 5.  1234 % 10 / 1 

3 / 5 

3 % 5  3 / 5.  1234 % 100 / 10 

0 / 5  0 % 5  0.0 / 5  1234 % 1000 / 100 

2) int amount = Integer.parseInt (―143‖); amount int quarters = amount / 25; quarters amount = amount % 25; int dimes = amount / 10; dimes amount = amount % 10; int nickels = amount / 5; nickels int pennies = amount % 5; pennies int numCoins = quarters + dimes + nickels + pennies; numCoins

3) Given totalSeconds, the total number of seconds, write assignment statements to store the number of

minutes and the number of extra seconds. For example, if totalSeconds = 121, then min = 2 and sec = 1.

4) Write an assignment statement to store the remainder when an integer is divided by 3.

5) Write the code for: if some number x is odd, set a label to print ―odd‖.

6) int x = Double.parseDouble(box.getText()); generates an error. What is it, and how

do you fix it?

Complete this scavenger hunt using Sun’s on-line Java API.

7) What class does the Integer class extend? _________________________

8) What kind of argument does the method parseInt require in the version that requires only a single

argument? _________________________

9) What kind of value does the method parseInt return? _________________________

10) What is the second field listed in the Double class? _________________________

Three-15

Java

Unit3

Discussion The Black Box

Most of us have no idea how buttons and listeners actually work. They are like a black box to us, in that their

private data and their implementation code are hidden and encapsulated. The client, often another

programmer, uses black box objects without knowing how they are doing what they are doing. Furthermore,

we don't care how they work! (If you really want to know how buttons and listeners work, ask your teacher

how to view Sun‘s Java source code.)

In Lab04, you get a chance to do your own black box programming. Driver04 looks exactly like all the other

drivers. It hasa Panel04, which has already been written and compiled for you, but is shown below. Do not

type it in!

Lines 4-6: Import some

mysterious classes.

Line 9: declares a private field

of type Odometer.

Lines 14-15: The panel

instantiates and adds an

odometer object to itself. What

is an odometer object? The

panel doesn‘t know!

Lines 17 – 19: create a button,

as usual.

Line 25: When the button is

clicked, the update method in

the odomoter object is called.

What is update? At this

point, we don't know. All we

know is that it is a method in the

odometer class.

Your job, should you decide to accept it, will be to program the Odometer class.

Draw the CSD scope lines in the code above. Give an example of a global variable: ___________ Give an

example of a local variable: ________________

Fill in the UML diagram for Lab04:

hasa

s

isa

4 import javax.swing.*;

5 import java.awt.*;

6 import java.awt.event.*;

7 public class Panel04 extends JPanel

8 {

9 private Odometer odometer;

10 public Panel04()

11 {

12 setLayout(new FlowLayout());

13

14 odometer = new Odometer();

15 add(odometer);

16

17 JButton button = new JButton("Step");

18 button.addActionListener(new Listener());

19 add(button);

20 }

21 private class Listener implements ActionListener

22 {

23 public void actionPerformed(ActionEvent e)

24 {

25 odometer.update();

26 }

27 }

28 }

JFrame

isa

Three-16

Java Unit3

Lab04 Odometer

Objective

Look inside a black box. Modulus and Integer Division ( % and / )

Background

The shell for the Odometer class is shown below.

An odometer is an object

that displays numbers.

Clearly, the Odometer class

should extend the JPanel

class. Since Odometer isa

JPanel, set the layout. Set

the background to black. Add the GUI components to

make the odometer panel look like an odometer.

Which kind of components do you need? How many?

Default labels are ―clear‖. There is a way to make the