Java程序辅导

Lab 3: Circle Applet
CMPT 183
Dr. Gutierrez
Topics for today …
• Simple syntax & math operations
• Random
• Interactivity (mouse)
• Basic structure of class (again)
• Hands-on Circle Applet
Created by Jerry Alan Fails
Simple Syntax: Assignment & Math Ops
• Assignment
– Used to store information in a variable
– Example: x = 0; // the variable x gets the value zero
• Math Operations
– Simple math operations:
• + : addition
• - : subtraction
• * : multiplication
• / : division
– Example: : x = 53 + 4 – 3 * (12 / 2);   // x gets     ???
• For today you’ll also need Math.PI (π)
Created by Jerry Alan Fails
39
The Random Class
• The Random class is part of the java.util
package
• It provides methods that generate pseudorandom 
numbers
• A Random object performs complicated calculations 
based on a seed value to produce a stream of 
seemingly random values
• See RandomNumbers.java 
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
//********************************************************************
//  RandomNumbers.java Author: Lewis/Loftus
//
//  Demonstrates the creation of pseudo-random numbers using the
//  Random class.
//********************************************************************
import java.util.Random;
public class RandomNumbers
{
//-----------------------------------------------------------------
//  Generates random numbers in various ranges.
//-----------------------------------------------------------------
public static void main (String[] args)
{
Random generator = new Random();
int num1;
float num2;
num1 = generator.nextInt();
System.out.println ("A random integer: " + num1);
num1 = generator.nextInt(10);
System.out.println ("From 0 to 9: " + num1);
continued
Copyright © 2012 Pearson Education, Inc.
continued
num1 = generator.nextInt(10) + 1;
System.out.println ("From 1 to 10: " + num1);
num1 = generator.nextInt(15) + 20;
System.out.println ("From 20 to 34: " + num1);
num1 = generator.nextInt(20) - 10;
System.out.println ("From -10 to 9: " + num1);
num2 = generator.nextFloat();
System.out.println ("A random float (between 0-1): " + num2);
num2 = generator.nextFloat() * 6;  // 0.0 to 5.999999
num1 = (int)num2 + 1;
System.out.println ("From 1 to 6: " + num1);
}
}
Sample Run
A random integer: 672981683
From 0 to 9: 0
From 1 to 10: 3
From 20 to 34: 30
From -10 to 9: -4
A random float (between 0-1): 0.18538326
From 1 to 6: 3
Examples – code to outcome
Copyright © 2012 Pearson Education, Inc.
Given a Random object named gen, what range of 
values are produced by the following 
expressions?
gen.nextInt(25)
gen.nextInt(6) + 1
gen.nextInt(100) + 10
gen.nextInt(50) + 100
gen.nextInt(10) – 5
gen.nextInt(22) + 12
Range?
0 to 24
1 to 6
10 to 109
100 to 149
-5 to 4
12 to 33
Examples – outcome to code
Copyright © 2012 Pearson Education, Inc.
Write an expression that produces a random 
integer in the following ranges:
gen.nextInt(13)
gen.nextInt(20) + 1
gen.nextInt(6) + 15
gen.nextInt(11) – 10
Range
0 to 12
1 to 20
15 to 20
-10 to 0
Mouse Events
• Events related to the mouse are separated into 
mouse events and mouse motion events
• Mouse Events:
mouse pressed the mouse button is pressed down
mouse released the mouse button is released
mouse clicked the mouse button is pressed down and released 
without moving the mouse in between
mouse entered the mouse pointer is moved onto (over) a 
component
mouse exited the mouse pointer is moved off of a component
Mouse Events
• Mouse motion events:
• Listeners for mouse events are created using the 
MouseListener and MouseMotionListener
interfaces
• A MouseEvent object is passed to the appropriate 
method when a mouse event occurs
Copyright © 2012 Pearson Education, Inc.
mouse moved the mouse is moved
mouse dragged the mouse is moved while the mouse 
button is pressed down
Making an Applet Interactive
• We need to listen for mouse events & handle them
• In addition to extending JApplet, implement 
MouseListener
• In init() method, register event listener by 
calling addMouseListener( this );
• Create empty definitions for mouse event methods:
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mousePressed(MouseEvent evt) { }
Created by Emily Hill
Responding to a Mouse Event
• One mouse event method should NOT be empty:
public void mousePressed(MouseEvent evt) {
Point p = evt.getPoint();
// Update the x/y coordinates
// refresh the applet
repaint();
}
• Example: HelloWorldNoOverlayEvent.java
Created by Emily Hill
Created by Emily Hill
Basic class structure
Three major components of a class:
– Fields – store data for the object to use
– Constructors – allow the object to be set up properly 
when first created
– Methods – implement the behavior of the object
public class ClassName
{
Fields
Constructors
Methods
} 
Fields
• Fields store values 
for an object.
• They are also known 
as instance variables.
• Fields define the 
state of an object.
public class Square
{
private int x;
private int y;
private int size;
private Color fillColor;
// Further details omitted.
} 
private int size;
visibility modifier
type
variable name
Constructors
• Constructors initialize an object.
• They have the same name as their class.
• They store initial values into the fields.
• They often receive external parameter values for this.
public Square(int x, int y)
{
this.x = x;
this.y = y;
size = 50;
color = Color.blue;
} 
/**
* Gets the size of the square.
*/ 
Methods
public int getSize()
{
return size;
} 
return type
method name
parameter list 
(empty)
start and end of method body (block)
return statement
visibility modifier
method header/signature
Constructor Practice
• Import the starter project. Download if from Course 
Materials in Blackboard
• Rename it to the same name plus your netID
• Add a constructor to the Circle class that accepts as 
parameters the initial x and y positions, radius, and 
color
• Alter your Picture class to use these constructors 
(instead of all the method calls you used before)
• What do you think of this approach?
Method Practice
• Add to (or modify) the Circle class:
– Add a getArea method that returns the area of the circle
– Add a getPerimeter method that returns the length of the 
circunference
– Modify paintComponent so it shows the area and 
perimeter of the circle (look at the first HelloWorld lab to remind 
yourself how to draw a string) 
– EXTRA: Use the DecimalFormat class to only show two 
significant digits (see book section 3.6)
• Use your Picture applet to test and make sure the 
area and perimeter are showing up and are being 
calculated correctly
Interactive Circle Applet
• Modify the provided CircleApplet starter class
– You need to store a circle and a random object
– Initialize/create the object to be stored in your init method
– Paint your circle in your paint method
• Modify the mouseClicked method body so the 
circle:
– Is moved to the point where the mouse was clicked
– Gets a new random radius that is at least 30 pixels
– Gets a new random color
What we did today …
• Concepts practiced
– Constructors
– Methods
– Classes
– Interactivity (mouse events)
• Modified the Circle and Square classes:
– Added a constructor that accepts as parameters the initial x and y positions, 
radius, and color
(should already be done from previous slide)
– Added a getArea method that returns the area of the circle/square
– Added a getPerimeter method that returns the area of the circle/square
– Modified paintComponent so it shows the area and perimeter of the circle/square
• Defined a new applet class – CircleApplet – that
– Stores a circle and a random object (fields)
– Paints the circle 
– Listens for mouse clicking and when it is clicked
• Change the location of the circle to click location
• And gets random radius and color
Homework
• Finish lab exercise
• CodingBat
• Read Chapters 2 and 3