Transcript Document

Java: Review
Java
Object-oriented programming
Introduction
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Java: Review
What is Java?
object-oriented language
platform independent
- JVM
- two-phase execution
Multi-threaded
HelloWorld program
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Java: Review
Java basics I
Java basics II
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Java: Review
Basics -I
• Basic concepts:
class declaration
class body, methods
variables
identifiers
comments
• Primitive data types
• Operators
arithmetic operators
logic operators
bitwise operators
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Java: Review
•
The modulus (%) operator gives the remainder
after integer division: 31%9 = 4.
-5%3 = -2
5%-3 = 2
- 5%-3 = -2
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Java: Review
Type Casting
• Example:
class GoodAssignment {
public static void main(String args[]) {
byte b;
int i=127;
b=(byte) i;
System.out.println(b); //display 127}}
class GoodAssignment {
public static void main(String args[]) {
byte b;
int i=258;
b=(byte) i;
System.out.println(b); //display 2}}
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8 bits
... 1 0 0 ... 1 0
16 bits
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Java: Review
Bitwise Operators
• Number can be represented by a set of bits (a series of 0s and
1s)
• Binary digits take on the value of 0 or 1.
• Example:
the binary number (110101)2 represents the decimal number 53.
Operator
Meaning
&
|
^
~
>>
>>>
<<
AND
OR
XOR
Bitwise complement
Shift right with sign extension
Shift right with zero fill
Shift left with zero fill
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Java: Review
Bitwise Operators
&
110101
101010
|
100000
110101
110101
110101
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110101
101010
111111
>>
>>>
<<
^
110101
101010
011111
|
111010
011010
101010
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Java: Review
import java.lang.*;
//bit operations
public class ShowBits {
public static void main(String args[])
{byte b = -5;
for (int i = 7; i>=0; i--) {
if ((b & 0x80) == 0)
System.out.println("bit " + i + "is 0");
else
System.out.println("bit " + i + "is 1");
b <<= 1;
}
}
}
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Java: Review
Java Basics - II
•
•
•
•
Modifiers (Specifiers)
Statements
Array
Control flow
- condition statements:
if
switch
- loop structure:
for
while
do
• others: String, print, new, constructor
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Java: Review
Modifiers
• Modifiers are special keywords that modify the definition of a
class, method, or variables.
modifiers
Modifiers
Modifiers
for methods for variables
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Modifiers
for classes
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Java: Review
Modifiers for Methods and Variables
•
•
•
•
•
static
final
private
protected
public
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access modifiers
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Java: Review
Modifiers for Classes
• There are three modifiers for classes in Java.
-
public
This makes a class accessible to outside of the package.
final
This prevents a class from being extended.
abstract
In an abstract class, some abstract methods (with only method
signature; no implementation) are defined. Therefore, an abstract class
can not be instantiated.
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Java: Review
Computing factorials
-
simple
-
recursive
- cache
Sorting
- simple
- Quick sorting
- Merge sorting
Computing primes
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Java: Review
Computing Factorials
• Recursive Factorials
public class Factorial2 {
public static long factorial(long x) {
if (x == 1) return 1;
else return x*factorial(x - 1);
}
}
public class ComputingFactorial {
public static void main(String arg[]) {
int a = Factorial.factorial2(Integer.parseInt(arg[0]));
System.out.println(a);}}
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Java: Review
Computing Factorials
• Caching factorials
public class Factorial3 {
//create an array to cache values 0! Through 20!
Static long[] table = new long[21];
Static {table[0] = 1;}
//factorial of 0 is 1
//Remember the highest initialized value in the array
static int last = 0;
public static long factorial(int x) {
while (last < x) {
table [last + 1] = table[last]*(last + 1);
last++;
}}
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Java: Review
Sorting Numbers
• Quick sort
The center element is 5.
main idea: from
1st step: 3
center
9 1
6
to
5
4
8
2
10
j
i
2nd step: 3
2
1
6
5
4
8
9
10
7
3rd step: 3
2 1
4
5
6
8
9
10
7
Smaller than 5
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i=j=5
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greater than 5
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Java: Review
Sorting Numbers
center
from
from center
4th step:
3
5th step:
1
to
2 41
2
3
to
5
6
8
9
10
7
4
i=2
j=2
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Java: Review
6th step: 1
The sequence contains only one element, no sorting.
center from to
7th step:
The center element is 4.
3 4
i=j=1
8th step:
4
The sequence contains only one element, no sorting.
1
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3
4
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Java: Review
Sorting Numbers
• Quick sort
3, 4, 6, 1, 10, 9, 5, 20, 19, 18, 17, 2, 1, 14, 13, 12, 11, 8, 16, 15
j
i
3, 4, 6, 1, 10, 9, 5, 15, 19, 18, 17, 2, 1, 14, 13, 12, 11, 8, 16, 20
3, 4, 6, 1, 10, 9, 5, 15,16, 18, 17, 2, 1, 14, 13, 12, 11, 8, 19, 20
3, 4, 6, 1, 10, 9, 5, 15, 16, 8, 17, 2, 1, 14, 13, 12, 11, 18, 19, 20
i=17
3, 4, 6, 1, 10, 9, 5, 15, 16, 8, 17, 2, 1, 14, 13, 12, 11
j=16
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Java: Review
• Another Java program for the quick sort:
public class Sorter {
public static void sort (int[] a, int from, int to) {
if ((a == null) || (a.length < 2)) return;
int i = from -1, j = to + 1;
int center = a[(from + to)/2];
do {i++; j--;
while ((i < to) && (a[i] < center)) i++;
while ((j > from) && (a[j] > center)) j--;
if (i < j) { int tmp =a[i]; a [i] = a[j]; a[j] = tmp;}
}while (i <= j);
if (from < j) sort(a, from, j);
if (i < to) sort(a, i, to);
}
}
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Java: Review
import java.lang.*;
public class sort {
public static void sort (int[] a, int from, int to) {
int tag = 0;
if ((a == null) || (a.length < 2)) return;
int i = from, j = to;
int center = a[(from + to)/2];
do {
while ((i < to) && (a[i] < center)) i++;
while ((j > from) && (a[j] > center)) j--;
if (j > i) tag = 1;
else if (j == i) tag = 0;
else tag = -1;
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Java: Review
switch (tag) {
case 1:
int tmp =a[i];
a [i] = a[j]; a[j] = tmp;
i++; j--;
break;
case 0: i++; j--; break;
default:}
}while (i <= j);
if (from < j) sort(a, from, j);
if (i < to) sort(a, i, to);}
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Java: Review
public static void main(String args[]) {
int array[]=new int[20];
for(int i = 0; i < 20; i++) //Generate random numbers
array[i] = (int)(Math.random()*100);
sort(array,0,19);
System.out.println("the final answer is:");
for (int i=0;i<array.length;i++)
System.out.print(array[i]+" ");
}
}
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Java: Review
• Sorting by merging
Merging means the combination of two or more ordered sequence into
a single sequence. For example, can merge two sequences: 503, 703, 765
and 087, 512, 677 to obtain a sequence: 087, 503, 512, 677, 703, 765.
A simple way to accomplish this is to compare the two smallest items,
output the smallest, and then repeat the same process.
503
087
703
512
765
677
087
087
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503
512
703
512
703
677
765
765
677
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Java: Review
• Merging algorithm
Algorithm Merge(s1, s2)
Input: two sequences: s1 - x1  x2 ...  xm and s2 - y1  y2 ...  yn
Output: a sorted sequence: z1  z2 ...  zm+n.
1.[initialize]
i := 1, j := 1, k := 1;
2.[find smaller] if xi  yj goto step 3, otherwise goto step 5;
3.[output xi]
zk.:= xi, k := k+1, i := i+1. If i  m, goto step 2;
4.[transmit yj  ...  yn] zk, ..., zm+n := yj, ..., yn. Terminate the algorithm;
5.[output yj]
zk.:= yj, k := k+1, j := j+1. If j  n, goto step 2;
6.[transmit xi  ...  xm] zk, ..., zm+n := xi, ..., xm. Terminate the algorithm;
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Java: Review
-
Merge-sorting
Algorithm Merge-sorting(s)
Input: a sequences s = < x1, ..., xm>
Output: a sorted sequence.
1. If |s| = 1, then return s;
2. k := m/2;
3. s1 := Merge-sorting(x1, ..., xk);
4. s2 := Merge-sorting(xk+1, ..., xm);
5. return(Merge(s1, s2));
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Java: Review
Computing Primes
• Finding the largest prime number smaller than a specified
integer:
Input integer m, find p  m such that p is a prime and if there is prime p’ >
p then p’ must be larger m.
than m.
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Java: Review
Computing Primes
Import java.lang.*;
public class Sieve {
public static void main(String[] args) {
int max = 100; //Assign a default value
try {max = Integer.parseInt(args[0]);}
catch (Exception e) {} //Silently ignore exceptions.
//Create an array that specifies whether each number is prime or not.
boolean[] isprime = new boolean[max+1];
//Assume that all numbers are primes, until proven otherwise.
for (int i = 0; < max; i++) isprime[i] = true;
//We know that that 0 and 1 are not prime. Make a note of it.
isprime[0] = isprime[1] = false;
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Java: Review
Computing Primes
//To compute all primes less than max, we need to rule out multiples of all
//integers less than the square root of max.
int n = (int) Math.ceil(Math.sqrt(max));
for (int i = 0; i <= n; i++) {
if (isprime[i]) { int k = 2;
for (int j = k*i; j < max; j = (k ++)*i)
isprime[j] = false; }
}
int largest;
for (largest = max - 1; !isprime[largest]; largest--); //empty loop body
System.out.println(“The largest prime less than or equal to “ + max + “is ”
+ largest); }}
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Java: Review
What is OOP?
Class, instance, field, method, ...
“this” key word
Method Overloading
Inheritance
Method Overriding
Abstract and Interface
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Java: Review
What is OOP?
• Procedural programming is where you would try to solve a
problem using pre-determined types: int, floats, strings and
arrays.
• In OOP, you create a model that best represents the problem.
• The programmer defined model is known a class.
• A class is a programmer defined type, together with a lot of
procedures manipulating over it. Such a type can be used as
template for instance of the class.
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Java: Review
“this” in Constructors (Example for Method Overloading)
• To invoke a constructor from another constructor in the same
class:
class Car { String licensePlate; double speed, maxSpeed;
public Car(String licensePlate, double speed, double maxSpeed) {
this.licensePalte = licensePlate;
this.speed = speed;
this.maxSpeed = maxSpeed;
}
public Car(String licensePlate, double maxSpeed) {
this(licensePlate, 0.0, maxSpeed); }
void accTomax ( ) { … } void accelerate ( ) {…}
}
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Java: Review
Interface
import java.util.*;
import java.lang.*;
interface CanFight { void fight ( );}
interface CanSwim { void swim ( );}
interface CanFly {void fly ( );}
class ActionCharacter { public void fight( ) { }}
class Hero extends ActionCharacter
implements CanFight, CanSwim, CanFly {
public void fight ( ) {System.out.println(“Can fight!”);}
public void swim ( ) {System.out.println(“Can swim!”); }
public void fly ( ) {System.out.println(“Can fly!”);}
}
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Java: Review
Interface
public class Adventure {
static void t(CanFight x) { x.fight();}
static void u(CanSwim x) { x.swim();}
static void v(CanFly x) { x.fly();}
static void w(ActionCharacter x) { x.fight();}
public static void main (String[ ] args) {
Hero h = new Hero( );
t(h); //Treat it as a CanFight
u(h); //Treat it as a CanSwim
v(h); //Treat it as a CanFly
w(h); //Treat it as an ActionCharacter
}
}
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Java: Review
Upcasting and Polymorphism
• Upcasting: Taking an object reference and treating it as a
reference to its base type is called upcasting, because of the
way inheritance trees are drawn with the base class at the top.
class Note {
private int value;
private Note(int val) {value = val;}
public static final Note
middle_c = new Note(0),
c_sharp = new Note(1),
b_flat = new Note(2);
}
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Java: Review
Upcasting and Polymorphism
class Instrument {
public void play(Note n) {
System.out.println(“Instrument.play()”)
}
}
class Wind extends Instrument {
public void play(Note n) {
System.out.println(“Wind.play()”);
}
}
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Instrument
Wind
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Java: Review
Upcasting and Polymorphism
public class Music {
public static void tune(Instrument i) {
// …
i.play(Note.middle_c);
}
public static void main(String[] args) {
Wind flute = new Wind();
tune(flute);
//Upcasting
}
}
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Java: Review
Upcasting and Polymorphism
• Polymorphism: In Java, the principle that the actual type of the
object determines the method to be called is called polymorphism.
Shape
draw()
erase()
class Shape {
void draw() {}
void erase() {}
}
Circle
draw()
erase()
class Circle extends Shape {
void draw() {
System.out.println(“Circle.draw()”);
}
void erase() {System.out.println(“Circle.erase()”);}}
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Square
draw()
erase()
Triangle
draw()
erase()
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Java: Review
Upcasting and Polymorphism
class Square extends Shape {
void draw() {
System.out.println(“Square.draw()”);
}
void erase() {System.out.println(“Square.erase()”);}}
class Triangle extends Shape {
void draw() {
System.out.println(“Triangle.draw()”);
}
void erase() {System.out.println(“Triangle.erase()”);}}
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Java: Review
Upcasting and Polymorphism
public class Shapes {
public static Shape randShape() {
switch((int) (Math.random()*3)) {
case 0: return new Circle();
case 1: return new Square();
case 2: return new Triangle();
default : return new Circle();}}
public static void main(String[] args) {
Shape[] s = new Shape[9];
for (int i = 0; i < s.length; i++)
s[i] = randShape();
//Make polymorphism method calls:
for (int i = 0; i < s.length; i++) s[i].draw();}}
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Java: Review
• Applet
-
HelloWorld Applet
import statement
Hypertext Mark Language (HTML)
• Graphic
-
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line
rectangle
polygon
ovals
arcs
color
font
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Java: Review
Applets
• There are two types of Java programs:
-
Applications and Applets
• An applet is a subclass of Applet class defined in “applet”
package.
• Using appletviewer to run a HTML file that contains an
applet class; or invoke it through an internet browser.
appletviewer HelloWorld.html
or
input the URL address of your HTML file by internet
explorer: H:\javaprog\EventTest1.html
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Java: Review
HelloWorld Applet
• The HelloWorld applet:
import java. applet.*;
import java. awt.*;
// A simple Java Applet
public class HelloWorld extends Applet
{
public void paint( Graphics g)
{
g. drawString(“ HelloWorld!”, 20,10);
}
}
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Java: Review
HelloWorld Applet
• HelloWorld.html:
<HTML>
<BODY>
<APPLET CODE = HelloWorld.class
WIDTH = 200
HEIGHT=200>
</ APPLET>
</BODY>
</HTML>
URL address: file://e:/javaprog/HelloWorld.html
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Java: Review
Life Cycle of an Applet
• An Applet executes within an environment provided by a
Web browser or a tool such as the applet viewer.
• It does not have a main() method
• There are four methods that are called during the life
cycle of an applet:
init(),
start(),
stop(),
destroy().
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Java: Review
import Statements
• import statements must appear before any of the names
defined in the import are used.
• It is a strong recommendation that all imports appear at
the
beginning of your program.
import java.applet.*;
import.java.awt.*;
• Package construction:
- Assume that there are classes in D:\ychen2\javaprog
- The first statement in each class: package javaprog
- in D:\ychen2, issue command: javac javaprog\*.java
- set classpath = %classpath%;D:\ychen2
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Java: Review
Graphics
• The java. awt package contains all the necessary
classes
you need to create graphical user interfaces (GUIs).
• Most of the graphics operations in Java are methods
defined in the Graphics class.
• You don’t have to create an instance of the Graphics
class
because in the applet’s paint() method, a Graphics object
is
provided for you. By drawing in that object, you draw
onto your applet which appears on the screen.
• The Graphics class is part of the java.awt package, so
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make
it into
Java code.
Java: Review
The Coordinate System
• Java’s coordinate system has the origin (0,0) in the top
left
corner of the applet.
- Positive x values are to the right and positive y values are
downward
• The coordinate
system is represented
by pixels.
+X
(0,0)
- Pixels in Java are integer values only
(20,20)
+Y
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Java: Review
Lines
• To draw a line onto the screen, use the drawLine()
method:
- void drawLine( int x1, int y1, int x2, int y2);
- This draws a line from the point with coordinates (x1, y1) to the
point with coordinates (x2, y2).
- Example:
import java. awt. Graphics;
public class MyLine extends java. applet. Applet {
public void paint( Graphics g) {
g. drawLine( 25,25, 75,75);
}
}
- There is no way to change the line thickness in Java.
So how do we make thicker lines?
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Java: Review
Rectangles
• To draw a rectangle on the screen, use the drawRect()
method:
- void drawRect( int x, int y, int width, int height)
• Example:
import java. awt. *;
public class MyRect extends java. applet. Applet {
public void paint( Graphics g) {
g. drawRect(120,20, 60,60);
g. setColor( Color. red);
g. fillRect( 120, 20,60, 60);
}
}
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Java: Review
Rounded Rectangles
• These are rectangles with the corners rounded according
to
the values of the arguments.
• Like the rectangle, there are two methods for round
rectangles:
- void drawRoundRect( int x, int y, int width, int height, int arcWidth,
int arcHeight)
- void fillRoundRect( int x, int y, int width, int height, int arcWidth,
int arcHeight)
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Java: Review
3D Rectangles
• You can also draw three dimensional rectangles in Java
- Warning: They really don’t look too good though
• There are two methods as well:
- void draw3DRect( int x, int y, int width, int height, boolean raised)
- void fill3DRect( int x, int y, int width, int height, boolean raised)
- The argument “raised”, when true, will paint the rectangle as if it
were raised from the surface.
- If it is false, the rectangle will appear as if it were depressed.
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Java: Review
Polygons
• Polygons are shapes with an unlimited # of sides.
• To draw a polygon, you need a set of x and y
coordinates.
• The polygon is then drawn by drawing a series of straight
lines from the first point to the second, to the third and so
on.
import java. awt. Graphics;
public class MyPolygon extends java. applet. Applet {
public void paint( Graphics g) {
int exes[]={ 39, 94,97, 142,53, 58,26};
int whys[]={ 33,74, 36,70,108,80,106};
int pts= exes. length;
g. drawPolygon( exes, whys, pts);
}}
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Java: Review
Polygons using the Polygon Class
• Example:
import java. awt. Graphics;
import java. awt. Polygon;
public class MyPolygon2 extends java. applet. Applet {
public void paint( Graphics g) {
int exes[]={ 39, 94,97, 142,53, 58,26};
int whys[]={ 33,74, 36,70,108,80,106};
int pts= exes. length;
Polygon poly= new Polygon( exes, whys, pts);
g.drawPolygon(poly);
g.fillPolygon( poly);
}
}
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Java: Review
Ovals
• Ovals are drawn with the drawOval() or fillOval()
methods
- void drawOval( int x, int y, int width, int height)
- void fillOval( int x, int y, int width, int height)
- This draws an oval within the bounding rectangle specified by the
arguments
- Example:
import java. awt. Graphics;
public class MyOval extends java. applet. Applet {
public void paint( Graphics g) {
g. drawOval( 20, 20, 60,60);
g. fillOval( 120, 20,60,60);
}
}
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Java: Review
Arcs
• An arc is basically part of an oval.
• Arcs are drawn using the method:—
-
void drawArc( int x, int y, int width, int height, int startAngle,
int arcAngle)
- void fillArc( int x, int y, int width, int height, int startAngle, int
arcAngle)
- This draws an arc within the rectangle specified starting from the
startAngle argument for a duration of arcAngle.
- Example:
g. drawArc( 10,10,100,80,45,210);
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Java: Review
Arcs
• Example:
import java. awt. Graphics;
public class MyArc extends java. applet. Applet {
public void paint( Graphics g) {
g. drawArc( 120,20, 60,60,90, 180);
g. fillArc( 120,20,60, 60,90,180);
}
}
Sept. 2003
Yangjun Chen
91.3902
58
Java: Review
The Color Class
• This class contains 13 constant values that can be used:
- black, blue, cyan, darkGray, Gray, green, lightGray, magenta,
orange, pink, red, white, yellow
• To address them we have to reference them through the
Color class
- eg. Color. black
- Too set the current color to blue:
g. setColor(Color. blue)
• Colors in Java are described by the RGB (Red, Green,
Blue) model.
- This model specifies the amount of red, green, and blue in a
color.
- The intensity of each component is measured as an integer
between
0 and 255, with 0 representing no light.
Sept. 2003 (0,0,0) is black Yangjun Chen
91.3902
(128,128,128) is medium gray
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Java: Review
The Color Class
• To declare a new color in Java, use the “new” operator
- Color myColor = new Color( 255, 0, 128);
- We now have a new color and since we know it is an object of the
Color class we can use it directly
g. setColor(myColor);
- You can also define the color “on the fly” or in line with the
setColor() method
g. setColor( new Color( 255,0,128));
Sept. 2003
Yangjun Chen
91.3902
60
Java: Review
The Font Class
• There are five basic fonts in Java
-
SanSerif (Helvetica), Serif (Times Roman), Monospaced (Courier),
Dialog, DialogInput
• There are some constant values associated with the Font
class
as well.
-
Font.BOLD, Font.PLAIN, Font.ITALIC
• Create a Font object by using the “new” operator
-
Font myFont = new Font(“Helvetica”, Font.BOLD, 12);
-
After creating a font, you have to set it before it can be used:
g.setFont(myFont);
You can also do this in line with the setFont() method
g.setFont(new Font(“Helvetica”, Font.BOLD, 12));
-
• You can also combine styles by adding them together, for
example
Sept. 2003
Font
Yangjun Chen
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myFont = new Font(“Helvetica”,
Font.BOLD+ Font.ITALIC, 12)
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