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Announcements
• P4 due tomorrow
• P5 handed out today
• References? Leave me a picture and a short
note
CS100
Lecture 18
1
Today’s Topics
• Pascal’s Triangle (fun with 2D arrays)
• Applets
• Parameter passing -- Examples galore!
CS100
Lecture 18
2
Pascal’s Triangle
1
1 1
Note: Elements of an array can be
1 2 1
arrays of different lengths!
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
int [] [] p;
p = new int[7][];
// p.length is 7
p[0] = new int[1];
// p[0].length is 1
p[1] = new int[2];
// p[1].length is 2
p[2] = new int[3];
// p]2].length is 3
CS100
Lecture 18
3
Compute Pascal’s Triangle
// Yield Pascal's triangle with size rows
public static int[][] calculatePascal(int size) {
int[][]p= new int[size][]; //the triangle
for (int r= 0; r != size; r= r+1) {
// Allocate row i of triangle --its r+1 values
p[r]= new int[r + 1];
// Calculate row r of Pascal's triangle
p[r][0]= 1;
for (int c= 1; c < r; c= c+1)
p[r][c]= p[r-1][c-1] + p[r-1][c];
p[r][r]= 1;
}
return p;
CS100
Lecture 18
}
4
Java and World Wide Web
HTML is the language in which files (or pages) are written
for the world wide web. Internet Explorer and Netscape,
the two most used browsers, read html files and display
them on the screen. Example html file:
<html><head>
<title> This goes at the top of the page</title>
</head>
<body>
This is a new paragraph. this is in italics.
This is in bold but this isn’t.
A new paragraph in a larger
font</body> </html>
CS100
Lecture 18
5
HTML is a markup language
• <command> begins a command, </command> ends it.
• There are commands for paragraphs, new line, font change,
font-style change, lists, tables, and many more.
• <A href=“http://www.cs.cornell.edu”>Cornell’s page</A>
produces a link on the page: Cornell’s page
CS100
Lecture 18
6
Applets in html
Command applet:
<applet archive=AppletClasses.jar
code=“className.class”
width=200 height=400
</applet>
Execute a java program. The window for it is 200 x 400.
All of the classes in it are in file AppletClasses.jar. The class
to begin execution is in file className.class.
Many examples of applets. To see one, bring up page
www.cs.cornell.edu, click on CSFacts, and click to get to
“Faculty over the years”
CS100
Lecture 18
7
Applets and OO
• Applets are a good advertisement for object
oriented programming
• Your classes inherit from Applet
• Applet contains all the details of how an applet
works and interacts with the system . . .
• . . . So you can just worry about what you want
your applet to do
• Note: no main method, already inside a running
program when applet is invoked.
CS100
Lecture 18
8
Example:
import java.awt.*;
import java.applet.Applet;
public class TrivialApplet extends Applet
{ public void init() {
repaint(); }
public void paint( Graphics g ) {
g.drawString( "Hello World!", 10, 10 );
g.drawString("Parking", 50, 50);
g.drawOval(45, 24, 43, 43);
g.drawLine(82,30,51, 61);
}} CS100
Lecture 18
9
Html for this applet:
<title>TrivialApplet</title>
<hr>
<applet archive="AppletClasses.jar"
code="TrivialApplet.class" width=200
height=200>
</applet>
<hr>
<a href="TrivialApplet.java">The source.</a>
CS100
Lecture 18
10
CS100
Lecture 18
11
Graphics
• Applets are most useful if you employ
graphics
• Check out the package java.awt in the Java
API -- lots of graphics classes there.
• Remember -- you can use all the
programming techniques you’ve learned so
far (loops, ifs, method calls, etc. etc.) when
you’re writing applets - this was just a
trivial example.
CS100
Lecture 18
12
Method Invocation
• 0. Allocate a new set of locations to contain
the parameters and local variables of the
method being called, in what we call a
frame.
• 1. Assign the arguments of the call to the
parameters of the method.
• 2. Execute the procedure body.
• 3. Delete the frame produced in step 0; if
the method is a function, return the value
of the function to the place of call.
• Terminology: parameter: formal parameter
argument: actual parameter
CS100
Lecture 18
MAKE THE
SPACE
ASSIGN (COPY!)
ARGUMENTS
TO
PARAMETERS
RUN THE
METHOD
DELETE FRAME
RETURN VALUE
13
Parameter-Passing Redux
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C();
c.myMethod(x, y);
System.out.println("x = " + x + " y = " + y);
}
public class C {
public void myMethod(int a, int b) {
a = 3;
b = 4;}
Lecture 18
} CS100
}
14
Names and boxes
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C();
c.myMethod(x, y);
System.out.println("x = " + x + " y = " + y);
}
public class C {
public void myMethod(int x, int y) {
x = 3;
CS100y = 4;}
Lecture 18
}
}
15
Names and boxes, 2
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C();
c.myMethod(x, y);
System.out.println("x = " + x + " y = " + y);
}
public class C {
public void myMethod(int y, int x) {
x = 3;
y = 4;}
Lecture 18
} CS100
}
16
Complications
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C(x, y);
c.myMethod(c.x, c.y);
System.out.println("x = " + x + " y = " + y);
public class C {
int x; int y;
public C(int xx, int yy) {x = xx; y = yy;}
public void myMethod(int a, int b) {
a = 3; b = 4;}
}
CS100
Lecture 18
}}
17
Complications, 2
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C(x, y);
c.myMethod();
x = c.x; y = c.y;
System.out.println("x = " + x + " y = " + y);
public class C {
int x; int y;
public C(int xx, int yy) {x = xx; y = yy;}
public void myMethod() {
x = 3; y = 4;}
CS100
Lecture 18
}
}}
18
Complications, 3
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C();
c.myMethod(x, x);
System.out.println("x = " + x + " y = " + y);
}
public class C {
public void myMethod(int y, int x) {
x = 3;
y = 4;}
Lecture 18
} CS100
}
19
...
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C(x, y);
c.myMethod(c);
x = c.x; y = c.y;
System.out.println("x = " + x + " y = " + y);
public class C {
int x; int y;
public C(int xx, int yy) {x = xx; y = yy;}
public void myMethod(C d) {
d.x = 3; d.y = 4;}
CS100
Lecture 18
}
}}
20
public class TrivialApplication {
public static void main(String args[]) {
int x = 1;
int y = 2;
C c = new C(x, y);
c.myMethod(c);
x = c.x; y = c.y;
System.out.println("x = " + x + " y = " + y);
public class C {
int x; int y;
public C(int xx, int yy) {x = xx; y = yy;}
public void myMethod(C d) {
x = 3; y = 4;}
} CS100
Lecture 18
}}
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