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CG0093: Application Design in Java
Weeks 1-2: Java Basics
Sajjad Shami
Michael Brockway
School of Computing, Engineering and Information Systems
Northumbria University
Java
1
Java
1995 Sun Microsystems
www.java.sun.com (several good tutorials)
J2SE: This module
J2EE
next term: CG0165
J2ME
Advanced Applications Development in Java
Java
2
Java Facts
Object oriented
Simple
– Small language
– Easy OO use, error handling
Platform independent
–
–
–
–
PC
Unix
Handheld
Smartcard
Java
3
Java Facts…
Network centric
– Designed for network environment
Large number of libraries (Packages)
– Graphics
– Phone
– Database
Java
4
Topics (Weeks 1-2)
From Deitel & Deitel: Java HTP 6e
Chapter 2 – Introduction to Java Applications
Chapter 20 – Introduction to Java Applets
Chapter 4 – Control Statements: Part 1
Chapter 5 – Control Statements: Part 2
Chapter 6 – Methods
Chapter 29 – Strings and Characters
Though not quite in that order
Emphasising main points
Java
5
Applications and Applets
Applications
– Run under host OS
– Full access to system resources
Applets
– Run from within web page
– Limited access to system resources
– Typically small
System Resources vary
– Console input/output
– User Interface
Java
6
Applications: Procedure
Write file say Example1.java that contains code
for class Example1
Compile as >javac Example1.java
– Statements terminate with a semi-colon
– // means line is a comment ( also /* ….*/ )
This will create bytecodes Example1.class
Execute by >java Example1
Java
7
Example1.java
//Adding integers using DialogBox
import javax.swing.JOptionPane;
public class Example1 {
public static void main (String args[])
{
String firstNumber = null,
secondNumber = null;
int n1 = 0,
n2 = 0,
sum = 0;
firstNumber = JOptionPane.showInputDialog("Enter first
integer");
n1 = Integer.parseInt( firstNumber );
Java
8
Example1.java …contd
n2 = Integer.parseInt(JOptionPane.showInputDialog(
"Enter second integer") );
sum = n1 + n2;
JOptionPane.showMessageDialog( null,
"The sum is: "+ n1 + "+" + n2 + " = " + sum,
"Equation",
JOptionPane.PLAIN_MESSAGE );
}
} // end Example1
Java
9
Example1.java (again)
//Adding Integers using DialogBox
import javax.swing.JOptionPane;
public class Example1 {
public static void main (String args[])
{
String firstNumber = null, secondNumber = null;
int n1 = 0, n2 = 0, sum = 0;
firstNumber = JOptionPane.showInputDialog("Enter first
integer");
n1 = Integer.parseInt( firstNumber );
n2 =Integer.parseInt(JOptionPane.showInputDialog("Enter
second integer"));
sum = n1 + n2;
JOptionPane.showMessageDialog(null,"The sum is: " +n1+
"+" +n2+ "=" +sum,"Equation",JOptionPane.PLAIN_MESSAGE);
}
}
Java
10
Style
Indent blocks {…}
Line up { and }
Split long lines
Be consistent
Easy to read (aids)
– Maintenance
• you may be some time before
revisiting
• others may maintain
– One parameter per line
– At “punctuation”
Remember ;
Names
–
–
–
–
– Use by others
• extending
• using from library
CapitaliseClasses
notFirstWordForVariables
simple_data_types
CONSTANT_VALUES
Clarity
Good choice of names
Java
– describe meaning
– don’t abbreviate
11
Starters
Make library class available
import javax.swing.JOptionPane;
• Declare class, note same name as file
public class Example1 {
• Declare main function (entry point)
public static void main ( String args[] )
• Declare and initialise variables
String
firstNumber = null,
secondNumber = null;
int n1 = 0,
n2 = 0,
sum = 0;
Java
12
Get data
Get a string using library dialog box
firstNumber =
JOptionPane.showInputDialog(
"Enter first integer"
);
• Convert to Integer (another library class)
n1 = Integer.parseInt( firstNumber );
Java
13
Get data …
• get second number,
• note how one method (function) can be
parameter of second
n2 =
Integer.parseInt(
JOptionPane.
showInputDialog(
"Enter second integer"
)
);
Java
14
Process and Output
Process data
sum = n1 + n2;
• Output results
JOptionPane.showMessageDialog(
null,
"The sum is: "+
n1 + "+" + n2 +
" = " + sum,
"Equation",
JOptionPane.PLAIN_MESSAGE
);

Breaking between parameters and operators can aid clarity if
done well…
Java
15
Built in Data types
8 types
Have wrapper class supplying utility functions
Type
Contents
Size
boolean
true or false
1 bit
char
Unicode character
2 byte
byte
signed integer
1 byte
short
signed integer
2 byte
int
signed integer
4 byte
long
signed integer
8 byte
float
IEEE 754 floating point
4 byte
double
IEEE 754 floating point
8 byte
Java
16
Operators (some)
operator
action
associativity
++
increment decrement
rightleft
multiply division remainder
leftright
addition subtraction
leftright
equal not-equal
leftright
AND XOR
(bitwise)
OR
leftright
&&
||
AND (conditional)
OR (conditional)
leftright
?:
conditional ternary
rightleft
=
*= /= %= += -=
assignment
assignment with operation
rightleft
--
*
/
+
-
==
&
%
!=
^
|
Java
17
Increments et.al.
pre and post
int
a =
y =
y =
z =
a,b,y,z;
b = 3;
z = 2;
a++;
++b;
a
b
y
z
=
=
=
=
4
4
3
4
• assignment and operation
int a,b,y,z;
a = b = 3;
y = z = 5;
y += a;
z %= b;
y = 8
z = 2
Java
18
Conditionals
And && Or ||
Short-circuit evaluation
int a = 5, b = 6
– evaluates left to right as far as needed to get result
• AND &&

second test is not evaluated as false and anything is false
• OR ||

a>=6 && b==6
a<=6 || b==6
second test is not evaluated as true or anything is true
• be aware of side effects
r = b==6 || ++a >= 42

side effect of increment
Java
19
Exercises Chapter 2
All examples in text
2.14
2.17
2.26
2.32
Java
20
Applets (Ch 20)
Java programs that can be embedded in HTML documents
(web pages)
When a browser loads a web page containing an applet, the
applet downloads into the browser and begins execution
J2SDK comes with appletviewer for testing applets before
embedding
> appletviewer example1.html
Three methods: init( ), paint( ) and start( )
.java and the .html files are usually stored in the same
directory
Java
21
Applets: Procedure
Write file SampleApplet.java
Also write file SampleApplet.html
In the same directory
> javac SampleApplet.java
– produces SampleApplet.class
> appletviewer SampleApplet.html
Java
22
SampleApplet.java
//Class SampleApplet.java: displays two strings
import java.awt.Graphics;
import javax.swing.JApplet;
public class SampleApplet extends JApplet{
public void paint ( Graphics g)
{
super.paint(g);
g.drawString( “Welcome to”, 25, 25 );
g.drawString( “Java Programing!”, 25, 40 );
}
}
Java
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SampleApplet.html
//SampleApplet.html loads class SampleApplet into the appletviewer
<html>
<applet code = “SampleApplet.class” width = “300” height = “60” >
</applet>
</html>
> appletviewer
SampleApplet.html
Java
24
Exercises Chapter 20
All examples in text
20.4
20.5
20.9
Java
25
Control Structures (Ch 4 & 5)
Decision
Selection
Repetition
– counter
– logic test
• top
• bottom
Exits
Java
26
if…then…else
Simple decision
else is optional
if ( a==7 )
do_a();
else
do_b();
if ( a==7 )
do_a();
• Ternary conditional operator
s = a%2==0 ? "even" : "odd"

evaluates to a value that can be assigned to a variable
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switch … selection
Must test integral type
byte, short, int, long
once matched falls through until
break.
default is optional
(but good practice)
a:
1.
2.
3.
4.
•
Java
this, that
that
that
then
other
switch(a)
{
case 1:
do_this();
case 2:
case 3:
do_that();
break;
case 4:
do_then();
break;
default:
do_other();
}
28
while
and
tests at TOP of loop
while ( a<100 )
a*=2;
– loop may never
execute
do
tests at BOTTOM of loop

do…while
a*=2;
while ( a<100 );
loop will execute once
• Ensure loops terminate!
Java
29
for
•
equivalent to…
i. initialisation
ii. test
iii. <body>
iv. increment
for(int a=0 ; a<=10 ; ++a)
b+=a;
int a=0;
while(a<=10)
{
b+=a;
a++;
}
• be aware
 test occurs after increment and before
next iteration!
Java
30
for loops…
• counter controlled loops


need not be simple increments
can use complex tests
for(
Enumeration e=a.getElements();
e.hasMoreElements();
e.nextElement()
);
A common idiom for
traversing sets of data
(Vector class)
for( int a=0,b=2; a<100; a*=2,b++ )
multiple counters and
increments
for( int a=1,b=1; b<=10; a*=++b )
geometric progression
Java
31
Loop Exits
break and continue
break
exits loop
continue
skips to next
iteration
line:
while(a>=1)
{
if (b==2) break;
for(int c=10 ; c>0 ; c-=b)
{
if(b==c) continue;
if(b==a) break line;
if(c%2==0) break;
b=a%4;
}
--a;
}
Java
32
Blocks and Scoping
“visibility”
of variables
public class Scope {
int a;
– from declaration
– until end of enclosing block
{
int x;
/*
*/
scope of a
}
scope of x
int b;
for(int c=10;c>0;--c)
{
a ^= c/b;
}
scope of b
scope of c
}
Java
33
Strings
Are Objects not built-in types
– are useful so a special case is made
– anonymous strings
• Creation
String s = new String("some text");
• Joining (Concatenation)
"some text"+" more text";
• Conversion from built-in types

automatic
"value is..." + variableName + "km";
 done by calling the toString() method of the class
Java
34
Packages
(libraries)
n2 = java.lang.Integer.parseInt(
javax.swing.JOptionPane.showInputDialog(
"Enter second integer");
No import
– Use full package path and class name
– long winded
– clear in context what class is
Java
35
Package: single class import
import javax.swing.JOptionPane;
n2 = Integer.parseInt(
JOptionPane.showInputDialog(
"Enter second integer"
));
Single class import
– import whole class
– use class name alone in code
– explicit what classes are used
• at top of file
• clear
• easy maintained
Java
36
Package: Package import
Package import
–
–
–
–
import javax.swing.*;
public class Sample2 extends JApplet
{
makes all classes available
JOptionPane. showInputDialog(
use class name
easier to manage as class use increases
get “free”
• java.lang.*
note: there is no overhead on the program code, the compiler
includes just those classes actually used.
good to import java.lang.* in any case.
Java
...
37
Exercises Chapter 4 & 5
All examples in text
4.17, 4.20, 4.22, 4.30
5.11, 5.17, 5.24
Java
38
Methods (functions)
Methods are the functions that form a Java program
– function is usually a term in non object-oriented programming
– method is the term in object-oriented programming
Already seen some of the predefined methods in
various Java library classes
– see Fig 6.2, page 235 Math class methods
How to define your own methods
Java
39
Invocation of methods
Anonymous
– called from within same class as declaration
a = solve(f,g,"loose");
• Object

called as a property of an existing object
• Class

d.drawString("Welcome...", 25, 25);
called as a property of a class
f = Math.sin( angle/3 );
Java
40
The Anatomy of a Method
Definition
public double cube( double x )
{
return x*x*x;
}
– public double cube( double x )
visibility
return type
method name
parameter list, type and name
• body




parameters are local variable
non-void methods must have a return and a value
can return from any point in a method
good practice to use return in a void method
Java
41
An Aside – Type Casting
conversion between variable types
promotion
– automatic for built-in types as parameter or in expressions
• float  double
• long  float, double
• int  long, float, double
• char  int, long, float, double
• short  int, long, float, double
• byte  short, int, long, float, double
explicit cast
– required where no promotion exists
• (long)Math.exp(f);
Java
42
Recursion
Factorial
– n! = n×(n-1)×(n-2)×…×2×1
• loop
long factorial(long n){
long f=1;
for(long p=n;p>=1;p--)
f*=p;
return f;
}
• recursive
long factorial(long n){
if(n<=1)
return 1;
else
return n * factorial( n-1 )
}
Java
43
Recursion…
Fibonacci series
• What would this be as a
for loop?
– f(0) = 0
f(1) = 1
f(n) = f(n-1)+f(n-2)
long fibonacci(long n)
{
switch(n)
{
case 0:
case 1:
return n;
default:
return fibonacci(n-1) + fibonacci(n-2);
}
}
Java
44
Method Overloading
A method may be given one or more definitions
– example java.lang.String.indexOf
• int
• int
• int
• int
indexOf(int ch)
indexOf(int ch, int fromIndex)
indexOf(String str)
indexOf(String str, int fromIndex)
The compiler uses the supplied parameter list to select
which version to use.
– typically the simpler definitions call the others with default
parameters
– e.g…
int indexOf(int ch)
{
return this.indexOf(ch,0);
}
Java
45
Extensions
public class Sample2
extends JApplet
implements Runnable, ActionListener
{
Extending a Class
– gain default methods for class
– may redefine override methods
• overridden methods must be given the same declaration as in
the parent class.
– additional methods may be added
• e.g. Applet has
– init(), start(), stop(), destroy()
• override these to define the applet behaviour
Java
46
Implementations
public class Sample2
extends JApplet
implements Runnable, ActionListener
{
Implementing an Interface
– Must provide an implementation of the defined methods
• e.g. Runnable interface
– run() provides the code that is the behaviour of the thread
• e.g. ActionListener interface
– actionPerformed (ActionEvent e) provides the code
that responds to GUI events
Java
47
Example: Casino game ‘Craps’
Game of chance
Played by rolling two dice
Rules on p.250
Application code in Fig 6.9 simulates the game
of craps
Uses several methods to define the game’s logic
Java
48
Craps.java (Applet version)
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
imports classes needed for Applet
– awt – Container and FlowLayout
– event – ActionListener and ActionEvent
– swing – JApplet, JLabel, JButton, JTextField
public class Craps extends JApplet implements ActionListener
• declares Applet


extends JApplet – gains basic Applet behaviour
implements ActionListener – responds to GUI events
Java
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Craps.java: JApplet
final int WON = 0, LOST = 1, CONTINUE = 2;
Initialise constants
– the final qualifier indicates that these are constants, their
value cannot be changed by the program.
public void init()
• called when the Applet is loaded

used to create the GUI and perform any other tasks needed
in setting up the Applet to run.
Java
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Craps.java : GUI
Container c = getContentPane();
A Container is an AWT class into which GUI items are
placed
The getContentPane() method of the JApplet returns
a reference to the Container that forms the Applet
– note how the method becomes available by extending the
JApplet class
c.setLayout( new FlowLayout() );
• the setLayout() method of a Container is used.
•
a “new” FlowLayout is created and passed to the Container
Java
51
Craps.java: GUI bits
create a new label and add it to the Container
–
add() is a method defined in JApplet
create a new text box
–
–
make it un-editable
add it to the Applet
die1Label = new JLabel( "Die 1" );
c.add( die1Label );
firstDie = new JTextField( 10 );
firstDie.setEditable( false );
c.add( firstDie );
• create a push button with a text label
• register a class to receive events from the button

this refers to the Applet

the class receiving events must implement an ActionListener.
• add it to the applet
roll = new JButton("Roll Dice" );
roll.addActionListener( this );
c.add( roll );
Java
52
Craps.java: Events
public void actionPerformed(ActionEvent e )
called by a GUI component when an event occurs, generated by
that component
the ActionEvent object passed to the method provides information
about the event and the object that generated it
Java
53
Exercises Chapter 6
All examples in text
6.8 – calculation
6.14 – loops
6.22 – multiple event sources
6.38 – calculation
Java
54
Strings and Characters (Ch 29)
Constructors
– String
– Byte and Char arrays
• with indexes
s1
s2
s3
s4
s5
=
=
=
=
=
new
new
new
new
new
String(
String(
String(
String(
String(
"Hello World" );
charArray);
byteArray);
charArray, 2, 3);
byteArray, 2, 3);
Once created the size (length) of a string is fixed
int l = s1.length();
methods for
–
–
–
–
–
char c = s2.charAt(5);
determining the length
extracting characters
comparisons
extracting sub-strings
changing the characters in the string
Java
55
Comparisons
Methods compare strings using the value
(ACSII/Unicode) of the characters
– lexicographical comparison – case sensitive!
s1 = new String( "Hello" );
s1.equals("hello"); //false
s1.equalsIgnoreCase("hello");
//true
• compareTo() method returns
 0 if equal
 <0 if string is less than argument
 >0 if string is greater than argument

Note: A<a
Java
56
Comparing regions
Parameters
1.
2.
3.
4.
s1 = new String( "Hello Worlds" );
s2 = new String( "world" );
s1.regionMatches(6, s2, 0, 5);//false
is position to start match at
is string to match
is position in parameter string to start at
number of characters to match
s1.regionMatches(true, 6, s2, 0, 5);//true
•
Optional first parameter

true – match with case insensitivity

false – match using case sensitive test
Java
57
A note on ==
The == test when applied to strings (or any Object)
does not test the value of the objects
It tests to see if the variables contain references to the
same object
String s1 = new String("hello");
Suppose
then
s1 == "hello"
is FALSE, s1 is not the same object as “hello”
Java
58
==
Now if
then
String s1 = "hello";
s1 == "hello"
is TRUE,
The compiler has reused the anonymous
string “hello” which s1 is a reference to
This test is very useful in handling events in an actionListener. A
test can be made to see who caused the event.
if(e.getSource()==button)
Java
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Other methods
These all create a new string object
–
–
–
–
concatenating – joining strings
to UPPER case
to lower case
trim – removes white space at beginning and end of strings
• Other tests



String s1 = new String(" Hello");
s1.concat(" World ");//" Hello World
s1.toUpperCase();
//" HELLO WORLD
s1.toLowerCase();
//" hello world
s1.trim();
//"hello world"
startsWith('a') tests if string begins with char of string
endsWith("sub") tests if string ends with char of string
lastIndexOf('s') finds position working back down the string
from the end
Java
"
"
"
60
StringBuffer
A StringBuffer can have its contents and size changed
– length() is the length of the current string
– capacity() is the length that can be stored without needing
more memory
– append(String) joins the string to the end of the buffer,
getting more memory if needed
– setLength(int) sets the length of the string, truncating if
necessary
– insert(int, String) inserts the string at the index
– delete(int from, int to) deletes the sub-string from
the first index, up to but not including the second
Java
61
StringTokenizer
A StringTokenizer splits a string into tokens
– just as a sentence is split into words
• creates a tokeniser for the sentence
import java.lang.utils.StringTokenizer;
StringTokenizer st = new StringTokenizer("Some sentence or other“);
• count() returns the number of tokens
• The while loop shows how to work
through the string extracting tokens in turn
Java
int count =
st.countTokens();
while(st.hasMoreTokens())
st.nextToken();
62
StringTokenizer…
StringTokenizer st = new StringTokenizer(
"Some sentence or other",
",.()-“
);
The tokenizer uses whitespace. A
second string can be used to define
the splitting characters
Java
63
Character tests
method
test (true if)
isDigit()
is a digit [0-9]
isDefined()
is defined in the Unicode character set
isLetter()
is a letter upper and lower case [a-z A-Z] and others
(international character set)
isUpper()
is an upper case letter
isLower()
is a lower case letter
isJavaIdentifierStart()
is valid to start a Java identifier, class, method or
variable name
isJavaIdentifierPart()
is valid to be in a Java identifier
•and many others…
the letter tests work with the Unicode set so they also test for
Ù ù Â â æ etc…
Java

64
Exercises Chapter 29
All examples in text
29.3 compareTo
29.4 regionMatches
29.7 StringTokenizer
29.9 StringTokenizer
– hint: try s = (String)st.nextToken().concat( s)
if time try
– 29.8
– 29.12
Java
65