Download Characteristics of Java

CSC 224 Java for Programmers
Winter 2002 (2002/01/07 -- 2002/03/13)
Monday/Wednesday 3:10 -- 4:40 pm
Instructor: John Petlicki
Office hours: Monday and Wednesday
1:30 – 3:00 pm (CST Building)
E-mail: [email protected]
Home page:
http://www.depaul.edu/~jpetlick
Review of Resources
CD-ROM of "Java Software Solutions"
contains:
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Java(TM) 2 SDK, Standard Edition (for
Microsoft Windows)
Forte for Java Community Edition (for
Microsoft Windows)
Slides in PowerPoint 97 format
Keyboard materials
Example programs
Characteristics of Java
Simple
Object-oriented
Strongly typed
Interpreted
Architecture neutral
Garbage collected
Multithreaded
Robust
Secure
Portable
Simple
Many programmers are already familiar with
Java syntax (borrowed from C and C++).
Difficult aspects of C++ are removed.
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No header files
No pointer arithmetic (pointer Syntax)
No structures or unions
No operator overloading
No virtual base classes
Etc.
Object-oriented
A technique of developing programs by
creating entities that contain logically
related
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data and
methods that interface to the data
Strongly typed
Every variable must have a declared
type.
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Java has eight primitive types
 4 types of integers
 2 types of floating numbers
 1 character type (for Unicode characters)
 1 boolean type for truth values
Interpreted
The Java source code is compiled into
Java bytecode.
The Java interpreter can execute Java
bytecodes on any machine to which the
interpreter has been ported.
Architecture neutral
Java bytecode can be run on many
different processors.
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Bytecode
 is not dependent on the machine code of any
particular computer.
 is designed to be easily translated into the
native machine code of any machine.
Garbage Collected
The programmer does not have to
explicitly return allocated memory to
the system.
Java performs automatic garbage
collection when object references are
lost.
Multithreaded
A program can be designed as
multithreaded in order to perform
multiple tasks at the same time
Can take advantage of multiprocessor
systems
Robust
Emphasis on early checking of possible
problems.
Dynamic run-time checking.
Eliminate situations that are error-prone
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Pointer model eliminates possibility of
overwriting memory and corrupting data
Secure
Security features
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Java will not corrupt memory outside its
own process space.
Web browser can prevent Java applets
from reading or writing local files
And more
Portable
The size of primitive data types and the
behavior of arithmetic on them is
specified and constant.
The libraries have portable interfaces –
the abstract Window class has
implementations for UNIX, Windows
and Macintosh.
A Java Program
/*
* Example HelloWorld.java
* Prints the message "Hello, World!“
*/
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world.”);
}
}
Java programming steps
Create a source code program with a text editor
(HelloWorld.java)
Compile the source code into Java bytecode
(javac HelloWorld.java)
Run the bytecode with a Java interpreter
(java HelloWorld)
Java interpreters translate bytecode instruction to
machine code
 and then carry out the instruction.
A compiler and interpreter are in the Java 2 SDK
(Software Development Kit )
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Installing JDK 1.3
For Microsoft Windows platform, run
the executable j2sdk1_3 file on the CD
For other platforms other than Microsoft
Windows, obtain the SDK directly from
Sun, at www.java.sun.com
Java Development Environments
DOS command line interface
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basic with SDK 2. (javac, java)
Forte -- heavyweight IDE
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forte_ce_2.exe on the CD ROM
Bluej – lightweight IDE
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Download from www.bluej.org
And others
Installing Bluej (optional)
Be sure the JDK is installed
Download from http://www.bluej.org/
Download the Bluej tutorial
Follow the installation instructions
Java Basics
Program structure
Naming conventions
Package
Identifiers
Reserved words
Data types
Type conversions
Arithmetic operators and expressions
Variable declaration and initialization
Assignments
Packages
Java provides mechanisms to organize
large-scale programs in a logical and
maintainable fashion.
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Class --- highly cohesive functionalities
File --- one class or more closely related
classes
Package --- a collection of related classes
or packages
Java Class Library
The Java class library is organized into a
number of packages:
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java.lang --- general
java.awt --- GUI
java.io --- I/O
java.util --- utilities
java.applet --- applet
java.net --- networking
Example using import
// Example Time
// Prints a greeting message with
// the current time.
import java.util.*;
public class Time {
public static void main(String[] args) {
System.out.println("Hello!”);
System.out.println("The time is " +
new Date());
}
}
Fully qualified class name
public class Time1
{
public static void main(String[] args)
{
System.out.println("Hello!");
System.out.println("The time is "
+ new java.util.Date());
}
}
Java Reserved Words
abstract
boolean
break
byte
byvalue
case
cast
catch
char
class
const
continue
default
do
double
else
extends
false
final
finally
float
for
future
generic
goto
if
implements
import
inner
instanceof
int
interface
long
native
new
null
operator
outer
package
private
protected
public
rest
return
short
static
super
switch
synchronized
this
throw
throws
transient
true
try
var
void
volatile
while
Identifiers
are the words a programmer creates in
a program
made up of letters, digits, underscore
character (_), and the dollar sign
cannot begin with a digit
Java is case sensitive, therefore Total
and total are different identifiers
White Space
Spaces, blank lines, and tabs are collectively
called white space
White space is used to separate words and
symbols in a program
Extra white space is ignored
A valid Java program can be formatted many
different ways
Programs should be formatted to enhance
readability, using consistent indentation
Variables
A variable is a name for a location in memory
A variable must be declared, specifying the
variable's name and the type of information
that will be held in it
data type
variable name
int total;
int count, temp, result;
Multiple variables can be created in one declaration
Variables
When
A variable can be given an initial value in the
declaration
int sum = 0;
int base = 32, max = 149;
a variable is referenced in a program, its current value is used
Data Types
Type
Storage
Min Value
Max Value
byte
short
int
long
8 bits
16 bits
32 bits
64 bits
-128
-32,768
-2,147,483,648
< -9 x 1018
127
32,767
2,147,483,647
> 9 x 1018
float
32 bits
Double
64 bits
+/- 3.4 x 1038
with 7 significant digits
+/- 1.7 x 10308
with 15 significant digits
Boolean Type
boolean
boolean constants:
true
false
Character Type
char
16-bit Unicode character.
ASCII is a subset of Unicode --- ISO-8859
(Latin-1)
Examples of character constants:
'A'
'y'
'8'
'*'
' ' (space)
'\n' (new line).
Escape Sequences
\b
backspace
\f
\n
\r
\t
form feed
new line (line feed)
carriage return
tab
\"
\'
\\
double quote
single quote
backslash
\uhhhh: hex-decimal code, e.g. \u000A
\ddd: octal code, e.g. \040
Data Conversions
Sometimes it is convenient to convert data from one type
to another
For example, we may want to treat an integer as a
floating point value during a computation
Conversions must be handled carefully to avoid losing
information
Widening conversions are safest because they tend to go
from a small data type to a larger one (such as a short
to an int)
Narrowing conversions can lose information because they
tend to go from a large data type to a smaller one (such
as an int to a short)
Data Conversions
In Java, data conversions can occur in three ways:
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assignment conversion
arithmetic promotion
casting
Assignment conversion occurs when a value of one type
is assigned to a variable of another
Only widening conversions can happen via assignment
Arithmetic promotion happens automatically when
operators in expressions convert their operands
Data Conversions
Casting is the most powerful, and dangerous, technique
for conversion
Both widening and narrowing conversions can be
accomplished by explicitly casting a value
To cast, the type is put in parentheses in front of the
value being converted
For example, if total and count are integers, but we
want a floating point result when dividing them, we can
cast total:
result = (float) total / count;
Legal data conversion
(No Information Loss)
String -- a class type
Examples of string literals:
"watermelon"
"fig"
"$%&*^%!!"
"354"
" " (space)
"" (empty string)
Constructor
String name = new String(“John”);
Concatenation operator +
methods: (page 75)
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charAt(int i)
length()
Arithmetic Operators and
Expressions
+
*
/
%
addition
subtraction
multiplication
division
remainder
precedence and association
Example Time2
import java.util.*;
public class Time2 {
public static void main(String[] args) {
Calendar calendar = Calendar.getInstance();
int hour = calendar.get(Calendar.HOUR_OF_DAY);
int minute = calendar.get(Calendar.MINUTE);
int second = calendar.get(Calendar.SECOND);
System.out.println("The time is "
+ hour + ":" + minute + ":" + second);
System.out.println("The time is "
+ hour / 10 + hour % 10 + ":"
+ minute / 10 + minute % 10 + ":"
+ second / 10 + second % 10);
}
}
Variable declaration / initialization
int x, y, z;
long count;
char a, b;
boolean flag;
float massInKilos;
short timeInSeconds = 245;
char ch1 = 'K', ch2 = '$';
boolean isNew = true;
double maxVal = 35.875;
Assignments
total = quantity * unitPrice;
count = count + 1;
count += 1;
count++;
Constants
an identifier that is similar to a variable except that it holds one
value for its entire existence
The compiler will issue an error if you try to change a constant
In Java, we use the final modifier to declare a constant
final int MIN_HEIGHT = 69;
Constants:
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give names to otherwise unclear literal values
facilitate changes to the code
prevent inadvertent errors
Operator Precedence
What is the order of evaluation in the
following expressions?
a + b + c + d + e
1
2
3
4
a + b * c - d / e
3
1
4
2
a / (b + c) - d % e
2
1
4
3
a / (b * (c + (d - e)))
4
3
2
1