CSC 224 Java for Programmers

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CSC 224 Java for Programmers

• Winter 2002 (2002/01/07 -- 2002/03/13)
  Monday/Wednesday 310 -- 440 pm
• Instructor John PetlickiOffice hours Monday
  and Wednesday
• 130 300 pm (CST Building) E-mail
  jpetlick_at_condor.depaul.edu Home page
  http//www.depaul.edu/jpetlick

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Review of Resources

• CD-ROM of "Java Software Solutions" contains
• 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

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Characteristics of Java

• Simple
• Object-oriented
• Strongly typed
• Interpreted
• Architecture neutral

• Garbage collected
• Multithreaded
• Robust
• Secure
• Portable

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Simple

• Many programmers are already familiar with Java
  syntax (borrowed from C and C).
• Difficult aspects of C are removed.
• No header files
• No pointer arithmetic (pointer Syntax)
• No structures or unions
• No operator overloading
• No virtual base classes
• Etc.

5
Object-oriented

• A technique of developing programs by creating
  entities that contain logically related
• data and
• methods that interface to the data

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Strongly typed

• Every variable must have a declared type.
• 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

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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.

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Architecture neutral

• Java bytecode can be run on many different
  processors.
• 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.

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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.

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Multithreaded

• A program can be designed as multithreaded in
  order to perform multiple tasks at the same time
• Can take advantage of multiprocessor systems

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Robust

• Emphasis on early checking of possible problems.
• Dynamic run-time checking.
• Eliminate situations that are error-prone
• Pointer model eliminates possibility of
  overwriting memory and corrupting data

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Secure

• Security features
• Java will not corrupt memory outside its own
  process space.
• Web browser can prevent Java applets from
  reading or writing local files
• And more

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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.

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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.)

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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

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Java Development Environments

• DOS command line interface
• basic with SDK 2. (javac, java)
• Forte -- heavyweight IDE
• forte_ce_2.exe on the CD ROM
• Bluej lightweight IDE
• Download from www.bluej.org
• And others

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Installing Bluej (optional)

• Be sure the JDK is installed
• Download from http//www.bluej.org/
• Download the Bluej tutorial
• Follow the installation instructions

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Java Basics

• Program structure
• Naming conventions
• Package
• Identifiers
• Reserved words
• Data types
• Type conversions
• Arithmetic operators and expressions
• Variable declaration and initialization
• Assignments

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Packages

• Java provides mechanisms to organize large-scale
  programs in a logical and maintainable fashion.
• Class --- highly cohesive functionalities
• File --- one class or more closely related
  classes
• Package --- a collection of related classes or
  packages

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Java Class Library

• The Java class library is organized into a number
  of packages
• java.lang --- general
• java.awt --- GUI
• java.io --- I/O
• java.util --- utilities
• java.applet --- applet
• java.net --- networking

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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("He
llo!) System.out.println("The time is "
new Date())
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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())

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Java Reserved Words
abstract boolean break byte byvalue case cast catc
h char class const continue
default do double else extends false final finally
float for future generic
goto if implements import inner instanceof int int
erface long native new null
operator outer package private protected public re
st return short static super switch
synchronized this throw throws transient true try
var void volatile while
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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

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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

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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

int total
int count, temp, result
Multiple variables can be created in one
declaration
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Variables

• A variable can be given an initial value in the
  declaration

int sum 0 int base 32, max 149

• When a variable is referenced in a program, its
  current value is used

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Data Types
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Boolean Type

• boolean
• boolean constants true false

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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).

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Escape Sequences
 
\uhhhh hex-decimal code, e.g. \u000A \ddd octal
code, e.g. \040
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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)

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Data Conversions

• In Java, data conversions can occur in three
  ways
• 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

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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

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Legal data conversion(No Information Loss)
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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)
• charAt(int i)
• length()

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Arithmetic Operators and Expressions

•   addition
• -  subtraction
•   multiplication
• /  division
•   remainder
• precedence and association

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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)
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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
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Assignments
total quantity unitPrice count count 1
count 1 count
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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
• give names to otherwise unclear literal values
• facilitate changes to the code
• prevent inadvertent errors

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Operator Precedence

• What is the order of evaluation in the following
  expressions?

a b c d e
a b c - d / e
1
4
3
2
3
2
4
1
a / (b c) - d e
2
3
4
1
a / (b (c (d - e)))
4
1
2
3