Lesson B - Basic Java Elements

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Lesson B - Basic Java Elements

• Unit B1 Program Elements

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Hello World Program, Again
// My First Program!! public class HelloWorld
public static void main(String args)
System.out.println(Hello World!)
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Identifiers Syntax

• Identifiers are the words a programmer uses in a
  program
• Identifier syntactic rules
• Can be made up of any length of
• letters
• digits
• underscore character (_)
• dollar sign ()
• Cannot begin with a digit
• Java is case sensitive
• User and user are completely different
  identifiers

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Identifiers Semantics

• Identifiers names can come from the following
  sources
• Fixed in Java as reserved words
• public, class, static, void, method,
• Chosen by the programmer to denote something
• HelloWorld, main, args
• Chosen by a programmer whose code we use
• String, System, out, println

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Naming style

• The correctness of the program is not affected by
  the names used
• public class X7_65Tx
• Names play a central role in the readability of
  the program
• They are part of its documentation
• They should thus be chosen carefully
• BankAccount, size, numberOfElements
• Follow conventions in choosing names!

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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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Valid, but bad Indentation
public class HelloWorld
public static void main(String
args)
System.out.println(Hello World!)

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Comments

• Comments are ignored and are treated as white
  space
• They should be written to enhance readability
• Explain what a piece of code does (its interface)
• Explain any special tricks, limitations,
• Java has three comment formats
• // comment to end of line
• / comment until
• closing /
• / API specification comment /

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Lesson 2 - Basic Java Elements

• Unit B2 - Variables and Data Types

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Variables

• A variable is a location in memory that can hold
  values of a certain data type
• Each variable must be declared before it is used
  
• The declaration allocates a location in memory to
  hold values of this type
• Variable types can be
• primitive
• reference to an object

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VariableExample Program
public class VariableExample public static
void main(String args) int x x 3
System.out.println(x) x 4
System.out.println(x)
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VariableExample Program (2)
public class VariableExample public static
void main(String args) int x x 3
System.out.println(x) x 4
System.out.println(x)
x
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VariableExample Program (3)
public class VariableExample public static
void main(String args) int x x 3
System.out.println(x) x 4
System.out.println(x)
3
x
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VariableExample Program (4)
public class VariableExample public static
void main(String args) int x x 3
System.out.println(x) x 4
System.out.println(x)
3
x
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VariableExample Program (5)
public class VariableExample public static
void main(String args) int x x 3
System.out.println(x) x 4
System.out.println(x)
4
x
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VariableExample Program (6)
public class VariableExample public static
void main(String args) int x x 3
System.out.println(x) x 4
System.out.println(x)
4
x
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Primitive Data Types

• A data type is defined by a set of values and the
  operators you can perform on them
• The Java language has several predefined types,
  called primitive data types
• The following reserved words represent the eight
  different primitive data types
• byte, short, int, long, float, double, boolean,
  char

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Integers

• There are four integer data types. They differ
  by the amount of memory used to store them

Type Bits Value Range
byte 8 -127 128
short 16 -32768 32767
int 32 about 9 decimal digits
long 65 about 18 decimal digits
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Floating Point

• There are two floating point types

Type Bits Range (decimal digits) Precision (decimal digits)
float 32 38 7
double 64 308 15
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Characters

• A char value stores a single character from the
  Unicode character set
• A character set is an ordered list of characters
• A, B, C, , a, b, ,0, 1, ,
  ,
• The Unicode character set uses 16 bits per
  character, allowing for 65,536 unique characters
• It is an international character set, containing
  symbols and characters from many world languages
• The ASCII character set is a subset of Unicode
• ASCII is the current standard (outside of Java)

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Boolean

• A boolean value represents a true/false
  condition.
• It can also be used to represent any two states,
  such as a light bulb being on or off
• The reserved words true and false are the only
  valid values for a boolean type

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Variable Declarations

• The syntax of a variable declaration is
• data-type variable-name
• For example
• Multiple variables can be declared on the same
  line
• Variables can be initialized (given an initial
  value) in the declaration

int total
long total, count, sum
int total 0, count 20 double unitPrice
57.25
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Variable Declaration Example
public class DeclarationExample public static
void main (String args) int weeks 14
long numberOfStudents 120 double
averageFinalGrade 78.6 System.out.println(w
eeks) System.out.println(numberOfStudents)
System.out.println(averageFinalGrade)
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More Variable Examples
double pi, conversionRate, temprature long
salary boolean isOn char c pi 3.14159 isOn
false c A salary 34000 isOn true
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Constants

• We may declare that a variable is a constant and
  its value may never change.
• Advantages
• readability
• efficiency
• error detection

final double PI 3.14159 final int
CHINA_OLYMPICS_YEAR 2008
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Lesson 2 - Basic Java Elements

• Unit B3 - Expressions

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Assignment Statements

• An assignment statement takes the following form
• variable-name expression
• The expression is first evaluated
• Then, the result is stored in the variable,
  overwriting the value currently stored in the
  variable

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Arithmetic Operators

• An operator is a mapping that maps one or more
  values to a single value
• Binary Operators
• a b adds a and b
• a - b subtracts b from a
• a b multiplies a and b
• a / b divides a by b
• a b the reminder of divining a by b
• Unary Operator
• -a The negation of a

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Pounds to Kg conversion
public class PoundsToKg public static void
main(String args) double weightInPounds
200.0 final double KILOS_IN_POUND 0.455
double weightInKg weightInKg
weightInPounds KILOS_IN_POUND
System.out.println(weightInKg)
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Pounds to Kg conversion 2
public class PoundsToKg2 public static void
main(String args) final double
KILOS_IN_POUND 0.455 System.out.println(200
.0 KILOS_IN_POUND)
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Integer Division

• When division is performed on integers (byte,
  short, int, long), the result is truncated to an
  integer.

int j 5 double x 5.0, y System.out.println(j
/ 2) // 2 System.out.println(x / 2.0) //
2.5 System.out.println(5 / 2) // 2 y j / 2
// 2
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Complex Expressions

• Expressions can combine many operators and
  operands
• Examples
• x
• -34
• weight 2.73
• 2 PI r
• a - (7 b)
• 1 2 3 4
• (x y) (2 - z (5 - q)) -(1-x)

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Operator Precedence

• Multiplication, division, and remainder () have
  a higher precedence than addition and
  subtraction.
• Operators with same precedence evaluate from left
  to right.
• Parenthesis can be used to force order of
  evaluation.

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Operator Precedence Examples
Expression Result
10 - 7 - 1 2
10 - (7 - 1) 4
1 2 3 7
(1 2) 3 9
1 - 2 3 4 5 15
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Conversions

• Data types can be mixed in an expression
• When the expression is evaluated one type is
  converted to another
• Data is converted to a wider type in three cases
• assignment conversion
• arithmetic promotion
• casting
• Can be converted to a narrower type only by
  casting
• List of types from narrowest to widest
• Narrow
  Wide
• byte short int long float double

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Conversion Examples
double f, x int j f 5 f 5.0 / 2 f x
j f 5 / 2 f (float) j / 5 j (int) f j
(int) 5.0 / 2.0
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Lesson 2 - Basic Java Elements

• Unit B4 - Objects and Method Invocation

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Reference Types

• Variables can be declared to be of an object
  type. In this case they hold a reference to an
  object of this type (class).

Turtle t String myName Date today
myName
t
today
Memory
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Creating Objects

• Objects are created by invoking a constructor of
  the class. Constructors may accept parameters.

Date today today new Date(12345)// mSec since
1.1.1970 Turtle t new Turtle()
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Creating Objects (2)

• Objects are created by invoking a constructor of
  the class. Constructors may accept parameters.

Date today today new Date(12345)// mSec since
1.1.1970 Turtle t new Turtle()
today
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Creating Objects (3)

• Objects are created by invoking a constructor of
  the class. Constructors may accept parameters.

Date today today new Date(12345)// mSec since
1.1.1970 Turtle t new Turtle()
today
12345
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Creating Objects (4)

• Objects are created by invoking a constructor of
  the class. Constructors may accept parameters.

Date today today new Date(12345)// mSec since
1.1.1970 Turtle t new Turtle()
t
today
12345
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Strings

• Strings are objects that are treated by the
  compiler in special ways
• Can be created directly using xxxx
• Can be concatenated using

String myName John Jones String hello hello
Hello World hello hello !!!! int year
2008 String s See you in China in year
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Method invocations

• You may invoke methods on an object. Methods may
  receive parameters.
• Methods may also return values.

Turtle leonardo new Turtle() leoardo.moveForwar
d(100) String lennon John Lennon int len
lennon.length() char initial lennon.charAt(5)
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APIs

• To use an object you only need to know its
  application programmer interface (API).
• The API of an object class includes a description
  of
• all available constructors and methods and what
  they do
• the parameters they take and the values that they
  return
• The API is usually given in a special format
  called javadoc.

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Javadoc example
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Javadoc example (2)
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Class Libraries

• A class library is a collection of classes that
  we can use when developing programs
• There is a standard class library that comes with
  every Java environment.
• Class Libraries are organized into packages
• java.net, java.lang, java.io, ...
• To use classes from a package you must either
• Import the package
• Or, use a fully qualified class name

import java.io. File f new
File(John)
java.io.File f new java.io.File (John)
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RandomNumbers.java
import java.util.Random public class
RandomNumbers public static void main
(String args) Random generator new
Random() int num generator.nextInt()
System.out.println ("A random int " num)
num generator.nextInt() System.out.print(A
nother one num)
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Lesson 2 - Basic Java Elements

• Unit B5 - Input and Output

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Types of Input and Output (I/O)

• Terminal-based I/O
• System.out.println()
• System.in.xxxxx()
• Graphic User Interface
• Windows, Buttons, Mouse,
• Stream based I/O
• Files, Web, Communication, Terminal-based I/O
• In this course we also provide our own classes
• InputRequestor, OutputWindow

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The InputRequestor Class

• To use the input requestor, you must first create
  the object

InputRequestor inp new InputRequestor()

• Then, you may use it to read primitive data types

int i inp.requestInt() int i
inp.requestInt(Enter age) float f
inp.requestFloat()
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InputRequestor Behaviour

• A window will pop up every time you use the
  requestXXX() method
• The window will disappear only after you have
  typed a legal input. The method returns this
  input.

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The OutputWindow Class

• Just as System.out enables you to display output
  in the MS-DOS window, you may display output in
  your own window.
• Create an OutputWindow object
• Useful methods in the output window
• println() - just as regular println().
• clear() - clears the output window.
• showMessage() - pop up a message on the desktop.

OutputWindow outwin new OutputWindow()
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The Output Window
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The showMessage() Pop-up Window
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Circle Area and Circumference
public class CircleCalc public static void
main(String args) final double PI
3.14159 InputRequestor in new
InputRequestor() OutputWindow out new
OutputWindow() double r
in.requestDouble("Radius ")
out.println("Radius " r)
out.println("Area " PI PI r)
out.println("Circumference " 2 PI
r)
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Lesson 2 - Basic Java Elements

• Unit B6 - Introduction to Applets and Graphics

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The Web
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My First HTML Example
ltH3gtMy First HTML Filelt/H3gt ltpgt Hello
World!!! ltpgt Here is a link to lta
hrefhttp//www.cnn.comgtCNNlt/agt!
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My First Web Page
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Running programs in a web page

• Java was invented so that running programs can be
  embedded in web-pages
• Such Java programs are called Applets
• This had to solve some problems
• Platform independence
• browsers have a built-in Java byte-code
  interpreter
• Security
• the program runs in the browser in a sandbox

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A Web Page with an Embedded Applet
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Writing Applets

• An Applet does not have a main method.
• Instead, it has a set of methods that control the
  interaction with the user.
• An Applet inherits this interface from the
  java.applet.Applet class
• The paint method has to paint the screen
• It receives a Graphics object, and uses it to
  paint
• The Graphics class has various drawXXX() methods

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Smiley.java
import java.applet.Applet import
java.awt. public class Smiley extends Applet
public void paint (Graphics page)
page.setColor(Color.yellow)
page.fillOval(50,50,300,300)
page.setColor(Color.black)
page.fillOval(125,125,50,50)
page.fillOval(225,125,50,50)
page.drawLine(150,275,250,275)
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Smiley.HTML
ltH3gtMy Smiley Appletlt/H3gt ltapplet
codeSmiley.class" width400 height400gt lt/applet
gt ltpgt Above this text you should see Smiley!!

• The Smiley.class file should be placed in the
  same directory with Smiley.HTML

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Smiley
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Graphics Coordinates
10
150
20
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page.drawLine (10, 20, 150, 45)
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Drawing an Oval
175
20
page.drawOval (175, 20, 50, 80)