CS2

1
CS2

• Category Java Basics
• Topic Creating basic programs
• Objectives
• Basic Program
• Comments and Conventions
• Data Types, Expressions, Operators
• Creating Objects
• Packages and the import statement

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CS 2

• Introduction to
• Object Oriented Programming
• Java Basics

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Basic Java Program

• Application execution begins at a main method
• Executes each statement one after the other and
  until reaches the end.
• Statements end with a semicolon.
• All Java programs have to have at least one class
• Usually there will be several

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Basic Class Definition

• public class Hello
• public static void main(String args)
  // display a greeting in the console
  window System.out.println("Hello,
  World!!")

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Documentation Comments

• Three ways to do it
• // Double slashes comment out everything until
  the
• // end of the line
• / This syntax comments out everything between
• the / and the /.
• / (There are no nested comments as in C.) /
• /
• This is syntax for Javadoc comments (similar
  to
• the second style of commenting. It has tags
  which
• allow for HTML formatting features.
• /

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Documentation and Commenting

• 70-80 of the cost of software is maintenance
• Program code is written for two audiences
• The compiler
• The programmer and his colleagues who will need
  to understand the code after it is written EVEN
  IF IT WORKS
• Comment to make your code clear!

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Javadoc

• Once commenting is complete the Javadoc program
  is run from the OS prompt.
• If for example a group of class files for a given
  project are located in the same directory then
  Javadoc may be run by typing
• javadoc .java
• When the program runs it will report any problems
  and will produce a series of HTML files
  documenting all classes, methods and fields.
• A Javadoc template is on the next slide followed
  by a sample

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• / A class called Template. Used to make
  template objects! /
• public class Template
• / Serial. The objects serial number. /
• public int serial
• / Pressure. The objects pressure in psi. /
• public double press
• / Temperature. The objects temperature in
  degrees C /
• public float temp
• / Time variables. Hours and minutes--the
  objects time. /
• int hours, minutes
• / Invisible. Why doesn't this one appear???
  /
• long hidden
• / Descriptive phrase. Longer statement of
  purpose.
• _at_param p1 A description of this parameter
• _at_param p2 A description of this parameter
• _at_ return A description of the return value
• /
• public int someMethod(double p1, String p2)

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Running javadoc

• Cgtjavadoc .java
• Loading source file Template.java...
• Constructing Javadoc information...
• Building tree for all the packages and classes...
• Building index for all the packages and
  classes...
• Generating overview-tree.html...
• Generating index-all.html...
• Generating deprecated-list.html...
• Building index for all classes...
• Generating allclasses-frame.html...
• Generating index.html...
• Generating packages.html...
• Generating Template.html...
• Generating serialized-form.html...
• Generating package-list...
• Generating help-doc.html...
• Generating stylesheet.css...

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Results
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Results
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Results
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Java Conventions

• No spaces between words for names
• Each word in a class name starts with an
  uppercase letter
• BankAccount
• All other names start with a lowercase letter
• aCheckingAccount
• balance

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Variable Declarations

• Simple form
• ltdatatypegt ltidentifiergt
• Example
• int total
• Optional initialization at declaration
• ltdata typegt ltidentifiergt ltinit valuegt
• Example
• int total 0

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Data Types

• Two main types of data in Java
• Primitive
• Numbers, Characters, Boolean
• Object
• Strings, Arrays, System.out (in standard package)
• Objects from libraries (other packages)
• Objects you define

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Primitive Type Facts
Type
Size
Min
Default
Max
boolean
false
1
false
true
char
'\u0000' (null)
16
byte
(byte) 0
8
-128
127
short
(short) 0
16
-32,768
32,767
int
0
32
-2,147,483,648
2,147,483,647
long
0L
64
-9,223,372,036,854,775,808
9,223,372,036,854,775,807
float
0.0F
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Approx 3.4E38 with 7 significant digits
double
0.0D
64
Approx 1.7E308 with 15 significant digits
void
Not truly min and max.
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Examples

• Local Variables
• int counter 0
• double batAvg 20d // notice this
• char gender f
• boolean isEmpty true
• Field Declarations
• private boolean flag
• public static final int
• MAX_STUDENTS200

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Specifying Numbers

• Note that whole integers appearing in your source
  code are taken to be ints. So, you might wish
  to flag them when assigning to non-ints
• float fMaxGrade 100f // now holds 100.0
• double dTemp 583d // holds double
• // precision 583
• float fTemp 5.5 // ERROR! Java thinks
• // 5.5 is a double
• Upper and lower case letters can be used for
  float (F or f), double (D or d), and long
  (l or L, but we should prefer L)
• float fMaxGrade 100F // now holds 100.0
• long x 583l // holds 583, but looks
• // like 5,381
• long y 583L // Ah, much better!

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Space Time

• Why so many data types?
• Performance
• Smaller data items (byte, short, float) take up
  less space
• Integer calculations are faster than floating
  point calculations
• Nevertheless, sometimes we need to convert data
  from one type into another

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Type Checking

• What happens when data is changed from one
  representation to another?
• Some languages (such as Java) enforce a strict
  set of rules to try and catch problems early
• It is a complier error to move data from a larger
  type to a smaller type without an explicit cast
  to the new type.

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Explicit Casting Intentional Loss of Precision
int x 45 byte b b (byte) x // cast
needed

Symbol Picture of
Memory
x
b
cast needed!
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But wait...

• int i 42
• byte b
• b i
• Surely 42 will fit in a byte???
• Yes, we just need to tell Java we know what we're
  doing!!! How? Casting

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Precise Rules

• Consider
• float f
• int i 5
• int j 2
• f i/j
• Result?
• f 2.0

But what if we wanted 2.5 as a result?
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One way

• Consider
• float f
• int i 5
• int j 2
• float temp1 i
• float temp2 j
• f temp1/temp2
• Result?
• f 2.5

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Better (Easier?) Way

• Consider
• float f
• int i 5
• int j 2
• f (float)i/(float)j
• Result?
• f 2.5

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Casting

• We say "cast i to a float"
• (float)i
• Are we actually changing i???
• No, we are telling Java to convert the value of i
  to a float before making the calculation.
• We are not actually changing the contents of
  variable i.

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We can even...

• Consider
• float f
• int i 5
• int j 2
• f (float)i/j
• Result?
• f 2.5
• When dividing two different types Java will
  automatically cast the j for us

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Expressions

• Formed by combining
• Variables
• Selected reserved words
• Methods (Java/OO speak for procedures and
  functions)
• Literal values
• Operators
• Syntax mostly borrowed from C/C

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Reserved Words(Keywords)

• abstract default if private throw
• boolean do implements protected
  throws
• break double import public transient
• byte else instanceof return try
• case extends int short void
• catch final interface static
  volatile
• char finally long super
  while
• class float native switch
• const for new synchronized
• continue goto package this

Don't worry about what all these words mean or
do, but be aware that you cannot use them for
other purposes like variable names.
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Operators

• Assignment (, , -, , /, etc.)
• Arithmetic , -, , /, (modulo) and unary
  minus
• String concatenation (overloaded)
• Increment and Decrement Operators
• Comparison , !, lt. lt. gt, gt
• Boolean (and), (or), ! (not) plus others
  (, , )
• Others which we will introduce as appropriate
• instanceof, . , , (), new, etc.
• Not used in CS1322
• Bitwise Not used in CS2 (, , , , ltlt, gtgt,
  gtgtgt)
• Conditional operator ?

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Operator Issues

• Are applied to operands (one or more)
• Different type operands may change meaning of
  operator ()
• Operators always return a value
• Operators may also have side effects
• Pay attention to the order of evaluation

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Assignment

• Basic assignment
• Compute value of expression on right hand side
  of operator
• Store result in variable on left hand side
• Combination assignment operators
• - / etc.
• Calculate value of expression on right hand side
• Perform operation (first part of symbol (, -, ,
  /) using variable on left hand side and value of
  expression on right hand side
• Store result in variable on left hand side
• a 1 is equivalent to a a 1
• Note The format of these operators is
• "operation then "equals sign
• x 2
• Not x 2 which just means assign to x the
  value positive 2

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Arithmetic

• , -, , /, (modulo), unary minus
• Precedence
• Unary minus (negation)
• , /,
• , -

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High precedence postfix operators .
(params) expr expr-- unary operators
expr --expr expr -expr ! creation or cast
new (type)expr multiplicative /
additive - shift
ltlt gtgt gtgtgt relational lt gt lt gt
instanceof equality ! bitwise
AND bitwise exclusive OR bitwise
inclusive OR logical AND
logical OR conditional
? assignment - /
ltlt gtgt gtgtgt Low
precedence
When operators of equal precedence appear in the
same expression, a rule must govern which is
evaluated first. All binary operators except for
the assignment operators are evaluated in
left-to-right order. Assignment operators are
evaluated right to left.
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String Concatenation

• Strings are objects in Java that hold textual
  data like "abc"
• Use \ to put a double quote in a string
• Strings separated by a plus () sign indicate
  concatenation of the Strings
• String temp abc def // abcdef

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Pre and Post Increment and Decrement Operators

• counter // same as counter counter 1
• counter-- // same as counter counter - 1
• counter // same as counter counter 1
• --counter // same as counter counter - 1
• What's the difference?
• counter counter increments after the
  statement containing it is evaluated.
• counter counter increments before the
  statement containing it is evaluated.
• counter 7
• a counter
• b counter
• What's the value of a, b and counter?

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Increment and Decrement Operators

• These operators are more than just a way to save
  typing.
• Be careful when using them.
• Misuse can introduce bugs!

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Declaring Variables

• When you declare a primitive you get space
  allocated for that primitive
• int x 0 32 bits allocated and set to 0
• When you declare an object you get an object
  reference (not an object!)
• String name
• name new String(Fred)

null
name
FredString
name
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Creating Objects

• Create objects use the new keyword
• Address myAddress new Address(123 Main, Apt
  2, Bedrock, GA, 30219)
• The Address class will create an address object
  and call a constructor that takes 5 String
  objects
• Constructors initialize the fields of an object

Address
myAddress
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String Objects

• Strings can be created with the new keyword
• String name new String(Fred)
• Strings can also be set to literals
• String name Fred
• Strings are immutable (cant change)
• Changes result in a new string

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Some String Methods

• How do I get the number of characters in a
  String?
• How do I create a substring?
• How do I change a string to all uppercase
  letters?
• How do I change a string to all lowercase
  letters?
• How do I find a replace a character in a string?

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Java Packages

• Java organizes groups of related classes into
  packages
• java.lang (String and System are in here)
• java.io
• java.sql
• java.net

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Import Statement

• If you wish to use a class that isnt in
  java.lang you can
• Use the full name of the class everywhere
• java.util.Random randomGen
• new java.util.Random()
• Or use an import statement giving the full name
  or (giving access to all in package)
• import java.util.Random // allow Random
• import java.util. // allow all in
  package
• Import statements go before the class definition
  in a file

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Useful Classes

• Random
• In java.util
• Pseudorandom number generator
• NumberFormat
• In java.text
• Format numbers for currency and percentages
• DecimalFormat
• In java.text
• Format decimal numbers for output

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Class Methods

• All java code must belong to a class
• How to provide general math functions?
• Like absolute value (abs)
• Calculate the square root (sqrt)
• Class methods on the Math class
• Invoked by ClassName.function()
• Math.abs(-7)
• Math.sqrt(4)

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Questions
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