Programs%20and%20Models

1
Programs and Models

• Almost all computer programs model some artifact
• Artifact product of civilization, either
  concrete or abstract, simple or complex
• Model a simplified representation of something
• It includes features that are considered
  important to the other
• Neglects other features not considered important
• Characteristics of a model
• Elements of the model represent other, more
  complex things
• Model elements exhibit consistent behavior
• Model elements can be grouped into different
  categories, depending on their behaviors
• Actions external to the model element cause the
  behavior of the model element

2
Objects, behavior, and classes

• In Java programming, model elements are called
  objects
• Objects that share common behavior are grouped
  into classes
• Defining a class in Java is writing code that
  specifies how objects of the class behave or act
• Once a class has been defined, objects of that
  class can be created
• Every object belongs to exactly one class and is
  an instance of that class
• Predefined objects and classes
• Java comes with some classes already defined
• We will also use classes created by other
  programmers

3
Thinking about classes and objects

• Think of a library
• To model a library, what would the relevant
  classes and objects be?

4
Modeling a monitor

• A device upon which information can be displayed
• For computers, monitors are devices to display
  information on
• (computers -gt monitors)
• For people, monitors are devices to read
  information from
• (monitors -gt people)

5
Some terminology

• Java uses a references to identify an object
• Messages are sent to references, specifying
  behavior with supporting details
• Reference any phrase that is used to refer to an
  object
• Messages a request for some desired behavior
  from an object

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Java monitor is predefined

• A monitor object is an instance of the
  PrintStream class (which is predefined)
• Does not model all features of a monitor (color)
• Used to display sequences of characters
• A predefined PrintStream object
• System.out refers to the predefined PrintStream
  object
• To display information on the monitor object, the
  Java language sends a message to the System.out
  reference
• println
• Specifies a desired bahavior (printing a line)
• Further details (the characters to be displayed)
  must also be sent

7
Sending a message

• To send a message we must specify the object and
  the behavior for that object
• In Java this means
• A reference to the receiver object
• A period
• The message to be sent
• To display Hello World on a monitor
• System.out.println(Hello World)

reference
message
8
A Java program

• A Java program needs
• A name (called an identifier)
• Identifier a sequence of letters, digits, or
  underscores
• The first character of an identifier must be a
  letter
• Surround it with additional notation
• Download FirstProgram.zip demo

9
Mechanics of Java programs

• Java programs are actually contained in a class
  that has a main() method
• Java source files have the same name as the class
  they contain, with the .java extension
• FirstProgram.java
• In CodeWarrior, we create projects that are the
  same name as the primary class
• FirstProgram.mcp

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Mechanics of Java Programs

• Source files (FirstProgram.java) need to be
  translated to a form that machines can understand
• These text files are compiled into Java Byte Code
  (an idealized machine language), which will be
  contained in a file with the same name as the
  text file but with the .class extendion
  (FirstProgram.class)
• CodeWarrior actually bundles all these class
  files into one package called AppClasses
• The Java Byte Code is interpreted by the Java
  interpreter for the particular machine you are
  working with
• Can be executed by any type of machine that has a
  Java interpreter
• Relatively easy to imbed Java interpreters into
  other programs (web browsers)

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Exercise

• Let us modify FirstProgram as an exercise

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PrintStream messages

• Message println()
• Two versions
• println(ltsequence of charactersgt)
• println(empty)
• In the first, the ltsequence of charactersgt will
  be displayed. Next character to be displayed will
  start on the next line
• In the second, nothing will displayed, and the
  next character to be displayed will appear on the
  next line
• Example
• System.out.println(Hello)
• System.out.println(World)
• Result
• Hello
• World

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PrintStream messages

• Message print()
• One versions
• print(ltsequence of charactersgt)
• The ltsequence of charactersgt will be displayed.
  Next character to be displayed will start on the
  same line, immediately after the last character
  in the ltsequence of charactersgt
• Example
• System.out.print(Hello)
• System.out.print(World)
• Result
• HelloWorld
• Next character to be printed will follow d

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The String class (2.5)

• A Java predefined class that models a sequence of
  characters
• Any group of characters in double quotes is a
  String object
• Hello World
• We have used these as arguments to println() and
  print() messages
• These are known as String Constants (cannot be
  changed)

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Messages for String objects

• println() and print() are messages for
  PrintStream objects
• What messages can we send to String objects?
• In other words
• What behavior does the String class provide?
• What methods do the String class provide?
• A beginning list can be found on page of the
  text
• A complete listing can be found in the Java API
  (Application Programming Interface)

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String references

• A String constant is a String reference
• Models precisely the character sequence
• References can be used as arguments to messages
• In other words, sending an object to another
  object
• Can also be used to send messages to objects they
  refer to
• Evoking the behavior of the object referred to

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Example toUpperCase()

• To send a message to the object, use the form
• reference.methodname(arguments)
• To send the toUpperCase() message (which has no
  arguments) to the String reference java
• java.toUpperCase()
• The receiver is the String object java refers
  to
• This message does not change the receiver
• A String method never changes the String object
• But produces a new String object with the desired
  change

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String messages create new String objects

• A new object is useless unless it can be
  referenced
• toUpperCase() creates a reference (return value)
  to the new object
• The message (java.toUpperCase() ) becomes the
  reference to the new object
• What to do with this reference?
• Send messages to the object referred to (not
  here)
• As an argument in a message to another object
• System.out.println(java.toUpperCase())

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Methods, arguments, and return values
Class Method Return value Arguments received
PrintStream println None None
PrintStream println None Reference to a String object
PrintStream print None Reference to a String object
String toUpperCase Reference to a String object None
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Methods, arguments, and return values

• Signature of a method
• Name
• Description of arguments
• Prototype of a method
• Signature
• Description of its return value
• Table on preceding page gives us information
  about four methods and their prototypes

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Reference variables

• Variable
• An identifier that can be given a value
• It can contain different values at different
  times
• Form
• type identifier
• type identifier1, identifier2,
• Reference variables
• An identifier that is given a reference
• To be used later or repeatedly
• Form
• class identifier
• class identifier1, identifier2,

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Assignment

• Assignment statements
• An assignment statement gives a variable a value
• Form
• identifier value
• Variable on the left-hand side
• Value on the right-hand side
• Variable must have already been declared
• Value must be consistent with the variable type
  or class

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Assignment and Equality

• String t Some String
• This is an imperative statement
• Tells the computer that the variable t refers to
  the String object Some String
• This is not a statement of equality
• Because t can be made to refer to another String
  object
• t A different String
• The previous object Some String is lost and no
  longer accessible to the program

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Multiple assignment of objects

• String t Some String
• String s t
• t A different String
• The variable t refers to the String object Some
  String
• The variable s refers to the same String object
  Some String as the variable t
• The variable t refers to a new String object A
  different String
• Variables are independent of each other
• Changing t did not affect s

25
Using String Methods

• Strings are immutable (they cannot be changed)
• String methods do not change the reference object
• They provide information about the object
• They create references to new String objects
  (such as toUpperCase() )
• Some commonly used String methods
• toUpperCase() - toLowerCase()
• length() - trim()
• concat(String) - indexOf(character)
• substring(number) - substring (number, number)
• Look at the String API for each of these methods

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Two demonstration programs

• Download MiddleCharacter.zip demo
• Download NameInitials.zip demo