Programming Basics: How To Create Java Programs

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Programming BasicsHow To Create Java Programs
Ankur M. Teredesai amt_at_cs.rit.edu www.cs.rit.edu/
amt
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Identifiers

• Identifiers in Java are composed of a series of
  letters and digits where the first character must
  be a letter.
• Identifiers should help to document what a
  classes, variables, or methods are used for.
• Upper and lower case letters are different in
  Java.
• ThisOne is not the same as thisOne.
• Identifiers should not include punctuation and
  can not include spaces.

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Identifiers

• A good programming standard for identifiers
  composed of multiple words is to capitalize the
  first character of the additional words.
• The case of the first letter of the first word
  will depend upon the use of the identifier.
• For Example,
• myCar, or bobAndSusieAreFriends

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Identifiers

• Class names begin with a capital letter.
• This standard makes it easier to understand which
  identifiers represent classes.
• In the MyFirstApplication we have two class
  names
• MyFirstApplication and MainWindow

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Identifiers

• Variable (object names) and method names begin
  with a lower case letter.
• In MyFirstApplication we have one object
• mainWindow
• Note that mainWindow is different from
  MainWindow.
• In MyFirstApplication we have one method
• main()

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Creating a Java Program

• The first step to creating a Java program is to
  define a class containing the main method.
• The MyFirstApplication program on page 39 of Wu
  is an example of a Java class containing a main
  method.
• This Java class definition can be thought of as
  the driver class or the ring leader class.

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MyFirstApplication
MyFirstApplication
main(String args)
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Declaring Objects

• In order to create objects to work within a Java
  program they must first be declared.
• The object declaration includes the class name
  that the object belongs to and the variable name
  for the object.
• The format for declaring an object is
• ltclass namegt ltobject(variable) namegt
• The class must be defined before you can declare
  the object.

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Declaring Objects

• Think about our cars.
• Cars was the class name and we had three
  instances of the this class.
• To declare each of the cars we had created we
  would type
• Cars clintsPorsche
• Cars jessicasBMW
• Cars johnsLamborghini

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Declaring Objects

• The MyFirstApplication program in Wu defines one
  object called mainWindow of the class type
  MainWindow.
• MainWindow mainWindow
• Note the differences in the text above.

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Declaring Objects

• Declaring an object only creates a name to refer
  to the object, it does not create the instance of
  that object.
• In order to use the named object instance, the
  object must be instantiated (or created).

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Object Instantiation

• Object instantiation (creation) actually creates
  a copy of the class and stores it in memory with
  the name defined in the object declaration.
• The format for object instantiation is
• ltobject namegt new ltclass namegt (ltargumentsgt)

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Object Instantiation

• ltobject namegt new ltclass namegt (ltargumentsgt)
• The ltobject namegt is the name for the object as
  you declared it in your program.
• The ltclass namegt is the name of the class that
  this object has been declared to belong to.
• The ltargumentsgt represent possible data that may
  be required to create the object instance.

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Object Instantiation

• We declared three types of Cars
• Cars clintsPorsche
• Cars jessicasBMW
• Cars johnsLamborghini
• To instantiate the instances of the Cars
• clintsPorsche new Car()
• jessicasBMW new Car()
• johnsLamborghini new Car()

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Object Instantiation

• The MyFirstApplication instantiates the object
  mainWindow on line two of the main method
• mainWindow new MainWindow()

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MyFirstApplication
MyFirstApplication
mainWindow
MainWindow
Main(String args)
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Message Passing

• Once an object exists, the program can begin to
  send messages to the object.
• The message must include the object to which the
  message is addressed, the name of the task to be
  completed, and any data required to complete the
  task.
• In Java speak
• ltobject namegt.ltmethod namegt(ltargumentsgt)

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Message Passing

• Lets create messages for our cars.
• We need to know how much gas Clint has.
• clintsPorsche.getGasLevel()
• We want to turn on Johns hazards.
• johnsLamborghini.turnOnHazards()

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Message Passing

• Now lets look at the last line of the main method
  in MyFirstApplication.
• mainWindow.setVisible(true)

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Method Declarations

• The method declaration provides the function
  implementation in the program.
• The basic format of a method declaration is
• ltmodifiersgt ltreturn typegt ltmethod namegt
  (ltparametersgt)
• ltmethod bodygt

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Method Declarations

• ltmodifiersgt ltreturn typegt ltmethod namegt
  (ltparametersgt)
• ltmethod bodygt
• Modifiers represent terms that determine what
  kind of method is declared. (Chap 4)
• The Return Type is the data type of the value
  returned by the method.
• If the method does not return a value this value
  is void.

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Method Declarations

• ltmodifiersgt ltreturn typegt ltmethod namegt
  (ltparametersgt)
• ltmethod bodygt
• The method name should identify the task to be
  completed by the method
• The parameters are the data that the method
  requires to complete the task.
• The method body includes the code statements that
  implement the task.

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Method Declarations

• Lets look at the main method in
  MyFirstApplication.

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Class Declarations

• Java programs are composed of one or more
  classes.
• Some classes are already defined in Java but we
  also need to be able to define our own classes.
• class ltclass namegt
• ltclass member declarationsgt

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Class Declarations

• class ltclass namegt
• ltclass member declarationsgt
• Every class must have a unique name.
• The class member declarations include method
  definitions and variable (data value).

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Class Declarations

• One class in every program must include the main
  method.
• This class is the driver class.
• The main method is the first method that is
  called when a program is executed.
• Lets look at the MyFirstApplication program.

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Import Statement

• The import statement is used to allow a class
  definition to access predefined Java classes.
• Related predefined Java classes are stored in
  packages.
• We will use the package called javabook provide
  by Wu.
• In order to import predefined classes
• import ltpackage namegt.ltclass namegt

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Import Statement

• import ltpackage namegt.ltclass namegt
• The package name represents the the particular
  package to look into for the class name.
• java. or javabook.
• The class name can be a specific class or every
  class in the package.
• import javabook.MainWindow
• import javabook.

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Comments

• Providing a description of what the program does
  is called commenting the code.
• Commenting is used to provide an overall
  description of the class and what it does, who
  the author is, when the class was created and
  modified, etc. Header comments
• Commenting is also used throughout your code to
  provide a description of a method, as well as to
  provide descriptions of complicated code.
• The use of meaningful variable and method names
  helps comment the code.

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Comments

• Java provides three ways to comment code.
• MyFirstApplication includes two types.
• The / . / is used for multiple line comments.
• The // is used for a single line comment.
• The final type is javadoc.

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Comments

• The javadoc comment appears before the class
  declaration.
• The javadoc comment begins with / rather than
  /.
• The javadoc comments end with /
• The benefit of using a javadoc comment is that
  the programmer can use a tool that will
  automatically create a web page from the comment.
• This is only true for javadoc comments.

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Comments

• Comments can not be put inside of another
  comment.
• Comments are for the programmer only, they do not
  represent executable code.
• The computer ignores comments.

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Commenting Rules of Thumb

• Do not state the obvious for self-evident
  information. For example if the code is
• x x y // it is not necessary that you are
  adding x and y and storing the result in x.
• Provide a brief statement about each variable and
  a brief description for each method.

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Commenting Rules of Thumb

• Do not comment code that is bad, if you are
  writing a long paragraph to describe what is
  going on, then re-write the code to make it
  simpler.
• Make sure your comments agree with the code.
• Use comments to Clarify not confuse the
  unsuspecting reader. Use them to help the reader.