Input and Java A Quick Intro

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Input and Java A Quick Intro

• In Java, we can do Input as well. Rather than go
  into details right now Ill introduce a class
  that does some input.

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DemoInput.java

• /
• Created on Jan 20, 2004
• _at_author Jerry Heuring
• This class is used to demonstrate some
  different input
• methods for different types. It isn't the
  best in all cases
• but should get you started in this class. It
  will depend
• heavily on some of the class equivalents of
  basic types.
• /
• import java.io.

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Constructors

• public class DemoInput
• private BufferedReader inputSource
• /
• The constructor -- it would prefer to get a
  BufferedReader
• as it will support what we want to do in the
  easiest way.
• we can convert from other types to a
  BufferedReader as well.
• _at_param source The source for input for this
  object.
• /
• public DemoInput(BufferedReader source)
• inputSource source

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• /
• Another constructor -- it gets an
  InputStreamReader instead of
• the BufferedReader.
• _at_param inStrRdr An InputStreamReader to use
  to do input
• with this object.
• /
• public DemoInput(InputStreamReader inStrRdr)
• inputSource new BufferedReader(inStrRdr)

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• /
• A Constructor taking an Input Stream -- it
  turns out that
• System.in is an InputStream so that this will
  be useful.
• _at_param inStr The input stream to be used for
  input by
• this object.
• /
• public DemoInput(InputStream inStr)
• inputSource new BufferedReader(new
  InputStreamReader(inStr))

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readLine Read a line

• /
• Reads a line from the input stream. It will
  return an
• empty string if an error (exception) occurs
  during the
• read.
• _at_return The next line of the file or an empty
  string
• (on error).
• /
• public String readLine()
• try
• return inputSource.readLine()
• catch (Exception e)
• return ""

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readInt reads an Integer

• /
• This routine will get a line and use the
  parseInt routine
• to convert it to an integer. It does handle
  the empty string
• by returning a zero.
• _at_return The integer on the next line of the
  file or zero
• if an error occurs.
• /
• public int readInt()
• String chunk
• chunk readLine()
• if (chunk "")
• return 0
• else
• return Integer.parseInt(chunk)

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readFloat read a float

• /
• Trying to do the same thing with a float. In
  this case
• I've decided to lose the String local and do
  the conversion
• directly. What happens if the read fails or
  the conversion
• fails?
• _at_return The float on the next line of the
  file or ??
• /
• public float readFloat()
• return Float.parseFloat(readLine())

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Main used for testing

• /
• The main program is going to try to test the
  input routines
• in the object. I'm not a big fan of this
  class but it does
• get us past some details and removes some
  initial guesswork.
• _at_param args The command line arguments. Not
  used in this version.
• /
• public static void main(String args)
• DemoInput testObject new DemoInput(System.in)
• String line
• int value
• float floatValue

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Main (continued)

• System.out.println("Input a line to read ")
• line testObject.readLine()
• System.out.println("The line input was
  "line)
• System.out.println("Input an integer ")
• value testObject.readInt()
• System.out.println("The value returned was
  "value)
• System.out.println("Input a floating point value
  ")
• floatValue testObject.readFloat()
• System.out.println("The value returned was
  "floatValue)
• System.exit(0)

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Program Design

• UML version

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Identify the Objects

• DotCom
• Company
• Size
• Hits
• GameGrid
• List of companies
• Where the companies are placed
• Oriented horizontal/vertical
• Player
• Turns/Guesses
• Rating
• Name

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Decide on the Methods/Procedures

• DotCom
• Set/Get Name, Size, Hits
• GameGrid
• Initialize
• Add Dot Com
• Check For Hit
• Dot Coms Left
• Player
• Take Turn
• Play Game
• Print Results

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UML For the Problem -- Player
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UML -- GameGrid
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UML -- DotCom
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Polymorphism Inheritance

• Much of the power in object oriented programming
  comes from polymorphism, inheritance, and
  interfaces
• Polymorphism is at the heart of all object
  oriented programming languages. It is the
  ability to treat different items in the same way.

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An Example of Polymorphism

• Assume you have a sound object, a movie object,
  and a game object.
• Because they are all objects they can be stored
  in a vector or any other collection of objects.
• Assuming they all have a play method (obviously
  doing different things) we could put them in a
  list, step through the list, and play each one.
• (I just told a lie it isnt quite this simple
  in Java but it is close)

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Templates are not Polymorphism

• Being able to handle different types of objects
  in a consistent manner is part of polymorphism.
• Polymorphism is NOT the same as a C template.
  We are not creating a separate class for each
  type of object.

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Inheritance

• In Java, C, and most (if not all) object
  oriented languages you can inherit from another
  class.
• Consider it as extending or enhancing an
  existing object. The existing object is still
  there but more features have been added to it.

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Inheritance Examples

• You buy a computer system and add on some
  specialized gaming equipment and I/O devices to
  extend its capabilities.
• The base computer system is still there it
  could probably be used without all the new bells
  and whistles youve added but the new object is
  of more use.

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More Inheritance

• Another common use of inheritance is to allow
  multiple objects share a common subset of methods
  and attributes.

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If You Look at Java There is All Kinds of
Inheritance Used

• The Stack class extends the Vector Class
• The Vector class extends the AbstractList class
• The AbstractList class exteds the
  AbstractCollection class
• The AbstractCollection class extends the Object
  class (All objects are descendents of the Java
  Object class)

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Opportunities for Inheritance

• Two or more subclasses that might need common
  behavior or that share some common underlying
  data.

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Is-a or Has-a?

• The book sites the Is-a test as a method to
  determine if something should be extended.
• A vector is a list
• A stack is a list (and in Javas case is a Vector
  as well)
• Has-a would mean that the object has an instance
  variable of a given type.

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Keep in Mind

• A subclass extends its superclass
• All public instance variables and methods are
  inherited from the superclass. Private variables
  and methods are not.
• Inherited methods can be overridden. Instance
  variables can be redefined but not overridden.

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Interfaces

• An interface is a group of calls/methods.
• Anybody that implements the interface must
  provide all the methods in the interface.
• Objects implementing an interface dont need to
  be a subclass of the same superclass but will
  have a common set of methods.

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Abstract Classes

• Abstract classes can be used in a hierarchy and
  as a type but cant be instantiated (created via
  new).
• Methods may also be abstract they have no body
  and look like a function prototype in C.
• Any class with an abstract method must also be
  declared as abstract.

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Where are Abstract Classes Used

• Used heavily in the collections inside of Java to
  enforce a common set of methods and of behavior.

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An Example of Inheritance