Building Java Classes

1
Building Java Classes

• Class Definition
• Standard Methods
• Static Variables and Methods
• Robert Stumpf, Ph.D.
• CIS Department

2
Class Definition

• A Java class is simply specified by
• class ltnamegt
• private lttypegt ltnamegt // attribute
• public lttypegt ltname ( ) // method

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Class Definition

• A Java class example
• class Temperature
• private int celsius // attributepublic
  int getCelsius ( ) // accessor
• return celsius
• public void setCelsius (aCelsius)
  // mutator celsius aCelsius

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Class Definition

• Getters are called accessors
• public int getCelsius ( )
• return celsius
• Setters are called mutators
• public void setCelsius (aCelsius)
• celsius aCelsius

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Class Definition

• Naming of arguments
• Your instructor uses the a or an before the
  name
• This facilitates ease of reading code
• Resolves conflicts when local variables might
  have the same name
• For example aCelsius and celsius
• The default for IBM Visual Age is acceptable
  (newValue)

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Class Definition

• Testing the temperature example class
• int celsius // declaration
• Temperature temperature // declaration
• temperature new Temperature ( ) //
  instantiation
• temperature.setCelsius (20) // populate
• celsius temperature.getCelsius ( ) //
  retrieve
• Note a default constructor exists called
• Temperature ( )

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Standard Methods

• In addition to the default constructor one may
  create additional constructors
• Constructors allow one to instantiate and
  populate a class in one statement rather that
  having to perform mutations to the attributes
  after instantiation
• One can then instantiate and populate a new
  temperature without having to use a mutator

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Standard Methods

• A Java constructor example (with no parameters)
• private int celsius
• Temperature ( )
• celsius 0
• Constructor has the same name as the class

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Standard Methods

• A Java constructor example (with parameters)
• private int celsius
• Temperature ( int aCelsuis)
• celsius aCelsuis
• This constructor is more convenient as it both
  instantiates and populates

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Standard Methods

• It is standard procedure to build a constructor
  for each class that populates all of its instance
  variables
• If a class has ten instance variables the
  constructor will then have ten arguments
• The exception to this is date instance variables
  which are often set to today's date from the
  system

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Standard Methods

• In addition to constructors there is a toString
• A toString allows one to obtain a printable copy
  of the object for display purposes
• It is standard to always include a toString
• The inherited toString delivers a cryptic string
  so the designer must write their own

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Standard Methods

• A toString converts object to a string
• class Temperature
• private int celsius
• public String toString ( )
• return Integer.toString (celsius) " o
  C"
• Returns "20 0 C" if value was 20

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Standard Methods

• A toString that includes the class name
• class Temperature
• private int celsius
• public String toString ( )
• return this.getClass ( )
  Integer.toString (celsius)
• Returns "package.Temperature 20"

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Standard Methods

• Testing the Java toString exampleString string
• Temperature temperature
• temperature new Temperature (20)
• string temperature.toString ( )
• System.out.println(string)
• System.out.println(temperature) // Gives the
  same answer
• Note that either using the toString explicitly or
  implicitly works equally well

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Static Variables Methods

• A Java static (class) variable example
• class Temperature
• private int celsius
• private static int instanceCount
• Temperature (int aCelsius)
• celsius aCelsius
• instanceCount

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Static Variables Methods

• A Java static (class) method example
• public static int getInstanceCount ( )
• return instanceCount
• Note that there is only one value of
  instanceCount per class even if there are many
  instances
• Note the word static in method declaration

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Static Variables Methods

• Testing the Java static variable example
• int celsius, numberInstances
• Temperature temperature
• temperature new Temperature (20)
• celsius temperature.getCelsius ( )
• numberInstances Temperature.getInstanceCount
  ( )
• Note that the static method is sent to the class
  name

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Cloning Classes

• The ability to copy an object is needed
• This is because Java uses pointers to objects
• For example
• Temperature t1 new Temperature (20)
• Temperature t2 t1
• This results in only one object and both
  variables are referencing it

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Cloning Classes

• What needs to be done is
• Temperature t1 new Temperature (20)
• Temperature t2 (Temperature) t1.clone ( )
• (Temperature) is a cast (a conversion from an
  object to Temperature)
• clone ( ) method must be provided by the
  programmer

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Cloning Classes

• A typical clone method
• public Object clone ( ) throws
  CloneNotSupportedException
• Temperature copy
• copy (Temperature) super.clone ( )
• return copy
• Note that this method overrides the parents
  method

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Cloning Classes

• Notes on cloning
• The actual method should allow for exceptions
  using try and catch
• This, however, is a later topic
• Note that super refers to parent class of Object

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Summary

• Classes are needed for model objects
• Classes can also be used to extend GUI classes
• Or one may develop or extend the java library
• An example of this might be to build a better
  toString for GregorianCalendar

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Summary

• Occasionally static variables and methods are
  needed for such things as counters
• Standard methods are expected in your business
  object model
• Constructors
• Accessors and Mutators
• toString ( )
• clone ( ) // Sometimes

24
Thank You

• A little in the right direction
• can make a big difference.Thank you
• Robert Stumpf
• email rvstumpf_at_csupomona.edu
• url http//www.csupomona.edu/rvstumpf