CS 303E Class 6 Classes and Class Behavior

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CS 303E Class 6Classes and Class Behavior
Don't know much geography Don't know much
trigonometry Don't know much about algebra Don't
know what a slide rule is for -Sam Cooke
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Object Behavior

• The book uses the example of a bank account
  class.
• READ IT!! Very good.
• In class we will use a time class so you have two
  different examples to look at.
• The time class is contained on the first page of
  notes.
• The class is used to represent times of the day.
• It is not complete only a start

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The Time Class

• Represents a time of day to nearest minute.
• What can we do with a time of day or time object
• increment it to the next minute
• increment it by some specified amount
• find out how many minutes have passed since
• get a String version of the time
• Not a complete list. Only a starting point.
  Much more could be added.

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The TimeTester Class

• A driver program to exercise the capabilities of
  the Time class.
• Only creates one object but could create many
• Time timeOne new Time()
• Once the object created methods may be called to
  manipulate the object or find out about it.
• timeOne.setTime(1000)

Time Object
timeOne
1000
myMinutes
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Classes

• Class
• are factories used to create objects
• specify the methods you can use for objects that
  belong to the class
• contain the implementation detail. The data
  layout for objects and the code for methods.
• Hold static methods, objects, and primitives.
• Today we will look at the 3rd item, the
  implementation details

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The main Method

• Up till now the only method has been main
• In Java programs there are normally many classes,
  but only one of them has the a main method.
• main is where the program starts to execute.
• The vast majority of classes do not have a main
  method.

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Other Methods

• Object behavior is defined by the method in the
  class that created the object
• Method headers contain the following parts
• access specifier (public, private, or protected)
• the return type of the method(int, double,
  String, etc., void)
• the name of the method (all lower case except
  internal words)
• A list of parameters the method requires
  (parameters receive the arguments sent when the
  method was called or executed)

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Method Implementation

• Inside the method is the code that carries out
  the function or job of the method
• return is a keyword which causes the method to
  end and send back whatever data (object variable
  or primitive variable) that follows the word
  return.
• the data type after return must match the return
  type in the method header
• return in usually the last statement in the
  method unless the return type is void.

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Instance Variables

• Instance Variables are contained inside objects
  and are used to describe the status or state of
  the object.
• Instance variables are defined with
• an access modifier (private, public, or
  protected)
• the data type of the variable (double, int,
  String, etc.)
• The name of the variable (descriptive name, all
  lowercase except capital letters at the start of
  internal words.)

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More on Instance Variables

• Instance variables are declared inside the class,
  but outside all of the class methods.
• Instance variables are almost always private.
• The variable can only be seen or used by the
  methods inside the class, not by any other
  classes.
• An objects instance variables may only be changed
  by calling methods on the object.
• Instance variables may be object variables or
  primitive variables
• There is no limit on the number of instance
  variables a class / object may have.

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Parameters

• Parameters receive the information and data sent
  via arguments.
• The arguments and parameters must match up in
  number, type, and order
• Otherwise a syntax error or logic error can
  occur.
• The parameters listed in the method header are
  the explicit parameters
• The object the method was called on is sent as an
  implicit parameter.

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Calling Methods

• In TimeTester
• timeOne.increment(10)
• In Time
• public void increment(int minutesToAdd)

minutesToAdd
10
this
Time Object
1000
myMinutes
timeOne