Classes and Objects

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Classes and Objects

• CSCI-1302

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Remember

• We covered reference types in the last few
  lectures.
• Objects, Strings, and Arrays are reference types.
• Now, we will be narrowing our focus on Objects,
  Classes, and Object-oriented programming.

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Object Oriented Principles

• Should know the following terms and definitions
  when working with Java
• Information hiding
• Encapsulation
• Generic Code
• Inheritance (Chapter 4)
• Polymorphism (Chapter 4)

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Objects

• Objects are reference types!
• A constructor must be called to create a new
  object. i.e. Allocate space in memory and
  assign an address to the variable.
• An object is an Instance of a given class.
• Objects have operations that may access or
  manipulate its state.

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Objects

• Objects have state and behavior.
• State - the variables that describe the object
• The label in a button object describes the state
• The x y coordinates describe state for Points.
• The behavior is the methods that allow actions to
  be performed on the object.
• This can be to get/set the state or any other
  action on the object. (setLabel for a button)

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Objects

• Methods operate on an object's internal state
• An object is an atomic unit Its parts cannot be
  dissected by general users.
• The user should not know what is inside of an
  object!

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Objects
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Encapsulation / Information Hiding

• Encapsulation and information hiding are two
  separate, orthogonal concepts
• Information hiding conceals how an object
  implements its functionality.
• Think of private data members (more on this
  later)
• Accessed indirectly by supplied methods
• How is a StringTokenizer object implemented?
  What data does it contain? Do we know?

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Encapsulation / Information Hiding

• Encapsulation ensures that the behavior of an
  object can only be affected through the object's
  methods.
• How is this accomplished?
• Think about this.
• More on this later in the class!

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Encapsulation / Information Hiding

• Information hiding lets one build higher level
  abstractions on top of lower level details.
• Ignore details of the system that are unrelated
  to the task at hand.
• Ex. - Use a Button object without worrying how it
  is created!

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Objects

• Try to compare objects to real-world objects.
  What is their state?
• Objects vary in the difficulty of their state.
• Dog object - name, color, breed, hungry
• Bicycle object - current gear, current pedal
  cadence, current speed
• Lamp object on/off

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Objects

• How do we create a new type of object, like a
  bike?
• First, we have to have a class.
• Then we instantiate an object of that class.
• By calling the constructor (Using new).

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Classes

• In object-oriented terms, we say that your
  bicycle is an instance of the class of objects
  known as bicycles.
• Since an object is one instance of a class, it
  has specific values for its members!
• A class is the blueprint from which individual
  objects are created.
• Bike object w/ specific values for its variables ?

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Bicycle Object
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Bicycle Class

• The Bicycle class had no constructors
• What is the default?
• Bad practice to set values for data members
  outside of the constructor
• The Bicycle class had no access modifiers.
• What is the default?

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Bicycle Class

• The Bicycle class does not contain a main method.
  
• It's not a complete application
• it's just the blueprint for bicycles that might
  be used in an application.
• Created and used by some other class.

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Common Methods for Classes (not in bike class)

• Constructors
• Why can we have multiple versions?
• Mutators modify the data members
• Accessors return the data members
• Must be careful with these! Remember
  Encapsulation!

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Common Methods for Classes (not in bike class)

• equals
• Should be included to test 2 objects of the same
  type for equality!
• Equality objects contain same information
• How would we write this for strings??
• toString turns the object into a string and
  returns this string
• Automatically called with println(objectname).

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Class Variables

• A class contains different Variable types
• Data Members (fields)
• Local Variables (within a method)
• Parameters

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Access Modifiers

• Data Members (fields) and Methods can be either
  Public, Private or Protected.
• public modifier-accessible from all classes.
• private modifier-accessible only within its own
  class.
• protected modifier-discussed later

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Access Modifiers

• In the spirit of encapsulation, it is common to
  make fields private. This means that they can
  only be directly accessed from the Bicycle class.
  We still need access to these values, however.
  This can be done indirectly by adding public
  methods that obtain the field values for us

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Access Modifiers

• Can we obtain access to a private field if we are
  not careful coding?
• Think about this reference types.
• What would a get method do with a reference type?
• How should we avoid the problem?

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Examples from Book

• IntCell class
• Date Class
• Ticket Class

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