CIS 234: More on Inheritance

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CIS 234 More on Inheritance

• Dr. Ralph D. Westfall
• May, 2010

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Sample Code, Etc.

• use code in the Animal.zip file for demonstrating
  this in Eclipse
• see Eclipse Tips (Debugging) for how to
  configure Eclipse to Step Into different classes

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Constructors Review

• a constructor is a method that runs when an
  object is created from a class
• constructor has same name as the class
• constructors can have arguments
• constructor may initialize the object with
  variable values and/or execute other code
• a class can have more than 1 constructor, to
  initialize more/different values

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Constructors Review - 2

• public class MyClass
• private int myVariable
• MyClass(int number) // same name
• myVariable number)
• // initializing with 1 argument
• MyClass(int num, boolean ok)
• // same name, different arguments

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Subclasses and Constructors

• a parent and a child class can each have their
  own constructors
• both will run when a subclass object is created
• can use subclass constructors to add additional
  variables that only the subclass needs

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Superclass Arguments

• if a superclass constructor needs arguments, the
  subclass must have a constructor (no longer
  optional)
• subclass constructor must include a call to
  superclass constructor with its arguments
• call to superclass constructor must be the very
  1st code in the subclass constructor
• use keyword super, not superclass name
• super(arg1, etc.)

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Superclass Arguments - 2

• public class OtherClass extends MyClass
• OtherClass(int num)
• super(num)
• // num goes to MyClass constructor

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Calling Superclass Methods

• keyword super is a synonym for the actual name of
  the superclass
• super(arg1, etc.) is same as calling the
  superclass constructor method by its name
• super. (note . in front of method name) calls
  that method in the superclass
• super.toString()
• only need this if toString is in both classes

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Access Modifiers Review

• modifiers control which classes can use data and
  methods in another class
• public any class can use
• protected subclasses can use
• private no other class can use
• "friendly" is default if none of others used
• can be used by classes that have been identified
  as being in same "package"

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Information Hiding

• data in a class should only be accessible to
  other classes via methods
• avoids bugs other programs can't change data
  values in unexpected ways, or use data before it
  is ready to use
• declare variables as private
• programmer chooses which data to make accessible
  by providing public get and set methods

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Information Hiding - 2

• private data fields are not directly accessible
  within subclasses
• two ways to make private superclass data
  accessible to its subclasses
• declare as protected, rather than private
• use get or set methods in superclass that have
  been declared as public

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Using Parent Class Methods

• a subclass can access public methods in parent
  class (e.g., gets and sets)
• subclass can't directly access private methods in
  parent class
• but can access public methods in the parent
  class, which can then call its own private methods

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Overriding Methods

• method in subclass with same name can override
  some superclass methods
• can't override private methods in a superclass
  (get compiler error message)

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Overriding Methods - 2

• can put super. (super dot) in front of a method
  name to call that method in the parent instead of
  method in the subclass with the same name
• not always necessary but is a good idea
• super.myClassMethod() // runs in parent
• myClassMethod() // overrides if method
• // is in subclass

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Review

• A constructor is a method. T or F?
• The constructors name is different from the
  class name. T or F?
• A class can only have 1 constructor. T or F?
• Constructors always receive arguments. T or F?

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Review - 2

• What is a superclass?
• If all superclass constructors need to have
  arguments, what is necessary in the subclass
  constructors?
• What key word is used in a subclass to run
  methods in a superclass?
• Name one access modifier key word.

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Review - 3

• Which is more often declared as public methods
  or properties (variables)?
• Why?
• What does overriding mean?