Inheritance

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Inheritance
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Extending Classes

• Its possible to create a class by using another
  as a starting point
• i.e. Start with the original class then add
  methods, change what methods do, etc.
• Allows reuse
• Can extend a class in several ways for different
  purposes
• based on the same original code

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Inheritance

• Inheritance allows a software developer to derive
  a new class from an existing one
• The existing class is called the parent class, or
  superclass, or base class
• The derived class is called the child class or
  subclass
• As the name implies, the child inherits
  characteristics of the parent
• That is, the child class inherits the methods and
  data defined by the parent class

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Inheritance

• Inheritance relationships are shown in a UML
  class diagram using a solid arrow with an
  unfilled triangular arrowhead pointing to the
  parent class

• Proper inheritance creates an is-a relationship,
  meaning the child is a more specific version of
  the parent

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Example

• In the Blackjack design example, we had a Player
  class and a Dealer class
• similar classes
• each has a hand, takes turns,
• We could define a parent class Person that can
  then be extended to create appropriate subclasses

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Inheriting

• Suppose we have a simple class A
• class A
• private int count
• public int method()
• return 1
• And we want to define an extended class B with
  another method

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Inheriting

• Now B can inherit A and add a new method
• class B extends A
• public int newMethod()
• return 2
• B now has everything from the definitions of both
  A and B

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Inheriting

• Using A and B
• A myA new A()
• B myB new B()
• System.out.println(myA.method())
• System.out.println(myB.method())
• System.out.println(myB.newMethod())
• Output 112
• An instance of B also has all members from A

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Parents and Privacy

• Subclasses cannot access private members
• class B extends A
• .
• public void setCount(int count)
• this.count count
• ERROR count is private in A this is not A
• Just like any other code cant access private
  members
• We dont want count to be public

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The protected Modifier

• Encapsulation says that members should not be
  public
• But we want access to inherited members
• New visibibility modifer protected

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The protected Modifier

• A protected variable can be accessed from all
  code in the same package
• It provides more encapsulation than public
  visibility, but is not as tightly encapsulated as
  private visibility
• Key point Protected variables can be accessed
  from all subclasses of a given class
• In UML, use symbol to mean protected

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Example from Text
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Constructors

• Constructors are not inherited, but can be added
  in the usual way
• class A
• public A()
• count 1
• class B
• public B(int n)
• setCount(n)

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The super Reference

• Sometimes, we want to access something in the
  super class
• For this, we use the super identifier
• Similar to this, the meaning of super depends on
  the context
• Using super gives access to the methods of the
  super class
• This is particularly useful in constructors

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The super Reference

• A childs constructor is responsible for calling
  the parents constructor
• The first line of a childs constructor should
  use the super reference to call the parents
  constructor
• Why? To make sure the parent-part is
  initialized
• This happens whether you like it or not
• By default, super() is called automatically

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Constructors (again)

• class A
• public A()
• count 1
• class B
• public B(int n)
• ltsuper() is automatically called heregt
• setCount(n)

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Constructors (a more useful example)

• class A
• public A(int n)
• count n
• class B
• public B()
• super(1)

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Overriding Methods

• A child class can override the definition of an
  inherited method in favor of its own
• The new method must have the same signature as
  the parent's method, but can have a different
  body
• The type of the object executing the method
  determines which version of the method is invoked

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Overriding Example

• class Bird
• boolean flying
• void flapWings()
• flying true

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Overriding Example

• class Penguin extends Bird
• void flapWings()
• return

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Accessing Parent Methods

• Sometimes you may want to access the definition
  of a method from the parent class
• You can use super to do this
• public void method(int x)
• // can put extra stuff here
• super.method(x)

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Overloading vs. Overriding

• Overloading deals with multiple methods with the
  same name in the same class, but with different
  signatures
• Overriding deals with two methods, one in a
  parent class and one in a child class, that have
  the same signature
• Overloading lets you define a similar operation
  in different ways for different parameters
• Overriding lets you define a similar operation in
  different ways for different object types

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Single Inheritance

• Java supports single inheritance, meaning that a
  derived class can have only one parent class
• Java does not support multiple inheritance
• e.g. a motorhome is a vehicle and a house
• We use interfaces to fake multiple inheritance

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The Object Class

• Single inheritance means that we can think of all
  classes as a single family tree
• The built-in class Object is the root of this
  tree
• A class that doesnt explicitly extend anything
  has Object as its parent class
• i.e. these are equivalent
• class Foo class Foo extends Object
• Object contains some methods (toString, equals)
  that are used unless overridden

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Built-in Classes

• Its also possible to inherit from built-in
  classes
• Even if you dont know the original
  implementation, can still add new methods
• e.g. list with extra data (ArrayList with
  additional instance variables) Scanner than can
  also read in a custom type .

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When to Inherit

• Inheritance is a powerful tool in OO design
• Easy reuse of code works naturally with problems
  that have a hierarchy of objects
• Dont confuse inheritance with aggregation
• A should extend B only if A is-a B or A is a
  more specific version of B or A can be
  subsituted for B
• not just because you need to use the parts of B

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Example Shapes

• Suppose we want to represent a collection of
  shapes (in a drawing program)
• square, rectangle, circle (maybe others later)
• All of these have some things in common
• position the x-y coordinates of the shape
• getters and setters for the position
• translate(x,y) move the shape by this much
• Create a class Shape that implements these

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Example Shapes

• Each of these will inherit Shape
• Rectangle
• add instance variables height and width
• Square
• inherit Rectangle and ensure heightwidth
• Circle
• add radius, interpret position as the centre

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Example Shapes

• The class definitions
• class Shape
• class Rectangle extends Shape
• class Square extends Rectangle
• class Circle extends Shape
• Note this is not right
• class Rectangle extends Square
• A square is a certain kind of rectangle, and this
  should be evident in your code

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Example Shapes (partial) UML
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Abstract Classes

• Not every class can be instantiated
• use reserved word abstract to indicate that a
  class cannot be instantiated
• used for place-holders in hierarchy
• make contain abstract methods
• e.g. Shape
• Abstract classes are just for code reuse, and for
  designing a nice class hierarchy

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Using final

• Methods may be marked final
• public final myMethod()
• cannot be overridden
• forces a method to have fixed behaviour
• Classes may be marked final
• public final class MyClass
• cannot be inherited
• forces all methods to have fixed behaviour