Polymorphism and access control

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Polymorphismand access control
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What is polymorphism?

• In general the ability of some concept to take
  on different forms
• In Java the ability to make identical method
  calls invoke different behaviors
• This is one of the fundamental concepts in
  Object-Oriented programming!
• Enables code reuse, encapsulation, etc.

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What is polymorphism?

• We have already seen it more or less
• Can be achieved either through interfaces or
  inheritance (no fundamental difference)
• The principle is the same a method is defined
  in a fundamental class, and several other classes
  can contain different implementations of that
  method

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What is polymorphism?
Animal
makeSound()
Duck
Mouse
makeSound()
makeSound()
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What is polymorphism?

• public class Duck extends Animal
• public void makeSound() print(Quack)
• public class Mouse extends Animal
• public void makeSound() print(Squeak)

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What is polymorphism?

• public void makeAnimals()
• ArrayListltAnimalgt zoo
• zoo new ArrayListltAnimalgt()
• zoo.add(new Duck())
• zoo.add(new Mouse())
• zoo.add(new Mouse())
• zoo.add(new Duck())
• zoo.add(new Mouse())
• makeNoise(zoo)

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What is polymorphism?

• public void makeNoise(ArrayListltAnimalgt zoo)
• for (Animal a zoo)
• a.makeSound()
• Output
• Quack
• Squeak
• Squeak
• Quack
• Squeak

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What is polymorphism?

• A bit strange, when you think of it
• The method makeNoise knows nothing at all about
  the subclasses
• but still, the subclass-specific methods are
  invoked!
• How is that possible?

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What is polymorphism?

• In Java, method calls are always determined by
  the type of the actual object, not the type of
  the object reference
• This is polymorphism

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What is polymorphism?

• The method makeNoise will still work, even if we
  add new subclasses! (closed for modification,
  open for extension)
• It is the Java Virtual Machine (JVM), which picks
  the correct method to call
• The JVM knows the true type of any object
• This principle is know as late binding

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Choosing methods

• Multiple versions of the same method can also be
  created through overloading
• println(String output)
• println(int output)
• However, method signatures are different
• Compiler can pick correct method at compile-time,
  this is early binding

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Abstract classes

• An interface class only provides method
  definitions, no implementation at all
• A super-class provides an implementation for all
  its methods, which can be over-written or used
  as-is
• Not the entire truth
• We can choose only to provide an imple-mentation
  for some of the methods

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Abstract classes

• public class DefaultShape
• public void draw() // do nothing
• public double getArea() return 0.0

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Abstract classes

• public abstract class DefaultShape
• public void draw() // do nothing
• public abstract double getArea()
• This is known as an abstract class
• getArea is an abstract method

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Abstract classes

• You can never create an instance of an abstract
  class (just like interface)
• You can have a variable of this type
• You force the user to provide an implementation
  of the abstract methods

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Abstract classes

• Is an abstract class just an interface?
• No, because they can have
• Instance fields
• Methods with implementation
• Constructors
• Why constructors?
• Cannot be called by a user, but
• can be called from a sub-class

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Preventing subclassing

• Maybe we do not want a user to be able to create
  a sub-class, or override a method
• Could be for security reasons, etc.
• We can prevent this by declaring a class or
  method as being final

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Preventing subclassing

• public class SecureAccount extends BankAccount
• ...
• public final boolean checkPassword(String pwd)
• ...
• public final class String ...

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Access control

• Why could we not do this?
• // Overridden method in CheckingAccount
• public void deposit(double amount)
• balance balance amount
• transactionCount

private!
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Access control

• Even a sub-class does not gain access to private
  methods or instance fields
• Private really means private!
• Sub-classes must use get/set-methods, just like
  any other user
• Not specifying public or private will provide
  access at package level

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Access control

• public class Person
• // Access
• private String name // class only
• public int height // everybody
• int weight // all in package
• Unless a very good reason exists, stick to public
  and private

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The Object class

• The system class Object is a superclass of all
  classes in Java!
• Polymorphism to the extreme!?
• Which methods could be meaningful to all Java
  classes?

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The Object class

• Not many
• String toString() returns a string
  representation of the object
• boolean equals(Object other) tests if the
  object equals another object
• Object clone() makes a full copy of the object

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The Object class

• String toString()
• Mostly used for debugging
• If not overwritten, outputs the class name plus
  an object identifier (memory location)
• BankAccount_at_a122c09
• Usually overwritten to print out the values of
  the instance fields, i.e. the state of the object
• Subclasses can print their own part

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The Object class

• boolean equals(Object other)
• Often useful to know if two objects are equal
• Equal by content, not by reference!
• We must first cast argument to correct class,
  then check if instance fields are equal
• NOTE For instance fields of non-primitive types,
  we need to call their equals(...) methods

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The Object class

• Object clone()
• Useful, if we want a real copy of an object, not
  just a copy of the reference to the object
• Clone() method should return a new object, with
  the same state as the given object
• Not trivial to clone an object, if the object
  contains references to other objects
• Can make a shallow copy or deep copy

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The Object class
Original
Copy
Deep copy
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Copy
Copy
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Copy
Shallow copy
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