SOEN 343 Software Design

1
SOEN 343Software Design

• Section H Fall 2006
• Dr Greg Butler
• http//www.cs.concordia.ca/gregb/home/soen343h-f0
  6.html

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Announcements

• Tutorials start this Friday
• Tutorial/Lab location
• Still TBD watch web page
• Additional tutorial slot
• Fridays 1645 to 1735 in H-929

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OO Review

• OO programming
• Java class, interface, static
• Static and dynamic typing
• Polymorphism, delegation
• exceptions
• OO development using UML models

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Java some details to be read on your own.

• Primitive types include
• Floating point float, double.
• Integral byte,short,int,long,char.
• boolean, is not assignment compatible with
  integral types explicit conversion required.
• boolean b (i 0 ? false true)
• Class types for boxing
• Float, Double, Byte, Boolean.
• E.g. Boolean o new Boolean(b)

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java.util.Vector

• Vector class
• Implements a growable array of objects.
• Example of use
• String s
• Vector v new Vector()
• v.add(s) // adds to end
• String t v.elementAt(0)
• v.remove(s)
• int i v.size()

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Java Basics Hello World!

• Illustrates
• How to write the main method to be called when a
  program is invoked.
• Output (console).

• public class HelloWorld
•   public static void main(String args)
•      System.out.println("HelloWorld!")
•   

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Object and Variables Run-time Initialization

• Local variables
• Are not implicitly initialized.
• Must be explicitly initialized.
• Object fields always initialized by default to
• Integral types (including char) 0.
• Floating point types 0.0.
• Reference types null.

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Object Creation and Variable Declarations

• Basic declaration (no object creation)
• Animal a
• null initialized declaration (no object
  creation)
• Animal a null
• Only using new will create (instantiate) objects
• a new Duck()

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Objects and Variables Key Points

• What you should know
• Run-time memory model
• Has two parts Stack, Heap.
• You should know what gets allocated where.
• Object creation and variable declarations.
• Declarations do not cause objects to be created.
• Know the role of null.
• Object creation / instantiation via new.

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Stack and Heap Quiz Question

• Given the following declarations
• int i 2
• int b new int2
• String r
• String s abc
• String t null
• What will be the state of the stack and heap
  after they have all been processed?

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Stack and Heap
Heap
Stack
i b r s t

2 ? null
0,0
abc
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Disjointedness of Primitive and Reference Types

• You cannot assign a primitive type to a variable
  of a reference type.
• You cannot assign a reference to a variable of a
  primitive type.
• The remaining few slides discuss only reference
  types.

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Type Hierarchy

• Every class is a subclass of Object.
• If S is a subclass of T then we can use an
  instance of S where ever a T is expected
• T t new S()
• Object o new S()
• // Do not do this (it is wasteful)
• Object o new String(abc)

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Each Reference Variable Two Types

• Each variable of a reference type has
• Declared type fixed.
• Run-time type can change at run-time.
• Synonyms
• Declared type static type, or apparent type.
• Run-time type dynamic, or actual type.

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Each Variable Two Types, Example

• Object m new Movie()
• m has two types
• Declared type Object.
• Run-time type Movie.

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Type Hierarchy Object, root of all
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Type Hierarchy Supertypes, Subtypes
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Unrelated Sub-hierarchies are Not Compatible (for
assignment, cast).

• Cannot
• Assign, or
• Cast
• A to K

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Declared Type Fixes Bounds

• Declared type fixed, e.g. A.
• Run-time type can change at run-time ...
• within bounds of declared type subhierarchy.

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Type Checking Assignment

• Always done relative to declared type.
• A a some-expression-of-type-X
• Legal? (or will cause a compile-time error?)
• Assignment is legal iff X is
• A,
• Subtype of A.

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Type Casting and Type Checking I

• A a (X) some-expression-of-type-K
• Legal? (will cause a compile-time error?)
• Same answer as given on previous slide because
  this is just a special case of the previous slide.

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OO Basics Static vs. Dynamic Typing

• Why not declare all fields or variables of
  reference types as
• Object
• void (C)
• What would we gain?
• What would we loose?

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Type Casting (in Statically Typed OO Languages)

• Purpose of Casting
• Inform compiler of (assumed) subtype of an
  object.
• Compiler can then perform better type checking.
• Type cast
• Like an assertion, it may fail e.g.
• Object i new Integer(0) String s (String)
  i// ? ClassCastException

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Polymorphism

• poly many
• morphism forms
• How does the meaning of this term apply to OO?
• Run-time type of a given expression can vary.
• Different types our concern
• Subtype polymorphism.

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Number of Legs, Another Solution
Issues?

• class Animal
• protected int numberOfLegs
• public int getNumberOfLegs()
• return numberOfLegs
• class Duck extends Animal
• public Duck() numberOfLegs 2

Best solution?
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Dynamic Dispatching

• Run-time type is only of concern when resolving
  non-static methods.
• Never for fields.
• Not for static methods.

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Overridden Non-static Field

• There are two fields (one in P, one in C).
• Moral do not do this!

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Static Methods

• No overriding
• P.m(), C.m() are distinct methods, both
  accessible.

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Exceptions, An Example Throwing

• public void m(String s)
• throws NullPointerException
• if(s null)
• throw new NullPointerException()
• ...

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Exceptions, An Example Catching

• public void m2()
• String t ...
• try
• m(t)
• catch(NullPointerException e)
• // t was null
• finally
• // code to exec no matter what

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Exception Class Hierarchy

• Two types of exceptions
• Checked
• Unchecked
• Checked
• Must be listed in method declaration.
• Must be caught or run-time error is reported.
• Unchecked
• No such restrictions.

Throwable
Exception
Error
RuntimeException
SampleChecked
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Exceptions

• Both
• methods
• constructors
• can throw exceptions.