L02 Software Design

Agenda
 Programming with Objects
– Classes
– Interfaces
– Generic programming
– Reflection
 Software Design
– Ducks…

Reading
 Don’t Repeat Yourself
 Polymorphism
 Separation of concerns
 Loose coupling
 Optional:
– http://docs.oracle.com/javase/tutorial/

Object Oriented Programming
 Object Oriented programming can be powerful
– One of the best ways for general purpose computing
 But
– The power of object oriented languages needs to be
used properly!
 However
– Programmers tend to forget the power of OO

Object Oriented Programming
 Programming languages with objects
– Objects hold data and methods
– Object variables (reference) point to objects
 Object Oriented
– Object are instances of classes, created with new
– Classes describe the objects
– Classes extend other classes – inheritance
– Instance variables are encapsulated
– Methods manipulate instance variable

Explain these concepts and why they are important in programming
Encapsulation
Interfaces
Polymorphism
EXERCISE

Think About This!
 Object-oriented programming
– Is not about class inheritance and creating advanced
class diagrams
 Remember
– Encapsulation – Hiding data
– Interfaces – Hiding implementation
– Polymorphism – Flexible and Generic Programming
 Powerful programming
– Separation of concerns
– Separating what varies from what stays the same

Separate Variations Design Principle
Identify the aspects of
your application that
vary and separate
them from what stays the
same

Don’t Repeat Yourself – DRY
 Single source of truth
Every piece of knowledge
must have a
single, unambiguous, authorit
ative representation within a
system

Object Oriented Design
 Design of classes and interfaces
– Class diagram shows relationships
– Sequence diagrams show flows
 Design Patterns
– Reoccurring solutions in design
– “Best practices” – known solutions for common
problems

Objects
 Object created
 Object used
Date day1;
Date day2 = new Date();
System.out.println (new Date ());
String s = new Date().toString ();
day1 null
day2
Date
Date now = new Date();
if (day2.before(now))
{
System.out.println (day2.toString());
}
before
tostring
...

Objects
 Operator new creates
memory
– String is an exception
 Reference variables
always point to some memory
day1 null
day2
Date
before
tostring
...
Date d = new Date();
Must be a concrete
class
Can be any supertype or
interface of the concrete class

Classes
Object
+getId() : String
-id : int
-name : String
-username : String
-password : String
-email : String
User
 Classes extend other classes
– Concrete Inheritance
– Subtype extends supertype
 Class contains
– Instance variables
– Methods
 Reference variables of type
Object points to any class
– Any supertype can reference subtype
Object obj = new User ();
User u = (User)obj;
supertype
subtype

Class methods
 Methods can be overridden
– Class extends a class and overrides a method
 Methods can be overloaded
– Same method with different parameters
 Methods can be
– public – any class can call the method
– private – only available within the class
– protected – only available within the class and
extended classes

Class A inherits class B. A overwrites method f. Variable b is created like
this:
B b = new A();
What happens when this line is run:
b.f();
A) The method in A is run
B) The method in B is run
C) First the method in B is run, then the method in A
D) The new statement is illegal and does not compile
QUIZ
✔

Classes
 References
– Point to concrete objects
– Must be same type or supertype of concrete object
– Can be interface or abstract class
Object obj = new Date ();
Date d = (Date)obj;

Constructors
 Classes have constructor
– Instantiation methods
– Same name as the class
 References
– this – refers to the class
– super – extended class
public Employee (String name,
double salary)
{
this (name);
this.salary = salary;
}
public Manager (String name)
{
super (name, 0.0);
...
}

class Point
{
private int x, y;
public Point ()
{
x = 0; y = 0;
}
public Point (int x, int y)
{
this.x = x; this.y = y;
}
public void move (intdx, intdy)
{
x+=dx;y+=dy;
}
public String toString ()
{
return "(" + x + "," + y + ")";
}
this used to refer to the
class variables
Override toString
method of Object
Class variables
Default constructors
Overloaded constructor
Example

Example
Bla
public class Test
{
public static void main (String[] args)
{
System.out.println ("Test");
Test test = new Test();
}
public Test ()
{
Point p0; // null reference
Point p1 = new Point ();
Point p2 = new Point (1,2);
Object obj = p2;
p0 = (Point)obj;
p0.move (1, 1);
System.out.println("p0=" + p0);
}
}
C:java>javac Test.java
C:java>java Test
Test
p0=(2,3)
X = 2
Y = 3

Java uses this method to pass objects to methods
A) Call by reference
B) Call by value
C) Call by object reference
D) Call by value reference
QUIZ
✔

Call By Value
 Methods use call by value
– Object references are passed by value
– The reference cannot change, but the object can
x:
y:
42
0
Point
void changeReferance(Point p)
{
while (p.x>0) p.x--;
}
p
Point p = new Point (42,0);
changeReferance(p);
System.out.println(p.x);
Reference p to Point
Local copy of a reference
p to Point
p is same as this.p
p

Inheritance
 Classes extend other classes
– Subclasses extend superclasses
 Reference variables of super types can
reference objects of subtypes
Employee
Manager
Empolyee e;
e = new Employee(. . .)
e = new Manager(. . .)
class Manager extends Employee
{ ... }
Polymorphism

public class Employee
{
private String name;
private double salary;
private Date hireDate;
public Employee()
{
}
public Employee(String name, double salary, Date hireDate)
{
this.name = name;
this.salary = salary;
this.hireDate = hireDate;
}
public Employee(String name)
{
this.name = name;
}

public String getName()
{
return name;
}
public double getSalary()
{
return salary;
}
public void setName(String name)
{
this.name = name;
}
public String toString()
{
return "Employee: " + getName();
}
}

Bla
{
String title;
double bonus;
public Manager (String name, String title)
{
super (name);
this.title = title;
}
public String getTitle ()
{
return title;
}
public double getSalary()
{
return this.bonus + super.getSalery();
}
public String toString ()
{
return "Manager: " + getName() +
", " + getTitle ();
}
}
New method
New variables
Overridden
methods

What does this program print?
public class Test2
{
public static void main (String[] args)
{
System.out.println ("Test2");
Test2 test2 = new Test2();
}
public Test2 ()
{
Employee e0 = new Employee ("Dilbert");
Employee e1 = new Manager ("Pointy Haired", "Boss");
System.out.println("e0: " + e0);
System.out.println("e1: " + e1);
}
}
C:java>java Test2
Test2
e0: Employee: Dilbert
e1: Manager: Pointy Haired, Boss
EXERCISE

Dynamic binding
 Decision on which method to run is taken at
runtime
– The virtual machine uses a method table for each
class
Manager m = new Manager();
m.setName(“P.H. Carl”); // Employee.setName
m.setTitle (“Boss”); // Manager.setTitle
m.getSalary (); // Manager.getSalary
Employee e1 = new Manager("Pointy Haired", "Boss");
e1.getSalary();
Is this manager
salary with bonus?

What does this program print?
public class Test1
{
public static void main(String[] args)
{
System.out.println("Test1");
new Test1();
}
public Test1()
{
Employee e0 = new Employee ("Dilbert");
Employee e1 = new Manager ("Pointy", "Boss");
System.out.println(e1.getTitle();
}
} Trick question!
Does not compile since getTitle is not in Employee
EXERCISE

Think About This!
 Why use Concrete Inheritance?
– Powerful implementation approach
– Layered Supertype Pattern
– Enables polymorphism if supertypes are used
– New classes can be added without recompile
 But remember
– Object oriented programming is not just about
concrete inheritance
– It has to be natural!
– Class hierarchies are rigid
– Not always good to force others to extend

Abstract Classes
 Abstract classes put the responsibility of
implementation on subclasses
– Classes extending an abstract class must implement
the abstract methods
– Can contain both concrete and abstract methods
– Normal classes are concrete classes
 Abstract classes cannot be instantiated
 Reference variables of abstract types are
allowed
– Object must be a concrete class

Abstract Example
abstract class Person
{
private String name;
public Person(String name)
{
this.name = name;
}
// get and set methods ...
public abstract String getDescription ();
}
class Employee extends Person
{
public String getDescription()
{
return "Employee called " + super.getName();
}
}
// Person p1 = new Person (); Does not work!
Person p2;
Person p3 = new Employee ("Dilbert");
System.out.println (p3.getDescription());
Key Concept: Polymorphism

Interfaces
 Interface is a class without implementation
– Declaration of how to implement class
– All methods and variables are static final
 Classes implement interfaces
– implements keyword
– Must implement all the methods – or be abstract

Interfaces
public interface Comparable
{
public int compareTo(Object other);
}
class Employee extends Person implements Comparable
{
public int compareTo(Object o)
{ ...

Example
public interface Comparable
{
public int compareTo(Object other);
}
{
public int compareTo(Object o)
{
Employee e = (Employee)o;
return this.getName().compareTo (e.getName());
} ...
Employee[] ale = new Employee[3];
ale[0] = new Employee ("Dilbert");
ale[1] = new Employee ("Wally");
ale[2] = new Employee ("Alice");
Arrays.sort(ale);
for (int j=0; j <ale.length; j++)
System.out.println(ale[j].getName());
Alice
Dilbert
Wally
How is this
possible?

Think About This!
 Class A calls class B -> A depends on B
 Class java.util.Arrays calls the
Employee.compareTo method
 Does Arrays depend on Employee?
Polymorphism
Separated interface

Many Faces
 Arrays looks at the class as Comparable, while
we regard it as Employee
Class Employee
implements Comparable
{
...
compare
Class Arrays
...
sort(Object[]
{
Comparable c ...
Test...
Arrays.sort(persons
Arrays does NOT call Employee.compareTo,
it calls Comaparable.compareTo
which happens to be Employee Polymorphism

Objects
class Employee extends Person
abstract class Person implements Comparable
Object o = new Manager()
Person p = new Manager()
Comparable c = new Manager()
Employee e = new Manager()
Manager m = new Manager()
Memory for Manager
reference
Must be
concrete class
Can be any
supertype

Using Interfaces
 Interfaces cannot be instantiated
– Variables of interface types can reference objects that
implement the interface
 Interface can extend interfaces
public interface Powered extends Movable
{
double milesPerGallon();
double SPEED_LIMIT = 95;
}
Comarable c = new Comparable (); // NO!!!
Comarable c = new Employee (); // OK!

Why interfaces?
 Why not use abstract classes?
– Only one class can be extended
– Class hierarchies are rigid and not always suitable
 Interfaces can improve software design
– Provide abstraction – hide the implementation
– Classes that use interfaces are not dependant on a
particular implementation
{
...

Example Pattern
 Table Data Gateway or Data Access Object
provide an interface to database table
– Decision on what database access methods to use
can be configured
 Example
public interface TeamDAO extends RuDAO
{
public void addTeam (Team team);
public Collection getTeams ();
} Interface
Implementation
Client
code

Example: Drawing system
public interface Drawable
{
public void draw ();
}
public class Rectangle extends Shape
{
private int h, w;
public Rectangle (int x, int y, int h, int w)
{
this.x=x; this.y=y; this.h=h; this.w=w;
}
public void draw ()
{
System.out.println ("Rectange (x="+x+",y="+y+",h="+h+",w="+w+")");
}
}
public abstract class Shape implements Drawable
{
protected int x,y;
}
public class Circle extends Shape
{
private int r;
public Circle(int x, int y, int r)
{
this.x = x; this.y = y; this.r = r;
}
public void draw()
{
System.out.println ("Circle (x="+x+",y="+y+",r="+r+")");
}
}

Example: Drawing system
 Drawing all objects
– All draw objects implement Drawable
public DrawTest()
{
List<Drawable> l = new ArrayList<Drawable>();
l.add(new Rectangle(1, 1, 1, 1));
l.add(new Circle(2, 1, 1));
l.add(new Rectangle(8, 4, 1, 1));
for (Drawable d: l)
{
d.draw();
}
}
Rectange (x=1,y=1,h=1,w=1)
Circle (x=2,y=1,r=1)
Rectange (x=8,y=4,h=1,w=1)

Think About This!
 All drawing objects in this Layer extend Shape
 Shape is abstract and implements Drawable
 Client code does not know about the classes
that implement Drawable
Shape is Layered Supertype
Shape is Template Method
Generic Programming

X extends Y. Which is true?
A) Correct if and only if X is a class and Y is an interface
B) Correct if and only if X is an interface and Y is a class
C) Correct if X and Y are either both classes or both interfaces
D) Correct for all combinations of X and Y being classes and/or interfaces.
QUIZ
✔

Generic Programming
 Programming in an data type independent way
– Same code is used regardless of the data type
 Example
– Sort can be applied to any data type
– Generic collection
• Java Collection Framework
 Design Principle
– Always use the most generic data type possible

Generic Programming
 All classes extend Object
– Allows generic algorithms and data structures
static int find (Object[] a, Object key)
{
int i;
for (i=0;i<a.length;i++)
if (a[i].equals(key)) return i;
return -1;
}
Employee[] staff = new Employee[10];
Employee e1 = new Employee("Dilbert");
staff[x] = e1;
int n = find(staff, e1);

Generic Programming
 Generic collections
– ArrayList is an example class that uses Object
ArrayList al = new ArrayList();
al.add (new Employee ("Dilbert"));
al.add (new Employee ("Wally"));
al.add (new Employee ("Alice"));
Iterator i = al.iterator();
Employee e;
while (i.hasNext())
{
e = (Employee)i.next();
System.out.println(e.getName());
}
Dilbert
Wally
Alice

Generic Programming
 Generic collections
– The Collections class is another example
List<Employee> list = new ArrayList<Employee>();
list.add (new Employee ("Dilbert"));
list.add (new Employee ("Wally"));
list.add (new Employee ("Alice"));
Collections.sort(list);
for (Employee e: list)
{
System.out.println(e);
}
Alice
Dilbert
Wally

Reflection
 Reflection allows examination and manipulation of
objects at runtime
– Get information about a class
• Fields, methods, constructors, and super classes
• Constants and method declarations belong to an interface
– Create an instance of a class whose name is not known
until runtime
– Get and set the value of an object's field, even if the field
name is unknown to your program until runtime
– Invoke a method on an object, even if the method is not
known until runtime

Reflection
static void showMethods(Object o)
{
Class c = o.getClass();
Method[] theMethods = c.getMethods();
for (int i = 0; i < theMethods.length; i++) {
String methodString = theMethods[i].getName();
System.out.println("Name: " + methodString);
String returnString = theMethods[i].getReturnType().getName();
System.out.println(" Return Type: " + returnString);
Class[] parameterTypes = theMethods[i].getParameterTypes();
System.out.print(" Parameter Types:");
for (int k = 0; k < parameterTypes.length; k ++)
{
String parameterString = parameterTypes[k].getName();
System.out.print(" " + parameterString);
}
System.out.println();
}
} }

Reflection
Bla
public class ReflectMethods
{
public static void main(String[] args)
{
Polygon p = new Polygon();
showMethods(p);
} Name: getBoundingBox
Return Type: java.awt.Rectangle
Parameter Types:
Name: contains
Return Type: boolean
Parameter Types: java.awt.geom.Point2D
...
Name: toString
Return Type: java.lang.String
Parameter Types:

Reflection
 Reflection is very useful in frameworks
– Infrastructure code
– “plumbing” – The “Noise”
 Examples
– Create Java objects from XML descriptions
– Load classes at runtime and invoke methods
– Tools and utilities for development

Dynamically Loading Classes
 Classes can be dynamically loaded at runtime
– Offers the flexibility to decide which class to run
dynamically
– Class names can be specified in configuration files
 Class class
Class instanceClass = Class.forName("RssFeedReader");
reader = (FeedReader)instanceClass.newInstance();

A) BD
B) DB
C) BDC
D) Compilation fails
QUIZ
✔
class Top {
public Top(String s) {
System.out.print("B");
} }
public class Bottom2 extends Top {
public Bottom2(String s) {
System.out.print("D");
}
public static void main(String [] args) {
new Bottom2("C");
System.out.println(" ");
} }

 Design and implementation of software needs to
be of quality
– Badly designed, well implemented = problem!
– Well designed, badly implemented = problem!
CODE
HORROR!!
CODE HORROR DUDE

 Good design
Is based on OO principles
Abstracts complex APIs such as J2EE
Is flexible and can be changed
Contains loosely coupled components

 Example from Head First Design Patterns

Getting Started
 SimUDuck is highly successful duck pond
simulation game
 Original design

Change Request
 But now we need the ducks to FLY

Problem!
 But not all duck fly – We forgot Rubber Duck!

How can we fix this?
 Just override fly and quack to do nothing

We even think ahead
 We fix all non-flyable and non-quackable ducks
as well
Code smell!

Which of the following are disadvantages of using inheritance to
provide Duck behavior?
A) Code is duplicated across subclasses
B) Runtime behavior changes are difficult
C) We can’t make ducks dance
D) Hard to gain knowledge of all duck behaviors
E) Ducks can’t fly and quack at the same time
F) Changes can unitentionally affect other ducks
QUIZ
✔
✔
✔
✔

The Problem
 The problem is this
– Derived classes (RubberDuck) are forced to inherit
behaviour they don’t have
– Derived classes (RubberDuck) needs to be exposed
to the inner workings of the superclass (Duck)
– Users of the base class (Duck) should expect same
functionality
– Violation of the Liskov Substitution Principle

Trying to fix the Problem
 Let’s try using interfaces
– Flyable and Quackable Code duplication!

What is the Problem?
 We tried this
– Inheritance changes all subcasses
– Interfaces cause code duplication
 The problem is we are mixing different types of
code in one type of classes
 Fix
– Separate Variation Design Principle
– Take what varies and encapsulate it so it wont affect
the rest of the code

Separation of Concerns
 Separate what changes from what stays the
same
– Move duck behavior to a separte classes
FlyWithWings flyBehavior = new FlyWithWings();
DATA TYPE IS TOO SPECIFIC

Separation of Concerns
 But the Duck classes cannot use the concrete
behavior classes!
– We need an interface or supertype
FlyBehavior flyBehavior = new FlyWithWings();
INTERFACE - POLYMORPHISIM

The Interface Design Principle
Program to an
interface, not an
implementation

Loose Coupling with Interfaces
 Advantages
– The ability to change the implementing class of any
application object without affecting calling code
– Total freedom in implementing interfaces
– The ability to provide simple test implementations and
stub implementations of application interfaces as
necessary

Program to an interfaces
 Program to an implementation
 Program to interface/subtype
 Program to unknown creation
Dog d = new Dog();
d.bark();
Animal animal = new Dog();
animal.makeSound();
Animal animal = getAnimal();
animal.makeSound();

Program to an interfaces
 Dependency Injection
– Make the caller responsible for setting the dependency
private Animal animal;
public setAnimal(Animal animal)
{
this.animal = animal;
}
...
animal.makeSound();
Injection happens
here, in the
set-method
LOOSE COUPLING = BEAUTIFUL!

Implementing Behavior
 We can add new behaviors without touching the
Duck classes

Integrating the Behavior
 The Duck classes will now delegate its flying
and quacking behavior
Behavior interfaces
Perform the
Bahavior

 Using the behavior
public class Duck
{
QuackBehavior quackBehavior;
...
public void performQuack()
{
quackBehavior.performQuack()
}
}
We don’t care what kind of object this
is, all we care is that it knows how to
quack!

 Setting the behavior
public class MallardDuck extends Duck
{
public MallardDuck()
{
quackBehavior = new Quack();
flyBehavior = new FlyWithWings();
}
}
This is not
programming
to an interface!

Setting Behavior Dynamically
 Add two new methods to the Duck class
 Dependency Injection
public void setFlyBehavior(FlyBehavior flyBehavior)
{
this.flyBehavior = flyBehavior
}
public void setQuackBehavior(QuackBehavior quackBehavior)
{
this.quackBehavior = quackBehavior
}
DuckFactory
{
public Duck getMallardDuck()
{
Duck duck = new MallardDuck()
duck.setFlyBehavior(new FlyWithWings());
duck.setQuackBehavior(new Quack());
return duck;
}
}

Setting Behavior Dynamically
 The idea
– Don´t think: Mallard is-a flying duck, think: it has-a
flying behavior
– Putting two classes together where one is a member
in the other is a composition
 Creating systems using composition give
flexibilty
– You can change the behavior at runtime

Composition Design Principle
Favor composition over
inheritance

Object Composition
 Problems with concrete inheritance
– Class hierarchy can get rigid
– Difficult to change the implementation
 Object Composition is more flexible
– Allows the behaviour of an object to be altered at run
time, through delegating part of its behaviour to an
interface and allowing callers to set the
implementation of that interface

Summary
 OO Programming is powerful
– If used correctly
– Remember Encapsulation, Interfaces, Polymorphism
 Generic programming
– Using classes, abstract classes and interfaces can
lead to powerful and flexible programs
 Reflection
– Powerful for building infrastructure

Job interview question
You are given the assignment of creating a component that needs to
know sales statistics of Lottery tickets. You know that there is a
another component in the system, Sale Server, that handles the sale.
You need real-time information. What would you suggest?
EXERCISE

Design Patterns
 Design pattern is a general solution to a common
problem in software design
– Systematic approach for problems that reoccur in software
development
– Not complete solution but starting point for design
– Not code ready to use
– Patterns have names and definitions
– Built on common practices
 Patterns should not be language dependant
– However patterns apply for types of programming
languages

L02 Software Design

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