Lecture 02Software 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/
Programming with Objects
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
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 programmingEncapsulationInterfacesPolymorphism
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
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, authoritative 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
Classes
Objects Object created
Object used
Date day1; Date day2 = new Date();
System.out.println (new Date ()); String s = new Date().toString ();
day1 null
day2Date
Date now = new Date(); if (day2.before(now)) { System.out.println (day2.toString()); }
beforetostring...
Objects Operator new creates
memory– String is an exception
Reference variablesalways point to some memory
day1 null
day2Date
beforetostring...
Date d = new Date();
Must be a concrete class
Can be any supertype or interface of the concrete class
ClassesObject
+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 runB) The method in B is runC) First the method in B is run, then the method in AD) 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 classObject 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 variablesDefault 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 TestTestp0=(2,3)
X = 2Y = 3
Java uses this method to pass objects to methods
A) Call by referenceB) Call by valueC) Call by object referenceD) 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 PointLocal copy of a referencep to Pointp 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
class Manager extends Employee{ 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 Test2Test2e0: Employee: Dilberte1: 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 classManager m = new Manager();m.setName(“P.H. Carl”); // Employee.setNamem.setTitle (“Boss”); // Manager.setTitlem.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 Exampleabstract 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
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) { ...
Examplepublic interface Comparable{ public int compareTo(Object other);}
class Employee extends Person implements Comparable{ 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());
AliceDilbertWally
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
class Manager extends Employee
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 interfacespublic 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
class Employee extends Person implements Comparable{ ...
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 ();}
Inte
rface
ImplementationClientcode
Example: Drawing systempublic 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 interfaceB) Correct if and only if X is an interface and Y is a classC) Correct if X and Y are either both classes or both interfacesD) Correct for all combinations of X and Y being classes and/or interfaces.
QUIZ
✔
Generic Programming
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 structuresstatic 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()); }
DilbertWallyAlice
Generic Programming Generic collections
– The Collections class is another exampleList<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);}
AliceDilbertWally
Reflection
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
Reflectionstatic 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) BDB) DBC) BDCD) 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(" "); } }
Software Design
Object Oriented Design Design and implementation of software
needs to be of quality– Badly designed, well implemented = problem!– Well designed, badly implemented = problem!
CODEHORROR!!
CODE HORROR DUDE
Object Oriented Design 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 wellCode smell!
Which of the following are disadvantages of using inheritance to provide Duck behavior?
A) Code is duplicated across subclassesB) Runtime behavior changes are difficultC) We can’t make ducks danceD) Hard to gain knowledge of all duck behaviorsE) Ducks can’t fly and quack at the same timeF) 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
Separate Variations Design Principle
Identify the aspects of your application that vary and separate them from what stays the same
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 interfacesProgram 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 interfacesDependency Injection– Make the caller responsible for setting the
dependencyprivate 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
Integrating the Behavior 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!
Integrating the Behavior 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
Next Design Patterns