Design Principles

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Design Principles

Hans Van Vliet, Software Engineering, Principles and Practice, 3rd edition, John Wiley & Sons, 2008. Section 12.1.Robert C. Martin, Clean Code: A Handdbook of Agile Software Craftsmanship, Prentice Hall, 2008.Joshua Bloch, Effective Java, 2nd edition, Addition Wesley, 2008.

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Outline

Encapsulate what varies Minimize access of members Favor composition over inheritance Program to interface Open-closed principle Liskov substitution principle

Why?

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Abstraction

“Abstraction arises from a recognition of similarities between certain objects, situations, or processes in the real world, and the decision to concentrate upon those similarities and to ignore for the time being the differences.” Tony Hoare

“An abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.” Grady Booch

Q: Examples in programming/code? Q: Why abstraction?

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Encapsulation

“Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior; encapsulation serves to separate the contractual interface of an abstraction and its implementation.” Grady Booch

“Encapsulation is a mechanism used to hide the data, internal structure, and implementation details of an object. All interaction with the object is through a public interface of operations.” Craig Larman

Q: Language constructs that support encapsulation? Q: Why encapsulation?

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Minimize the Accessibility of Classes and Members Classes should be opaque Classes should not expose their internal implementation details Use getters and setters

Compare

public double speed;

vs

private double speed; public double getSpeed() { return speed; } public void setSpeed(double speed) { this.speed = speed; }

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Minimize the Accessibility of Classes and Members Classes should be opaque Classes should not expose their internal implementation details Use getters and setters

Compare

public double speed;

vs.

private double speed; public double getSpeed() { return speed; } public void setSpeed(double speed) { this.speed = speed; }

Is this better?Why?

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Advantages of Minimizing Accessibility

Centralized checking of constraints Add useful side-effects (monitoring) Decouple internal representation from

function: Improved encapsulationReduced coupling of components

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Outline


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Composition vs. Subclassing

Two different methods for code reuse Delegation (prototype) vs inheritance (OO) Black box vs. white/grey box reuse Q: pros and cons? which one to use?

A B A B

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Composition

Method of reuse in which new functionality is obtained by creating an object composed of other objects New functionality obtained by delegating to objects being

composed Sometimes called aggregation or containment

Aggregation - when one object owns or is responsible for another object and both objects have identical lifetimes (GoF)

Aggregation - when one object has a collection of objects that can exist on their own (UML)

Containment - a special kind of composition in which the contained object is hidden from other objects and access to the contained object is only via the container object (Coad)

A B

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Advantages of Composition

Contained objects are accessed by the containing class solely through their interfaces

"Black-box" reuse, since internal details of contained objects are not visible

Good encapsulation Fewer implementation dependencies Each class is focused on just one task (cohesion) The composition can be defined dynamically at run-time

through objects acquiring references to other objects of the same type

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Disadvantages of Composition

Resulting systems tend to have more objects Interfaces must be carefully defined in order to

use many different objects as composition blocks

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Inheritance

Method of reuse in which new functionality is obtained by extending the implementation of an existing object

The generalization class (superclass) explicitly captures the common attributes and methods

The specialization class (subclass) extends the implementation with additional attributes and methods

A B

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Advantages of Inheritance

New implementation is easy, since most of it is inherited

Easy to modify or extend the implementation being reused

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Disadvantages of Inheritance

Breaks encapsulation, since it exposes a subclass to implementation details of its superclass

"White-box" reuse, since internal details of superclasses are often visible to subclasses

Subclasses may have to be changed if the implementation of the superclass changes

Implementations inherited from superclasses can not be changed at runtime.

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Example (from Effective Java by Bloch)

public class InstrumentedHashSet<E> extends HashSet<E> {

private int addCount = 0; // The number of attempted element insertions

public InstrumentedHashSet() { super(); }

public boolean add(E e) { addCount++; return super.add(e); }

public boolean addAll(Collection<? extends E> c) { addCount += c.size(); return super.addAll(c); }

public int getAddCount() { return addCount; }}

Variant of HashSet that tracks number of insertions

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Example Variant of HashSet that tracks number of insertions

Keep track of insertions

Call super constructor



public InstrumentedHashSet() { super(); }




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Example Variant of HashSet that tracks number of insertions

When we add, update the counter



public InstrumentedHashSet() { super(c); }




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Example: Test It---Add!public class InstrumentedHashSetTest { private InstrumentedHashSet<String> set;

@Before public void setUp() { set = new InstrumentedHashSet<String>(); } @Test public void testAdd() { assertEquals(0, set.getAddCount()); set.add("CS4311"); assertEquals(1, set.getAddCount()); set.add("Fun"); assertEquals(2, set.getAddCount()); }

% java org.junit.runner.JUnitCore InstrumentedHashSetTest

JUnit version 4.10

.

Time: 0

OK (1 test)

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Example: Test It---AddAll!public class InstrumentedHashSetTest { … @Test public void testAddAll() {

set.addAll(Arrays.asList(new String[] {"CS4311", "Fun"})); assertEquals(2, set.getAddCount()); }


JUnit version 4.10

..E

Time: 0.015

There was 1 failure:

1) testAddAll(InstrumentedHashSetTest)

java.lang.AssertionError: expected:<2> but was:<4>

at …

FAILURES!!!

Tests run: 2, Failures: 1

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Example: Test it---AddAll!public class InstrumentedHashSetTest { … @Test public void testAddAll() {

set.addAll(Arrays.asList(new String[] {"CS4311", "Fun"})); assertEquals(2, set.getAddCount()); }


JUnit version 4.10

..E

Time: 0.015

There was 1 failure:

1) testAddAll(InstrumentedHashSetTest)

java.lang.AssertionError: expected:<2> but was:<4>

at …

FAILURES!!!

Tests run: 2, Failures: 1

Test success or failure?

What went wrong?

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Example: Investigate It!

Look at the API document, e.g., JDK 8 says

public boolean addAll(Collection<? Extends E> c) of AbstractCollection

Adds all of the elements in the specified collection to this collection (optional operation). …This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn. …

Look at the source code. The internal implementation of addAll() in the superclass

superclass invokes the add() method! First we add 2 to addCount in InstrumentedHashSet’s

addAll(). Then we invoke super’s addAll().

For each element, this addAll() invokes the add() method, which as overridden by InstrumentedHashSet adds one for each

element.

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Example: Investigate It!

Look at the API document, e.g., JDK 7 says

public boolean addAll(Collection<? Extends E> c) of AbstractCollection

Adds all of the elements in the specified collection to this collection (optional operation). …This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.

Look at the source code. The internal implementation of addAll() in the superclass

superclass invokes the add() method! First we add 2 to addCount in InstrumentedHashSet’s

addAll(). Then we invoke super’s addAll().

For each element, this addAll() invokes the add() method, which as overridden by InstrumentedHashSet adds one for each

element.

Need to know the implementation

details to get this right!

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Example: Fix It! Variant of HashSet that tracks number of insertions







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Example: Fix It! Variant of HashSet that tracks number of insertions







Any concerns?

Hint: Program to interface

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Example: Composition

Let’s write an InstrumentedSet class that is composed of a Set object.

Our InstrumentedSet class will duplicate the Set interface, but all Set operations will actually be forwarded to the contained Set object.

The contained Set object can be an object of any class that implements the Set interface (and not just a HashSet)

InstrumentedSet Set

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Examplepublic class InstrumentedSet<E> implements Set<E> { private final Set<E> s;

private int addCount = 0;

public InstrumentedSet() { s = new HashSet<E>(); }

public boolean add(E e) { addCount++; return s.add(e); }

public boolean addAll(addAll(Collection<? Extends E> c) { addCount += c.size(); return s.addAll(c); }

public int getAddCount() { return addCount; }

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Example: Forwarding methods

public void clear() { s.clear(); } public boolean contains(E e) { return s.contains(e); }public boolean isEmpty() { return s.isEmpty(); } public int size() { return s.size(); } public Iterator iterator() { return s.iterator(); } public boolean remove(E e) { return s.remove(e); }public boolean containsAll(Collection<? Extends E> c) { return s.containsAll(c); } public boolean removeAll(Collection<? Extends E> c) { return s.removeAll(c); } public boolean retainAll(Collection<? Extends E> c) { return s.retainAll(c); } public E[] toArray() { return s.toArray(); } public E[] toArray(E[] a) { return s.toArray(a); } public boolean equals(E e) { return s.equals(e); } public int hashCode() { return s.hashCode(); } public String toString() { return s.toString(); }

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Coad's RulesUse inheritance only when all of the following criteria are satisfied:

A subclass expresses “is a special kind of” and not “is a role played by a.”An instance of a subclass never needs to become an object of another class.A subclass extends, rather than overrides or nullifies, the responsibilities of its superclass.A subclass does not extend the capabilities of what is merely a utility class.For a class in the actual Problem Domain, the subclass specializes a role, transaction or device.

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Outline


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Program to an Interface, not an Implementation Q: interface vs. implementation?

A sorted set class provides a “choose” operation that returns an arbitrary element contained in the set. You learned that the operation always returns the smallest element of the set. Is this an interface or implementation feature?

Why?

service provider

service user

Client <<interface>>List

<<uses>>

LinkedList ArrayList

X

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Interface

Interfaces express types in a limited way. An interface is the set of methods one object knows that

it can invoke on another object. An object can have many interfaces. Different objects can have the same type and the same

object can have many different types. An object is known by other objects only through its

interface.

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Example

public class InstrumentedSet<E> implements Set<E> { private final Set<E> s;


public InstrumentedSet(Set<E> s) { this.s = s; }




Which lines?

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Example

public class InstrumentedSet<E> implements Set<E> { private final Set<E> s;


public InstrumentedSet(Set<E> s) { this.s = s; }




Which lines?

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Advantages:

Clients are unaware of the specific class of the object they are using.

One object can be easily replaced by another. Object connections need not be hardwired to an object

of a specific class, thereby increasing flexibility Loosens coupling. Increases likelihood of reuse Improves opportunities for composition since contained

objects can be of any class that implements a specific interface

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Outline


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Open/Closed Principle

Software should be open for extension, but closed for modification

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The Open-Closed Principle (OCP) says that we should attempt to design modules that never need to be changed.

To extend the behavior of the system, we add new code. We do not modify old code.

Modules that conform to the OCP meet two criteria: Open For Extension - The behavior of the module can be extended to meet new

requirements. Closed For Modification - the source code of the module is not allowed to

change.

How can we do this? Abstraction Polymorphism Inheritance Interfaces Design patterns such as template methods New programming paradigms or languages such as AOP and AspectJ

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Open/Closed Principle Consider the following method that totals the price of each part in the

specified array of parts of some class:

public double totalPrice(Part[] parts) { double total = 0.0; for (Part p: parts) { total += p.getPrice(); } return total; }

If Part is a base class or an interface and polymorphism is being used, then this class can easily accommodate new types of parts without having to be modified!

It conforms to the OCP.

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Open/Closed Principle Suppose the Accounting Department decrees that motherboard

parts and memory parts should have a premium applied when figuring the total price.

public double totalPrice(Part[] parts) { double total = 0.0; for (Part p: parts) { if (p instanceof Motherboard) total += (1.45 * p.getPrice()); else if (p instanceof Memory) total += (1.27 * p.getPrice()); else total += p.getPrice(); } return total; }

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Is this OK?

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Does not conform to OCP.

totalPrice() must be changed whenever Accounting changes pricing policy.

What could we do instead?

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A better idea is to have a PricePolicy class which can be used to provide different pricing policies:

public class Part { private double price; private PricePolicy pricePolicy;

public void setPricePolicy(PricePolicy pricePolicy) { this.pricePolicy = pricePolicy; }

public void setPrice(double price) { this.price = price; } public double getPrice() { return pricePolicy.getPrice(price); } }

Part PricePolicy

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/** * Class PricePolicy implements a given price policy. */ public class PricePolicy { private double factor; public PricePolicy (double factor) { this.factor = factor; } public double getPrice(double price) { return price * factor; } }

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/** * Class PricePolicy implements a given price policy. */ public class PricePolicy { private double factor; public PricePolicy (double factor) { this.factor = factor; } public double getPrice(double price) { return price * factor; } }

Pricing policies can be set dynamically by changing the PricePolicy object.

Multiple pricing policy classes may exist.

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Pair; 2 minutes

An example of OCP application from standard libraries or frameworks like JDK?

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Outline


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Liskov Substitution Principle

An object of a subtype may be substituted for an object of its supertypes. An object of a subtype should behave like an object of

its supertypes.

Q: Why? A program that uses supertypes must be able to use

objects of the subtypes.

Barbara Liskov, Turing Award 2008

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Substitution Principle --- How?

Syntactically:If a method T1 m(T2 x) of class T is overridden in a subclass S,

T2_S is a superclass of T2_T (contra-variance of argument types)

T1_S is a subclass of T1_T (covariance of return type)

Q: Java rule?Q: How about exceptions that may be thrown?

Semantically:An overriding method should behave like an overridden methodPre_T Pre_S

Post_S Post_T

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Design Principles

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