L07 Design Principles

Lecture 07Design Principles

Agenda Why Principles? SOLID Principle

1. Single Responsibility Principle2. Open/Closed Principle3. Liskov Substitution Principle4. Interface Segregation Principle5. Dependency Inversion Principle

Reading Barbin Chapter 5 SOLID Principles

Big Ball of Mud Is this your code?

Why Principles? Based on knowledge and research Encourage writing high quality code Promote better communication

Why Principles? Should not be taken as dogmatic All design decision have context and

specific set of problems to solve

SOLID Principles Object oriented design and development

1. Single Responsibility Principle2. Open/Closed Principle3. Liskov Substitution Principle4. Interface Segregation Principle5. Dependency Inversion Principle

Single Responsibility Principle (SRP) Class should have one and only one

reason to change Violating this principle leads to low

cohesion and make the code brittle and loads to code bloat and confusion

Single Responsibility Principle Cohesion refers to the degree to which

the elements of a module belong together– if the methods that serve the given class tend

to be similar in many aspects, then the class is said to have high cohesion

High cohesion– Increased understanding of modules – Increased ease in maintaining a system– Increased ease in reusing a module

Single Responsibility Principle Example:

– Rectangle has two responsibities

The Open-Closed Principle (OCP) Design and write code in a fashion that

adding new functionality would involve minimal changes to existing code– Most changes will be handled as new methods

and new classes– Designs following this principle would result in

resilient code which does not break on addition of new functionality

public class ResourceAllocator{ ... public int allocate(intresourceType) { intresourceId; switch (resourceType) { case TIME_SLOT: resourceId = findFreeTimeSlot(); markTimeslotBusy(resourceId); break; case SPACE_SLOT: resourceId = findFreeSpaceSlot(); markSpaceSlotBusy(resourceId); break; ... } return resourceId; }...

Resource Allocator Example

Holy Buckets!!I need to change the class for new types!!! Horrible!

Resource Allocator Example Design for extensions

List resources = new ArrayList();...public int allocate(intresourceType){ int resourceId = findFreeResource(resourceType); markAsBusy(resourceId); return resourceId;}

The Liskov Substitution Principle (LSP)

All code operating with reference to the base class should be completely transparent to the type of the inherited object

It should be possible to substitute an object of one type with another within the same class hierarchy

Inheriting classes should not perform any actions that will invalidate the assumptions made by the base class

LSP Examplepublic class Rectangle {

protected int _width; protected int _height; public int getWidth() { return _width; } public int getHeight() { return _height; } public void setWidth(int width) { _width = width; } public void setHeight(int height) { _height = height; }}

LSP Examplepublic class Square extends Rectangle {

public void setWidth(int width) { _width = width; _height = width; }

public void setHeight(int height) { _height = height; _width = _height; } }

Implementation convenience

LSP Exampleimport junit.framework.Assert;import org.junit.Test;

public class RectangleTests {

@Test public void areaOfRectangle() {

Rectangle r = new Square(); r.setWidth(5); r.setHeight(2); // Will Fail - r is a square and sets // width and height equal to each other. Assert.assertEquals(r.getWidth() * r.getHeight(),10); }}

Interface Segregation Principle (ISP) Split large interfaces into smaller and

more specific interfaces– Role interfaces– The smaller the interface, the better

Large interfaces– Fat or polluted interfaces– Clients are forced to depend on methods they

do not use

Dependency Inversion Principle (DIP) Low-level components should depend on

high-level, not vice versa. This is dependency Inversion

The high-level components should be the one defining logic and enforcing change

High-level components depending on low-level components decreases the capability of the high-level components

Dependency Inversion Principle Separated interface pattern Program to interfaces Dependency inversion is the very heart of

framework design

Dependency Inversion Principle Dependency inversion means

– High-level components should not depend on low-level components, both should depend on abstractions

– Abstractions should not depend on details, details should depend on abractions

Summary Framework patterns

– Inversion of Control and Dependency Injection– Template Method– Strategy

From problems to patterns– Game Framework

Spring framework– Bean containers– BeanFactory and ApplicationContext