l07 design principles
DESCRIPTION
Look at the SOLID principlesTRANSCRIPT
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