Advanced Object Oriented Design Laboratory.

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Blaise Cook
6 years ago
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1 Advanced Object Oriented Design Laboratory

2 Message Chains

3 Law of Demeter Principle Object- Oriented style rule for builing systems (Dem et er Research Group 1987, published 1988) General Form: Each unit should only use a limited set of other units: only units closely related to the current unit Don t talk to strangers

4 Law of Demeter Principle cont. friends

5 Object Form of Law of Demeter Principle unit = method Closely related: 3. A parameter of the method 4. The enclosing object (this). 5. A static object. 6. An immediate part object (computed or stored): a. An object that the enclosing method returns b. At tributes of the enclosing object c. An element of a collection which is an attribute of the enclosing object 7. An object created within the method.

6 Object Form of Law of Demeter Principle unit = method Closely related: 3. A parameter of the method 4. The enclosing object (this). 5. A static object. public void dosth(object obj) { System.out.println(obj.toString()); this.dosth( LoD );

7 Object Form of Law of Demeter Principle 1. An immediate part object (computed or stored): a. An object that the enclosing method returns public class SthClass {... public Object getobj() { return new Object(); public void dosth() { System.out.println(getObj().toString());

8 Object Form of Law of Demeter Principle 1. An immediate part object (computed or stored): a. At tributes of the enclosing object b. An element of a collection which is an attribute of the enclosing object public class SthClass { public List collection;... public void dosth() { Object elem = collection.get(0); System.out.println(elem.toString());

9 Object Form of Law of Demeter Principle 1. An object created within the method. public void dosth() { Object obj = new Object(); System.out.println(obj.toString());

10 Law of Demeter Principle - Motivation Programmers can only keep a limited set of items in short- term memory and it is easier to keep them in memory if they are closely related Hiding the structure of navigation causes less changes to client. Reusability of m ethods (low coupling)

11 Law of Demeter Principle The Strong Law of Demeter The instance variables make up a given class. Inherited instance variable types may not be passed messages. The Weak Law of Demeter The instance variables and any instance variables inherited from other classes make up a given class

12 What is Message Chains? Message Chains is a violation of the Law of Demeter Principle

13 example public class Person { private Department department; public void setdepartment(department department){ this.department = department; public Department getdepartment() { return department;

14 example cont. public class Department { public Department(Person manager) { this.manager = manager; public Person getmanager() { return manager;

15 example cont Person manager = john.getdepartment().getmanager(); Violation of the Law of Demeter

16 example public class Person { private Department department; public void setdepartment(department department){ this.department = department; public Department getdepartment() { return department; public Person getmanager() { return department.getmanager(); Extract to method Move method Hide delegate Correct

CAS 703 Software Design

CAS 703 Software Design Dr. Ridha Khedri Department of Computing and Software, McMaster University Canada L8S 4L7, Hamilton, Ontario Other 1 2 3 4 Other Principle of Least Privilege Principle of Fail-Safe Defaults Principle of