1. Design Patterns – Chocolate Factory (from Head First Design Patterns)

2. Scenario Automated chocolate bar maker Large chocolate boiler
General process:
Fill with ingredients
Boil the chocolate
Drain the tank
Chocolate goes to the next step
Two data members
empty
boiled

3. Scenario (cont.) Method fill Method drain Method boil
If empty, empty = false, boiled = false
Method drain
If not empty and isboiled, empty = true
Method boil
If not empty and not boiled, boiled = true

4. Scenario (cont.)
Possible issue: Software shouldn’t be able to think that there is / create two ChocolateBoiler instances
Why not?

5. Scenario (cont.)
Possible issue: Software shouldn’t be able to create two ChocolateBoiler instances
Why not?
There is only one physical ChocolateBoiler instance
Steps could get done out of sync on two software instances (e.g. drain an empty boiler, two fills in a row)

6. Possible Solution Use Singleton pattern for ChocolateBoiler class
Only one ChocolateBoiler can be created
Always access that ChocolateBoiler

7. public class ChocolateBoiler {
private boolean empty;
private boolean boiled;
private static ChocolateBoiler uniqueInstance;
private ChocolateBoiler() { empty = true; boiled = false; }
public static ChocolateBoiler getInstance () {
if (uniqueInstance == null) {
uniqueInstance = new ChocolateBoiler(); }
return uniqueInstance; } }

8. Is this really a good solution? What possible problem can occur?
Hint: think about threads
Three columns:
Thread one
Thread two
Value of uniqueInstance

10. How to fix problem? Idea Make getInstance method synchronized
public static synchronized Singleton getInstance()

11. How to fix problem? Idea Make getInstance method synchronized
public static synchronized Singleton getInstance()

12. Problem with synchronization
Only used first time through
Later synchronization overhead is totally unnecessary
What can be done?
Ignore problem (if not mission-critical)
Use “eager creation” (static initialization of uniqueInstance) rather than “lazy creation” (creating at run-time, after code check for null instance)
Double-checked locking – put synchronization only around body of ‘if’ statement

13. Singleton
How to generalize Singleton pattern to support creation of up to N instances?

14. Singleton as an Anti-Pattern
Many criticisms of singleton have been put forth
Issues
Function like a global variable
Mix creation and structure
Difficult to test
Overall: may want to look for alternatives, only use Singleton as a last resort