Simple example app for design patterns.

Notes

• used to guarantee one instance of an object in an application
• static in nature
• thread safe
• a private instance
• a private constructor
• no parameters required for construction
• getInstance() doesn't necessarily mean it is a singleton

Pitfalls

• Overuse
• Difficult to unit test
• If not careful, not thread-safe
• Can be confused for Factory

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Notes

• Handles complex constructors
• Large number of parameters
• Immutability

Examples

• StringBuilder
• DocumentBuilder
• LocaleBuilder

Pitfalls/Considerations

• Immutable
• Inner Static Class
• Designed first
• Complexity

Notes

• Clone
• Guarantee unique instance
• Great to help with performance issues associated with creation of lots of objects
• Lightweight Construction
• Avoids keyword "new"
• Each instance is a copy, but unique
• Costly construction not handled by client
• Can do shallow(immediate properties) vs deep copy(object references)

Examples

• Cloneable

Pitfalls

• Sometimes not clear when to use
• Typically have to use it with another pattern like registry
• Shallow VS Deep Copy

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Notes

• Doesn't expose instantiation logic
• Defer to subclasses
• Common interface
• Specified by architecture, impelemented by user
• Solves complex creation
• Opposite of a Singleton

Example

• Calendar
• ResoureBundle
• NumberFormat

Pitfalls

• Complexity
• Creation in subclass
• Refactoring

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Notes

• Factory of Factories
• Factory of related objects
• Common Interface
• Defer to Subclasses
• Groups Factories together
• Factory is reponsible for lifecycle
• Common Interface
• Concrete Classes
• Parameterized create method
• Composition

Examples

• DocumentBuilder
• Frameworks

Pitfalls

• Complexity
• Runtime switch
• Pattern within a pattern
• Problem specific
• Starts as a Factory