Java Learning - Design Patterns

Creational Patterns

Singleton

• A class that has only one instance, but no clear owner.

• That instance is available everywhere.

• The instance is initialized only when it’s first used (lazy initialization).

Builder

• A mutable factory that constructs the to-be-created object property by property.

• Usually supports method chaining.

• Often used to create immutable data objects.

Abstract Factory

• The thing creating objects is also an object.

• Encapsulate construction of several related objects into a single Java interface.

• Hide construction details from callers.

Behavioral Patterns

Strategy

• You define an interface to represent a kind of task or problem.

• Each concrete implementation defines a different “strategy” for solving the task.

• The strategies can be swapped for each other.

Template Method

• You define a base class or interface (template) for a procedure or algorithm.

• But leave empty placeholders (blank or default method) for some parts of the procedure.

• Callers fill in the blanks by extending the base class and overriding the placeholder methods.

Structural Patterns

Adapter

• Transform one API or interface into another.

• Typically “wrap” an existing interface to adapt it to a different interface.

• One common use of the adapter pattern is to wrap legacy APIs.

Decorator

• These patterns both “wrap” another object, called the delegate.

• An Adapter returns a different interface than the delegate.

• A Decorator returns the same interface, but with added functionality or responsibilities.

Dependency Injection

• Add @Inject annotations to inject objects from a DI framework.

• Use modules to configure which classes or objects should be used when an interface is injected.

• Usually can be configured to return a specific instance of an object.

• Take care of indirect, or transitive, dependencies.