Item 01 Consider static factory methods instead of constructors
Advantages of static factory methods
- Static factory methods have names
- Not required to create a new object for each invocation
- Similar to flyweight pattern
- Return an object of any subtype of their return type
- An API can return objects without making their classes public
- Application: interface-based frameworks (item 20)
- The class of returned object can vary from call to call as a function of the input parameters
- The class of returned object need not exist when the class containing the method is written
- Application: service provider framework
Limitation of providing only static factory methods
- Classes without public or protected constructors cannot be subclassed
- Hard for programmers to find
Common static factory method names
| Name | Description | Example |
|---|---|---|
from | Type conversion | Date d = Date.from(instant) |
of | Aggregation | Set<Rank> faceCards = EnumSet.of(JACK, QUEEN, KING) |
valueOf | Verbose alternative to from and of | BigInteger primer = BigInteger.valueOf(Integer.MAX_VALUE) |
instance / getInstance | Return an instance | |
create / newInstance | Return a guaranteed new instance | Object newArray = Array.newInstance(classObject, arrayLen) |
getType | Return an instance of a specific type | FileStore fs = Files.getFileStore(path) |
newType | Return a new instance of a specific type | BufferedReader br = Files.newBufferedReader(path) |
type | Concise alternative to getType and newType | List<Complaint> litany = Collections.list(legacyLitany) |
Service provider framework
A system in which providers implement a service, and the system makes the implementations available to clients, decoupling the clients from the implementation.
| Component | Description | JDBC Example |
|---|---|---|
| Service interface | Represent an implementation | Connection |
| Provider registration API | Used by provider to register an implementation | DriverManager.registerDriver |
| Service access API | Used by clients to obtain instances of the service | DriverManager.getConnection |
| Service provider interface (optional) | Describe a factory object that produces instances of the service interface | Driver |