018: Effective Java for Android developers : Item 3

In this mini Fragment, we introduce Joshua's third Item: Enforce the Singleton property with a private constructor or an enum type.

Stay tuned for more items from our "Effective Java for Android developers" Fragment series.

• Effective Java (2nd Edition) - Joshua Bloch

Enforce the Singleton property with a private constructor or an enum type

Approaches

1. Create a public static final INSTANCE variable and privatize constructor
2. Same as 1 but privatize variable and expose access with provide factory method getInstance
3. Single element Enums

Considerations

• First two approaches are open to Serialization attacks (deserializing creates new instance)
• To protect from those declare the fields transient + provide readResolve method
• Enums are concise, provide free serialization and ironclad Singleton guarantees and are functionally equivalent to first approach

Supplemental reading (for the diligent ones that follow shownotes)

• Dianne Hackborn regretting exposing the base Application class
• DH again saying Application is a Singleton with crappier semantics
• Elvis sightings

• @fragmentedcast [twitter.com]
• @donnfelker [twitter.com]
• @kaushikgopal [twitter.com]