Item 03: Enforce the singleton property with a private constructor or an enum type

• A singleton is a class that is instantiated exactly once.
• Making a class a singleton can make it difficult to test its clients.

Singleton as final field

• The public field makes it clear that this class is a singleton.
• Simple.

public class Elvis {
  public static final Elvis INSTANCE = new Elvis();
  
  private Elvis() {
    // ...
  }
}

Singleton with static factory method

• Provide flexibility about whether the class is a singleton without changing its API.
• If necessary, can write a generic singleton factory (item 30).
• A method reference can be used as a supplier. Elvis::instance is a Supplier<Elvis>.
• To maintain the singleton guarantee for serializable class, declare all instance fields transient and provide a readResolve method (item 89).

private Object readResolve() {
  return INSTANCE;
}

public class Elvis {
  private static final Elvis INSTANCE = new Elvis();
 
  public static Elvis getInstance() {
    return INSTANCE;
  }
 
  private Elvis() {
    // ...
  }
}

Singleton as single-element enum

• Provide serialization machinery for free.
• Provide an ironclad guarantee against multiple instantiation.
• Best way to implement a singleton.

public enum Elvis {
  INSTANCE;
  
  // methods...
}