Question

I need some guidance on how to initialize this hash inside a singleton.

So according to the author of effective java, you declare a singleton using an enumeration: https://sites.google.com/site/io/effective-java-reloaded/effective_java_reloaded.pdf?attredirects=0

So I have this so far:

public enum MySingleton {
  INSTANCE;

  private final Hashtable<SomeEnumType, UserSettings> settings;

  public final UserSettings getSettings(SomeEnumType enumType) {
     return settings.get(enumType);
  }

}

So I think I have my singleton correct, I need help on how I can initialize the settings hashtable inline.

private final Hashtable<SomeEnumType, UserSettings> settings = new Hashtable<SomeEnumType, UserSettings>() {{
  put(SomeEnumType.Blah1, new UserSettings ???????  );
}};

Say my UserSettings has some fields, can I set them inline?
Any comments on the singleton, is it ok?

Note: I will not be updating this hash at all after I initialize it.

Answer 1

As you’re not updating the Hashtable, you don’t require synchronization, so I’d use a HashMap instead. You have 2 options to initialize the map.

1) In the enum constructor

public enum MySingleton {
  INSTANCE;

  private final HashMap<SomeEnumType, UserSettings> settings;

  private MySingleton(){
    settings = new HashMap<SomeEnumType, UserSettings>();
    settings.put(...,...);
  }

  public final UserSettings getSettings(SomeEnumType enumType) {
     return settings.get(enumType);
  }

}

2) In an initializer block

public enum MySingleton {
  INSTANCE;

  private final HashMap<SomeEnumType, UserSettings> settings;
  {
    settings = new HashMap<SomeEnumType, UserSettings>();
    settings.put(...,...);
  }

  public final UserSettings getSettings(SomeEnumType enumType) {
     return settings.get(enumType);
  }

}