How to initialize Java enum based Singleton?

Question

What is the proper way to initialize java enum based singleton, if I have to initialize it before I can use the object.

I have started writing the code , but I am not sure if I am doing it right. Could you help me to implement this singleton correct for me?

public enum BitCheck {
 INSTANCE;
 private static HashMap<String, String> props = null;
 public synchronized void initialize(HashMap<String, String> properties) {
 if(props == null) {
 props = properties;
 }
 }
 public boolean isAenabled(){
 return "Y".equalsIgnoreCase(props.get("A_ENABLED"));
 }
 public boolean isBenabled(){
 return "Y".equalsIgnoreCase(props.get("B_ENABLED"));
 }
}

• zengr: I don't like this, what are you trying to achieve?
• Dunes: Why do you need an enum with only one element when you are already using a singleton?
• java_mouse: @Dunes I want a singleton so that I can use this throughout my code base to check values. I will initialize with the properties during startup and will use the check methods all over the project.
• Dunes: I understand why you might want a singleton, but to also make it an enum seems bizarre.
• maba: @Dunes Please read this SO question for some enlightenment regarding using Enums as Singletons: What is an efficient way to implement a singleton pattern in Java?.

Answer 1

It's perfectly possible to create constructor for enum:

public enum BitCheck {
 INSTANCE;
 BitCheck() {
 props = new HashMap<String, String>();
 }
 private final Map<String, String> props;
 //..
}

Note that:

• props field can be final (we like final)
• props doesn't have to be static
• constructor is called automatically and eagerly for you

Pay attention to the last point. Since enum-singletons are created eagerly when the enum BitCheck class is loaded, you have no way to pass any arguments to the constructor. Of course you can through INSTANCE declaration:

public enum BitCheck {
 INSTANCE(new HashMap<String, String>());
 BitCheck(final Map<String, String> props) {
 this.props = props;
 }

but this doesn't make any difference, right? What do you want to achieve? Maybe you actually need lazy-initialized singleton?

• Amit Deshpande: I guess It will be bad Idea to create like this since constructor does not offer anything good. It can be simple declaration
• java_mouse: I want to initialize with a set of values, that I want to pass it to the singleton. How can I do that? Your example is initializing with empty hashmap.
• Tomasz Nurkiewicz: @java_mouse: that's the thing. There is no way to pass anything to the constructor from outside, enum singleton won't help you.
• java_mouse: @TomaszNurkiewicz Thanks, I think I got it. I should go for traditional singleton implementation.

Answer 2

You have to just initialize it in declaration.

public enum BitCheck {
 INSTANCE;
 private final Map<String, String> props = new ConcurrentHashMap<String, String>();
 public void putAll(HashMap<String, String> map) {
 props.putAll(map);
 }
}

Answer 3

You need to define a constructor like this:

public enum BitCheck {
 INSTANCE;
 private final HashMap<String, String> props;
 BitCheck() {
 }
}

A very good example is the Planet enum at: http://docs.oracle.com/javase/tutorial/java/javaOO/enum.html

Answer 4

public enum BitCheck {
 INSTANCE;
 private BitCheck() {
 // initialize here
 }
}

Answer 5

You should implement to interfaces and therefore change to:

private Map<...> props;

You can look at What is the best approach for using an Enum as a singleton in Java? which is similar to what you want.

Answer 6

You can try something like this within your Enum based singleton code. This will make sure that the Singleton can be initialized exactly once.

private static Properties props;
private static AtomicBoolean isInitialized = new AtomicBoolean(false);
public void init(Properties props){
 if(isInitialized.compareAndSet(false, true)) {
 this.props = props;
 }
}

NOTE: For more complex initialization, you would need 2 AtomicBooleans for initStarted and initCompleted (instead of single AtomicBoolean - isInitialized). Then the first thread sets initStarted and executes the initialization steps, the rest wait until initCompleted is set to true by first thread.

• Debajyoti Pathak: Not sure what exactly you mean by complex initialization and why would we need 2 atomic booleans. Correct me if I am wrong, but I think your above code should work fine as long as I have all the initialization code (however many steps) within that 'if' block.