declaring instance keyword in java

Question

I see a "instance" keyword being declared in my java code declaration of a ENUM.Can someone explain how this keyword works?

Java code:-

public enum TodoDao {
    instance;

    private Map<String, Todo> contentProvider = new HashMap<String, Todo>();

    private TodoDao() {
        Todo todo = new Todo("1", "Learn REST");
        todo.setDescription("Read http://www.vogella.com/articles/REST/article.html");
        contentProvider.put("1", todo);
        todo = new Todo("2", "Do something");
        todo.setDescription("Read complete http://www.vogella.com");
        contentProvider.put("2", todo);
    }
    public Map<String, Todo> getModel(){
        return contentProvider;
    } 
}

I dont see instance anywhere. Also the keyword is `instanceof` — Woot4Moo, Dec 29 '13 at 15:26

score 9 · Answer 1 · edited May 23 '17 at 12:02

9

It is the best way to implement the GoF singleton pattern, according to Joshua Bloch, see related question: What is an efficient way to implement a singleton pattern in Java?

According to the java style guide, it should be all caps, though, i.e. INSTANCE.

To use this, you can call the following from any part of the code:

Map<String, Todo> todos = TodoDao.INSTANCE.getModel();

And you can be certain that the initialization will happen only once.

edited May 23 '17 at 12:02

Community

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1

answered Dec 29 '13 at 15:40

GeertPt

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this response deserves the first place! – eeadev Oct 30 '15 at 15:00

score 3 · Answer 2 · answered Dec 29 '13 at 15:26

3

It's not a keyword. It's an enum constant, which should be in caps. You should be able to find that wherever you are learning Java from.

It's actually a really bad idea to have mutable statics like this. Also returning a mutable internal object is bad.

answered Dec 29 '13 at 15:26

Tom Hawtin - tackline

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score 1 · Answer 3 · edited May 23 '17 at 12:34

suppose you use an enum instead of a class if you know the number of instances in advance.

Enum use to serve as a type of fixed number of constants :

Eg.

public enum TodoDao {
    INSTANCE
};

INSTANCE is public static final fields of TodoDao class, and like all static fields they can be accessed by class name like

TodoDao b1 = TodoDao.INSTANCE;

Here INSTANCE is final static inner classes .

Read more:What's the use of "enum" in Java?

declaring instance keyword in java

3 Answers3