CODEWITHSUNDEEP
javasingleton
a Learner
a Learner

Reputation: 5042

declaring variable static in singleton

while implementing singleton as:

class MyConnection {
    private static MyConnection connection = new MyConnection();

    private MyConnection() {
    }

    public  static MyConnection getConnection() {
        return connection;
    }
}

1)why we give connection as static?

Is this only due to the fact that getConnection() is static and we cannot reference non-static in static context or there is any other reason?

2) is it necessary to declare connection as final?

Upvotes: 2

Views: 1208

Answers (3)

Jacob is on Codidact
Jacob is on Codidact

Reputation: 3750

A Singleton is a class of which you'll only ever have one instance. In Java the most common way to implement that pattern is to declare a private constructor to prevent normal initialization and a public static method to provide the single instance. (but aioobe's enum answer is cool!)

Static methods can be called without an instance of the class, allowing you to do things like MySingleton.getInstance() - however, since you're not guaranteed to have an instance of the class, there's no way to access the instance variables. By declaring a variable as static you declare it as a "class variable" - allowing access to it in the same way as you do static methods, and also allowing static methods to access it (again, static methods cannot access non-static variables, because there's no guarantee that they exist and we wouldn't know which instance to look in anyways). So to answer 1) it's because if you needed an instance of the singleton in order to look up the instance of the singleton, you'd have a catch 22.

As for 2) you don't need to declare it final, but it's good form to declare anything that can be final, final. That way you're explicit about your intentions and the compiler can help you if you make a mistake. It should be at least final or private.

Upvotes: 1

Ramesh PVK
Ramesh PVK

Reputation: 15446

1)why we give connection as static?

Because you want it to be singleton. Even though you are not accessing it through static method, you can access the static variable by making it public and final. But, the variable has to be static.

2) is it necessary to declare connection as final?

Not Required if you are accessing through static method like in your code, as it not exposed outside to get modified.

But if you make the singleton variable public , it has to be final also.

Upvotes: 1

aioobe
aioobe

Reputation: 421010

1) why we give connection as static?

If it was non-static, you would need to have an instance of MyConnection to get hold of the connection reference which sort of defeats the purpose. :)

Is this only due to the fact that getConnection() is static and we cannot reference non-static in static context or there is any other reason?

Yes. (Since getConnection() needs to be static, connection needs to be static.)

2) is it necessary to declare connection as final?

No, but it's good practice, since once initialized, it should not be changed.

But, an even better practice is to use an enum instead.

enum MyConnection {

    INSTANCE;

    // your methods...
}

and access it through MyConnection.INSTANCE.

Rule of thumb: If a class shall have a predefined number of instances, use an enum. In this case the number of instances is one.

Upvotes: 9

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