about static methods and variables

Question

why used this codes in AEinsam/HabReader:

private static User instance = null; // line:25

public static User getInstance() {
if (instance == null)
    instance = new User();
return instance;
}

// line:31

can I make class without this method?

Yes; it is possible to create a class with putting that code in it. — SLaks, May 03 '13 at 20:37
@SotiriosDelimanolis +1 for pointing out that it's bad, for several reasons even - here's a [JavaWorld article](http://www.javaworld.com/javaworld/jw-04-2003/jw-0425-designpatterns.html) explaining some of the issues. — fvu, May 03 '13 at 20:39

score 4 · Accepted Answer · edited May 23 '17 at 11:57

4

This is (a poorly implemented) Singleton. If you omit this static block but do not create a public constructor then your class might be useless. If you are looking for an alternative that behaves the same way I (and most others on SO) would direct you to the Enum Pattern

edited May 23 '17 at 11:57

Community

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answered May 03 '13 at 20:37

Jason Sperske

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Emphasis on the _poorly implemented_ – Sotirios Delimanolis May 03 '13 at 20:38
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One reason why it is 'poorly implemented' is because it does not define a private default constructor 'private User() {}'. This means there can still be multiple instances of this User class. – Barry NL May 03 '13 at 20:41

score 2 · Answer 2 · answered May 03 '13 at 20:37

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This is a singleton pattern which means that class is supposed to have only one instance existing.

answered May 03 '13 at 20:37

dratewka

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Barry NL · Answer 3 · 2013-05-03T20:58:57.140

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The writer of that code wants to make sure there is only a single instance of the User class. This design pattern is called 'singleton'.

Yes, you can make a class without the method, but there could be multiple instances of that class.

edited May 03 '13 at 20:58

answered May 03 '13 at 20:38

Barry NL

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about static methods and variables

3 Answers3