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i am confused about the usage of static methods in java , for example it makes sense if main method is static , but while coding we have got objects for example 

 JFrame frame= new JFrame(); 
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);// here why not frame.EXIT_ON_CLOSE

and same way when we use 

 GridBagConstraints c= new GridBagConstraints();// we have an object but still
 c.anchor = GridBagConstraints.PAGE_END;

so can anyone please explain me the is there any special reasons for it ?

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  • static means there is no instance of an object associated with it. That's all, nothing special. Commented Dec 2, 2012 at 13:45
  • "for example it makes sense if main method is static" no it doesn't, but I probably shouldn't start that kind of discussion in a Java-only question... Commented Dec 2, 2012 at 19:29

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Static methods and fields belong to all objects in a class, as opposed to non-static ones, which belong to a particular instance of a class. In your example, no matter how many JFrame frame objects you create, accessing frame.EXIT_ON_CLOSE would produce the same exact result. To state this fact explicitly, static members (also known as "class members") are used.

Same logic applies to static methods: if a method does not access instance variables, its result becomes independent of the state of your object. The main(String[] args) method is one such example. Other common examples include various factory methods, parsing methods for primitives, and so on. These methods do not operate on an instance, so they are declared static.

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I like to add, static methods are usually like math function. Means no side effects and always the same output for the same input.
One reason people do this is so that you don't need to create an instance of the object to access those fields. So, for example, if you wanted to present the various choices of JFrame exit behavior to the user in a dialog, you wouldn't need to new up a JFrame object just to see what those choices are.
@NabilA. This is not necessarily the case; for example if the static method changes a static variable. For example singleton id-generators.
@RobAu thats why I wrote usually. But maybe it would be better to say, if you want to create a function, you use static methods.
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JFrame.EXIT_ON_CLOSE is not a method. It is static field. See this doc.

If you don't want some functionality associated with class not object, then you can use static method.

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ok , i just want to ask is there any special purpose of not using objects (while we have)
It is design decision. In this case as dasblinkenlight explained, irrespective of how many JFrame objects you have EXIT_ON_CLOSE need to produce same behaviour, so it is better to associate with class than instance. Let us say if each instance need to behave differently for EXIT_ON_CLOSE, then we need to make at as instance specific than static one.
@SikanderNawaz you can use objects but that would be of no use.. as at comiplation that object reference gets replaced by Class name automatically
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- JFrame.EXIT_ON_CLOSE is a static variable (field) not method in JFrame Class.

- static methods are class methods, for example in Math class there is No instance variables, and its constructor is private. So static worked perfectly there...

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