Inner class Static

Hi all,
i just went through inner classes in java, and found inner classes cannot hold static variables unless the class declared as static. is there any reason or logic behind that?

we can find some logic behind all OOP concepts. whats apply here?

thanks in advance....

Quote:

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Originally Posted by jomypgeorge

i just went through inner classes in java, and found inner classes cannot hold static variables unless the class declared as static. is there any reason or logic behind that?

we can find some logic behind all OOP concepts. whats apply here?

---

An object of an inner class is like a planet orbiting a start (an object from the outer class). Should two different planets orbiting different stars have different static variables? If so those variables won't be really static to the inner class, they would be ordinary variables of the outer class. If not, (all static variables should be the same) they're actually static variables of the outer class.

kind regards,

Jos

thanks jos,

you are right. we can place that variable in base class.
then why there is an option for static inner class?
what is the use of it?

Quote:

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Originally Posted by jomypgeorge

thanks jos,

you are right. we can place that variable in base class.
then why there is an option for static inner class?
what is the use of it?

---

A static nested class is just a scope: the nested class (not objects thereof) only exist in the scope of the outer class. If it is not a private or protected class it could have been an outer class itself. Nested classes have all access to the static members of the outerclass, that's all and it has its use now and then.

kind regards,

Jos

You might want to check out Roedy Green's page on nested and inner classes. There's nothing on when to use static nested classes, but there's all the syntax laid out including the factoid that you can have static final compiletime constants. He's a bit skeptical about there being any reason for the restriction, but does give some ideas (of others) near the end of the page.

I have seen reference to static nested classes being used for usage/debugging/testing as part of the .java file of some class, but whose .class files are not shipped with the rest.

Sorry - that wasn't clear: you can have static final compile time constants in inner classes, RG is skeptical about the restriction on inner classes having static stuff.

Quote:

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Originally Posted by pbrockway2

I have seen reference to static nested classes being used for usage/debugging/testing as part of the .java file of some class, but whose .class files are not shipped with the rest.

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That's cute; do you happen to know how they did that?

kind regards,

Jos

Code:

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public class MyApp {
    private String recip = "World";

    public String getRecip() {return recip;}

    public static void main(String[] args) {
        MyApp app = new MyApp();
        System.out.println("Hello " + app.getRecip());
    }

    static class Test {
        public static void main(String[] args) {
            MyApp app = new MyApp();
            if(app.getRecip() == null) {
                System.out.println("Fail!");
            } else {
                System.out.println("OK");
            }
        }
    }
}

>javac.exe MyApp.java

>java MyApp$Test
OK

>del MyApp$Test.class

>java MyApp
Hello World

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Of course, you don't actually have to delete the test class file(s), but the app runs fine without them so that can be excluded from the jar or whatever.

Quote:

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Originally Posted by pbrockway2

Of course, you don't actually have to delete the test class file(s), but the app runs fine without them so that can be excluded from the jar or whatever.

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That's cute, I didn't know that a static nested class isn't loaded when the outer class code is loaded. I guess that no static initializers can be present in the nested class or a reference to it in the outer class can be present; I'll play a bit with it ;-) Thanks for sharing.

kind regards,

Jos

thanks to u both for your reply

what i got from this is

Inner class instance has existence only with an instance of outer class.
so inner class holding a static variable is just like saying "an non static variable is static"

static inner class is just a class put in another class.
we can made independent instances of it without having outer class instance.

right?

Quote:

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Originally Posted by jomypgeorge

thanks to u both for your reply

what i got from this is

Inner class instance has existence only with an instance of outer class.
so inner class holding a static variable is just like saying "an non static variable is static"

static inner class is just a class put in another class.
we can made independent instances of it without having outer class instance.

right?

---

Yep, the only thing I don't understand is "an non static variable is static"; see my first reply again.

kind regards,

Jos

yes i got it.

if we need a static variable in inner class then we can meet the same requirement by placing it in outer class.

also even though inner class is static, its not loaded when outer class is loaded.
i think inner class is loaded when we refer it first in code...
right?

Quote:

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Originally Posted by jomypgeorge

yes i got it.

if we need a static variable in inner class then we can meet the same requirement by placing it in outer class.

also even though inner class is static, its not loaded when outer class is loaded.
i think inner class is loaded when we refer it first in code...
right?

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Yep, for your last statement you have to check it with pbrockway2, I didn't find the time yet to play with it ;-)

kind regards,

Jos

ok jos thanks alot

Quote:

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Originally Posted by JosAH

That's cute, I didn't know that a static nested class isn't loaded when the outer class code is loaded. I guess that no static initializers can be present in the nested class or a reference to it in the outer class can be present; I'll play a bit with it ;-) Thanks for sharing.

kind regards,

Jos

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This is how lazy loading of singletons works.

Code:

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public class MySingleton {
    public static class MyHolder {
        private static MySingleton singleton = new MySingleton();
    }
    public MySingleton getInstance() {
        return MyHolder.singleton;
    }
}

---

MyHolder isn't loaded until a call to getInstance...and consequently singleton isn't initialised until then.

Quote:

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Originally Posted by Tolls

This is how lazy loading of singletons works.
MyHolder isn't loaded until a call to getInstance...and consequently singleton isn't initialised until then.

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Of course! I just didn't see the idiom; I'll go and bang my head againts a pointy stick and punish myself now ... (*)

kind regards,

Jos ;-)

(*) not really, I'll go and get myself a Grolsch ;-)