Initialisation of Static variables in java

Question

I was asked this question in an interview

If you do something like this,

    private int c = d;
    private int d;

It results in compile-time error that you

Cannot reference a field before it is defined.

Coming to the interview question,

    1  public static int a = initializeStaticValue();
    2  public static int b = 20;


    3  public static int initializeStaticValue() {
    4   return b;

       }

    5   public static void main(String[] args) {
           System.out.println(a);
           System.out.println(b);
        }

I gave the same response as a gets initialised by a call to initializeStaticValue() where it is referencing an undefined value b.

But the programs works fine, gets compile, and prints

0
20

I am confused why

Cannot reference a field before it is defined.

was not thrown.

Secondly, when i debug it, why the control lands at

3  public static int initializeStaticValue() {

I mean, why this is the starting position of the program.

not very clear about your question: 1. b is defined before initializeStaticValue() so no error of cannot reference a field before it is defined. 2. before you run your main(), your class needs to be loaded by JVM first, and you can treat the static field initialization is part of the load process, that's why your initializeStaticValue() starts before your main(). Is that what you are asking for? — Adrian Shum
– Adrian Shum, Commented May 29, 2015 at 1:08
@Adrian: As soon as the control comes to the line number 1(marked above), there is a call to initializeStaticValue(). How does the compile know that b exists at this moment? — Farhan stands with Palestine
– Farhan stands with Palestine, Commented May 29, 2015 at 1:13
@ShirgillAnsari The static variable b 'exists' as soon as the class is loaded. However it has not been assigned a value yet (so it has the default, which is why initializeStaticValue returns 0). The former case is a semantic language restriction because it never makes sense (and this behavior follows that for local variables, eg): however the compiler/language does not look into methods as these cases quickly become non-deterministic at compile time (see the halting problem). — user2864740
– user2864740, Commented May 29, 2015 at 1:16
It is just similar to : initializeStaticValue() is declared after a, why compiler knows that it exists when it is on line 1? — Adrian Shum
– Adrian Shum, Commented May 29, 2015 at 1:16
@user2864740: I agree that the static variable b 'exists' as soon as the class is loaded and it possesses a default value of 0. But if you debug the program, you will notice that the debugger landsitself at initializeStaticValue() --> public static int a = initializeStaticValue(); --> public static int b = 20. Why the call to initializeStaticValue is invoked before public static int a = initializeStaticValue()....? — Farhan stands with Palestine
– Farhan stands with Palestine, Commented May 29, 2015 at 1:20

Srini · Accepted Answer · 2015-05-29 01:55:45Z

5

If you are concerned about the order of initialization/execution, here is what is going to happen (I believe it is not very accurate, just giving you an idea):

JVM is asked to run the Java app (assume your class is named) Foo, it tries to load Foo class from classpath
Foo is loaded, with static variables assigned with default value (0 for int)
Static initializers will be executed, first running that on line 1 which in turn calls initializeStaticValue() which returns the value of b at this moment (0), and assigns it to a
Static initialization continues, and comes to line 2. It assigns b with 20.
Foo is successfully loaded & initialized, and JVM calls Foo.main()

edited May 29, 2015 at 1:55

Srini

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answered May 29, 2015 at 1:27

Adrian Shum

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5 Comments

Srini Over a year ago

@AdrianShum, it is not recommended to ask for acceptance, it puts pressure on the questioner. Good answer though.

Srini Over a year ago

@AdrianShum, wholeheartedly agree with your point. We've all been at the receiving end of upvoted answers that weren't accepted. I'm not quite sure what the protocol is to intimate new users of the accept feature. Hopefully they go through the site's introductory tour before asking questions.

Adrian Shum Over a year ago

@BruceWayne let's raise that in meta stack overflow maybe :)

Farhan stands with Palestine Over a year ago

@Adrian: I am aware of the accept functions. Thank you once again.

Srini Over a year ago

@AdrianShum, I'm near certain that this issue must have been raised before. I'll try looking for it, let me know if you find it first.

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