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Java Forum / General / April 2007Initialization order brain-damage
Hi y'all, So after spending a particularly frustrating afternoon debugging through constructors and browsing the Java Language Specification 3.0 in some detail, I have come to the conclusion that I actually know nothing and need to ask a question: What is the standard order of evaluation of expressions during object construction w/rt constructors, super-constructors, and instance variable initializers? Specifically with the code: class A { AType a1 = new AType(); A() { aDoesSomething(); } } class B extends A { AnotherType b1 = new AnotherType(); B() { bDoesSomethingElse(); } } B bongo = new B(); when do all of the various bits get executed? If there's a section of the language spec that tells me this, I would really appreciate the pointer, as well... david rush Dave Rush wrote: > Hi y'all, > [quoted text clipped - 26 lines] > the language spec that tells me this, I would really appreciate the > pointer, as well... Assuming you mean during the call to new B(), Space for new B is allocated superclass default constructor new AType() aDoesSomething() <== better not be significant or overridable or have to do with anything other than constructing an A new AnotherType() bDoesSomethingElse() <== better not be significant or overridable or have to do with anything other than constructing a B <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.8.7.1>  SignatureLew >What is the standard order of evaluation of expressions during >object construction w/rt constructors, super-constructors, and >instance variable initializers? Specifically with the code: instance initialization »memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass [...] All the instance variables in the new object, including those declared in superclasses, are initialized to their default values [...] evaluate the arguments and process [explicit or implicit] constructor invocation recursively using these same five steps. [...] Execute the instance initializers and instance variable initializers [...] in the left-to-right order in which they appear textually in the source code for the class. [...] Execute the rest of the body of this constructor.« JLS3, 12.5 http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.5 instance initializer »Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).« JLS3, 8 Classes http://java.sun.com/docs/books/jls/third_edition/html/classes.html instance variable initializer »Initialization expressions for instance variables« JLS3, 8.3.2.2 Initializers for Instance Variables http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.3.2.2 >JLS3, 12.5 Simplified and in my wording: 0 set fields to default values as they are specified by the language 1 invoke the (implicit or constructor-called) super-constructor and perform steps 1-3 for the superclass 2 do the explicit initializations (variable and instance initializers) in the source-code order 3 execute the rest of the constructor >0 set fields to default values as they are specified by > the language [quoted text clipped - 3 lines] > initializers) in the source-code order >3 execute the rest of the constructor e.g., class B { int a = Do.print( "B0" ); { Do.print( "B1" ); } int b = Do.print( "B2" ); public B(){ Do.print( "B4" ); } { Do.print( "B3" ); }} class A extends B { int c = Do.print( "A0" ); { Do.print( "A1" ); } int d = Do.print( "A2" ); public A(){ Do.print( "A4" ); } { Do.print( "A3" ); }} public class Main { public static void main( final java.lang.String[] args ) { new A(); }} class Do { public static int print( final java.lang.String text ) { java.lang.System.err.println( text ); return 1; }} /* prints B0 B1 B2 B3 B4 A0 A1 A2 A3 A4 */ >> JLS3, 12.5 > [quoted text clipped - 10 lines] > > 3 execute the rest of the constructor Excellent summary. Do you think it would be possible to avoid the appearance of unbounded recursion without making it too detailed and complicated? Something like this: 0 set fields to default values as they are specified by the language 1 If not in new Object() constructor invoke the (implicit or constructor-called) super-constructor and perform steps 1-3 for the superclass 2 do the explicit initializations (variable and instance initializers) in the source-code order 3 execute the rest of the constructor It also does not deal with this() on the first line of a constructor, but I can't see how to fold that in without making it too complicated. This is obviously a trade-off between level of detail and being clearer than the JLS description. Patricia >If not in new Object() constructor invoke the (implicit Thanks for the correction! I already was aware of this inaccuracy and am accepting it for the sake of simplification. Feel free to use and publish the list of steps including this modification regarding the class »Object«, while I might not add it to my own list. > >JLS3, 12.5 > 2 do the explicit initializations (variable and instance > initializers) in the source-code order This is the piece of the text in 12.5 that my brain just kept blipping over. The verbage is dreadful. Thank you so much for the help. I should add that I think this is broken behavior! If a subclass overrides a method used in a superclass contructor that needs an initialized field in the subclass, you're just completely hosed. You have to resort to copy&paste reuse or a lazy initializer via a getter method. Leaving the desirability of accessing all instance vars through getters aside for the moment, this leaves open a rather large hole for non-obvious bugs in systems with high-levels of black-box reuse as are commonly being built in Java today. david -- (and yes I coded my way around this days ago using a lazy init...just wanted to know if it was in fact mandatory to do so) Dave Rush wrote: >>> JLS3, 12.5 >> 2 do the explicit initializations (variable and instance [quoted text clipped - 6 lines] > overrides a method used in a superclass contructor that needs an > initialized field in the subclass, you're just completely hosed. You But if you want initialiser code to access this (say, an inner class), you would be hosed if super had not been called. Don't call overrideable methods from constructors (unless specifically designed otherwise). Tom Hawtin Poo. It looks like the Google groups interface just ate my well- thought-out and carefully reasoned post. This means that this one is my grumpy migraine-influenced version... > Dave Rush wrote: > >>> JLS3, 12.5 [quoted text clipped - 7 lines] > But if you want initialiser code to access this (say, an inner class), > you would be hosed if super had not been called. Well, potayto, potahto. Gripping hand is that there is no way to run anything before the superclass constructor code. This is good if you like flexibility in your reuse patterns, bad if you fear people reusing your code in surprising ways. So basically the standard supports incompetent programmers rather than enabling competent ones. > Don't call overrideable methods from constructors (unless specifically > designed otherwise). Well the call of the overridable method wasn't in *my* code. It was in a 3rd party constructor's and I had no control over that code. This particular control had gotten good reviews on various java community sites and has been around for more than a few years, so I expected the code to be relatively mature. So basically your preaching "never reuse any black-box code", which is reasonable, but terribly unfashionable just now. And do you really want the compiler to barf on any calls of non-final/ non-private methods in a constructor? david Dave Rush wrote: > Hi y'all, > [quoted text clipped - 26 lines] > the language spec that tells me this, I would really appreciate the > pointer, as well... The best overall description that I've found is at: http://java.sun.com/docs/books/jls/third_edition/html/execution.html#44670 12.5 Creation of New Class Instances Patricia Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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