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Java Forum / General / May 2007explain this pls
class Out { int x; { k = 5; //valid x = k; //invalid } public int k; } Out.java:6: illegal forward reference x = k; //invalid ^ 1 error On May 30, 2:27 pm, Jussi Piitulainen <jpiit...@ling.helsinki.fi> wrote: > Out.java:6: illegal forward reference > x = k; //invalid > ^ > 1 error so "forward assignment" is ok? k = 5; //"forward assignment" x = k; //compiler error because of forward reference ? >> Out.java:6: illegal forward reference >> x = k; //invalid [quoted text clipped - 7 lines] > > ? Seems so. A simplest web search suggests that the JLS, possibly in section 8.3.2, requires declaration before using a variable in an initializer "not on the left hand side of an assignment". >>> Out.java:6: illegal forward reference >>> x = k; //invalid [quoted text clipped - 10 lines] > section 8.3.2, requires declaration before using a variable in an > initializer "not on the left hand side of an assignment". I suppose that makes sense. k = 5; is correct because the assignment of k works, but if x=k; were allowed, this could happen: int x; { x = k; } int k = x; What is x and k? k is defined initially in terms of x, which is initialized to k, so no definite assignment can be given them. In short, forward assignment is harmless, so it is allowed; forward reference opens up a can of worms, so it is forbidden. >>>> Out.java:6: illegal forward reference >>>> x = k; //invalid [quoted text clipped - 27 lines] > In short, forward assignment is harmless, so it is allowed; forward > reference opens up a can of worms, so it is forbidden. Skipping the web search and going straight to the JLS we find: <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.3.2> > Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to class variables. and > The declaration of a member needs to appear textually before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold: > [quoted text clipped - 4 lines] > > A compile-time error occurs if any of the four requirements above are not met. I interpret that last sentence to implicitly include "and the usage is a forward reference." Instance initializers are covered in: <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.6> which repeats: > Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to instance variables. The JLS can be very useful in resolving Java syntax or semantics questions.  SignatureLew >>> Out.java:6: illegal forward reference >>> x = k; //invalid [quoted text clipped - 10 lines] > section 8.3.2, requires declaration before using a variable in an > initializer "not on the left hand side of an assignment". Skipping the web search and going straight to <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.3.2> we find > Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to class variables. Going in turn to <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#287406> we find: > The declaration of a member needs to appear textually before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold: > [quoted text clipped - 3 lines] > * C is the innermost class or interface enclosing the usage. > A compile-time error occurs if any of the four requirements above are not met. That last phrase I think is a typo; I interpret it to mean "A compile-time error occurs if all of the four requirements above are met and the variable is referred to before declaration." That the block is an instance initializer is clear from <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.6> which repeats > Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to instance variables. Reading the JLS is a useful way to resolve syntax questions. SignatureLew 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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