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Java Forum / General / July 2006a unusual codes
the codes: ------------------- Object[] stuff = new Object[5]; stuff[0] = "eggs"; stuff[1] = new StringBuffer( "flour" ); stuff[2] = 3.56; stuff[3] = 'c'; stuff[4] = 123; stuff[0]="33"; for( int i=0; i<stuff.length; i++ ) { System.out.println( stuff[i] ); } ------------------ It seems odd..... followings is what I guest. please tell me whether it's correct, thank you very much 1: what I think is because that the Object Class is any class's superClass, so it can point to any type (right?), but in these codes it only have Object Class's interface, not the interface which it point to. is it right? 2: stuff[0]="eggs" in this sentence,the JVM create a String object(right?), although it's a String object and of course it has all the String object's interface, but what the reference(stuff[0]) can access is only the Object Class's interface.not the String interfaces(just like concat(),charAt()....) is it right? 3: stuff[0]="eggs" System.out.println( stuff[0] ); and when print it,the print function call stuff[0]'s toString() function,and this toString() is the String object's , not the Object Class's is it right? thank you very much JTL yes yes and yes 100% congratulations :-) > stuff[2] = 3.56; > stuff[3] = 'c'; > stuff[4] = 123; But these assignments can't work at all, can they? These are primitives, not objects, so they don't derive from Object. stuff[2] = new Float(3.56); Would be the correct syntax? Or do primitives get promoted to objects somehow? >> stuff[2] = 3.56; >> stuff[3] = 'c'; [quoted text clipped - 7 lines] > Would be the correct syntax? Or do primitives get promoted to objects > somehow? You can have arrays of primitives. > stuff[2] = 3.56; > stuff[3] = 'c'; > stuff[4] = 123; it can be compiled in JBuilder and it print: ---------------- 33 flour 3.56 c 123 ---------------- stuff[4]=123; I think it is turn to "new Integer(123);" automatismly when in compiling I guest.... Mark Space schreef: > > stuff[2] = 3.56; > > stuff[3] = 'c'; [quoted text clipped - 7 lines] > Would be the correct syntax? Or do primitives get promoted to objects > somehow? Since Java 5 this is correct syntax. The auto-boxing feature will actually compile this to: ... stuff[0] = "eggs"; stuff[1] = new StringBuffer("flour"); stuff[2] = Double.valueOf(3.56D); stuff[3] = Character.valueOf('c'); stuff[4] = Integer.valueOf(123); stuff[0] = "33"; ... Regards, Bart so what I suppose is that any superClass's reference can point to its any subClass but what the reference can access is only the superClass's interface, not the subClass's and the method it called is exactly what the subClass has overrided. is it right? jtl.zheng schreef: > so what I suppose is that any superClass's reference can point to its > any subClass > but what the reference can access is only the superClass's interface, > not the subClass's > and the method it called is exactly what the subClass has overrided. > is it right? An array of Object's can contain any Object. As every class in Java extends Object, the array can contain every single instance of any class. Without casting you can only access the methods defined in Object, that's correct, and if the method you're trying to call is overridden by the subclass, the overridden method will be called. Example: String test = "a string"; Object o = test; test.substring(5); // Correct o.substring(5); // Incorrect ((String) o).substring(5); // Correct The reason the second one is incorrect is that the compiler does not know it actually is a String. In the third line you tell the compiler to trust you (the programmer) and assume the object o references actually is a String and cast it before executing the method. regards, Bart > > stuff[2] = 3.56; > > stuff[3] = 'c'; > > stuff[4] = 123; > > But these assignments can't work at all, can they? These are > primitives, not objects, so they don't derive from Object. You are correct, but... > stuff[2] = new Float(3.56); > > Would be the correct syntax? Or do primitives get promoted to objects > somehow? Yes, the compiler (since 1.5) automatically converts the original expression into something similar to what you have. It's called "autoboxing" and is a bloody stupid idea. The compiler actually generates code to call the static valueOf(<primitive>) method rather than using an explicit constructor, which can in some cases make use of cached lists of pre-allocated instances. E.g. the line stuff[4] = 123 will generate a call to the (new in 1.5) method Integer.valueOf(int), which (from the 1.5 code, not the spec) will in fact use the cache since 123 is in the cached range [-128, 127]. -- chris >> Would be the correct syntax? Or do primitives get promoted to objects >> somehow? > > Yes, the compiler (since 1.5) automatically converts the original expression > into something similar to what you have. It's called "autoboxing" and is a > bloody stupid idea. What's stupid in that idea? Can you argue please? > The compiler actually generates code to call the static valueOf(<primitive>) > method rather than using an explicit constructor, which can in some cases make > use of cached lists of pre-allocated instances. E.g. the line stuff[4] = 123 > will generate a call to the (new in 1.5) method Integer.valueOf(int), which > (from the 1.5 code, not the spec) will in fact use the cache since 123 is in > the cached range [-128, 127]. That's from spec (JLS 3rd ed.): "5.1.7 Boxing Conversion (...) If the value p being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2." piotr Piotr Kobzda schreef: > >> Would be the correct syntax? Or do primitives get promoted to objects > >> somehow? [quoted text clipped - 4 lines] > > What's stupid in that idea? Can you argue please? I'm not totally against autoboxing, but it should be used with great care. Specially when used in conjunction with generics. Take following code: List<Integer> carefull = new ArrayList<Integer>(); carefull.add(1); carefull.add(43); carefull.add(227); carefull.remove(43); System.out.println(carefull.size()); This compiles fine and the output seems pretty straightforward, but what you get is this: Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 43, Size: 3 on the remove line. Regards, Bart > > Yes, the compiler (since 1.5) automatically converts the original > > expression into something similar to what you have. It's called > > "autoboxing" and is a bloody stupid idea. > > What's stupid in that idea? Can you argue please? The basic problem is that it makes it look as if one thing is happening, when in fact something completely different is happening. That is just bad language design, for both beginners and experts. /Especially/ when it doesn't add to the expressive power of the language. (Of course the /real/ problem here, is the use of non-object primitive types at all -- at least as the "normal" way to express numeric values -- but that's a different debate...) It (autoboxing, or indeed any other sleight-of-hand[*] in the compiler) can cause the unwary to introduce performance problems, or semantic problems, without realising it. E.g, some time ago, there was a person posting about memory problems here -- it was only after some considerable amount of speculation and debate that we realised he was holding around a million "floats" in an ArrayList (or something like that). Since he didn't realise that his -- otherwise perfectly reasonable -- Java code wasn't doing what he thought it did, he didn't initially mention how he was storing all those values. That only emerged later. Autoboxing causes (yet more) ugly little glitches with generics too -- but I can't remember exactly what and it's too hot to go searching ;-) > That's from spec (JLS 3rd ed.): Aha! Thank you for the correction. -- chris [*] "Sleight-of-hand" as in the shell-and-bean game, or its variants, where the unwary punter is induced to bet, unwisely, that the bean is under the shell it /seemed/ to be placed under.... 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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