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Java Forum / General / March 2008Arrays and variables
Hello all, Can someone confirm this to me. -> All the arrays when created are filled with 0, or null. Or is it random numbers? Thanks, Francois > Hello all, > [quoted text clipped - 4 lines] > > Thanks, Francois http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#23658 >Then, if a single DimExpr appears, a single-dimensional array is >created of the specified length, and each component of the array is >initialized to its default value (§4.12.5). > Hello all, > > Can someone confirm this to me. > > -> All the arrays when created are filled with 0, or null. Or is it > random numbers? Java (unlike C or C++) never initializes anything to random values. Array members, like other object fields, are initialized to 0 for numeric types false for booleans null for reference types >> -> All the arrays when created are filled with 0, or null. Or is it >> random numbers? [quoted text clipped - 5 lines] > false for booleans > null for reference types Not quite. Local variables (including local arrays) do not have default values. You are required to initialize them yourself. <http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.12.5> But it's never "random." Variables will always be initialized somehow in Java. >>> -> All the arrays when created are filled with 0, or null. Or is >>> it [quoted text clipped - 9 lines] > Not quite. Local variables (including local arrays) do not have > default values. You are required to initialize them yourself. Local variables are neither array members nor object fields. There is no such thing in Java as a "local array". If you mean something like: void method() { int[] iarr = new int[20]; ... } the 20 members of iarr are, indeed, all initialized to 0. > <http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.12.5> > > But it's never "random." Variables will always be initialized > somehow > in Java. That is "Java (unlike C or C++) never initializes anything to random values." I won't say that I'm always right, and I welcome corrections when I'm wrong, but this isn't one of those times. > I won't say that I'm always right, and I welcome corrections when I'm > wrong, but this isn't one of those times. You said "anything." I'm not correcting you, per se, just clarifying for other readers who might get confused. >> I won't say that I'm always right, and I welcome corrections when I'm >> wrong, but this isn't one of those times. > > You said "anything." I'm not correcting you, per se, just clarifying > for other readers who might get confused. He said, > Java (unlike C or C++) never initializes anything to random values. That is truth. If something is not initialized to a known value, it generates a compiler error. <http://java.sun.com/docs/books/jls/third_edition/html/defAssign.html> You said, > Local variables (including local arrays) do not have default values. That is not truth. Locally declared arrays are initialized to default values.  SignatureLew >That is "Java (unlike C or C++) never initializes anything >to random values." In Java, in { final double d = java.lang.Math.random(); } , »d« will be initialized to a random value. (Run-time operations, of course, are not done by »Java«, but by the JVM.) In C, in { double d; } , »d« will not be initialized to a random value, it will hold an /indeterminate/ value (ISO/IEC 9899:1999 (E), 6.7.8, p10). An indeterminate value is not a random value. But then, actually, there are not random values (is »7« a random value?), only random sequences. >> That is "Java (unlike C or C++) never initializes anything >> to random values." [quoted text clipped - 4 lines] > > , »d« will be initialized to a random value. Pseudo-random. SignatureLew >> That is "Java (unlike C or C++) never initializes anything >> to random values." [quoted text clipped - 4 lines] > > , »d« will be initialized to a random value. Not at all; if you know the algorithm used and the value of its inouts, you can calculate that value precisely. It's no more random than l in long l = System.currrentTimeMillis(); > (Run-time operations, of course, are not done by »Java«, > but by the JVM.) [quoted text clipped - 5 lines] > , »d« will not be initialized to a random value, it will hold > an /indeterminate/ value (ISO/IEC 9899:1999 (E), 6.7.8, p10). It's random as far as the programmer is concerned: it could literally be any bit pattern at all. >-> All the arrays when created are filled with 0, or null. Or is it random >numbers? see http://mindprod.com/jgloss/initialisation.html http://mindprod.com/jgloss/array.html one of the features of Java that initially attracted me to it was the deterministic initialisation and in general precisely repeatable results from running a program more than once. I went nearly insane with C/C++ random/lack of initialisation causing bugs that appeared and disappeared at random. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >> -> All the arrays when created are filled with 0, or null. Or is it >> random numbers? [quoted text clipped - 8 lines] > with C/C++ random/lack of initialisation causing bugs that appeared > and disappeared at random. My favorite thing was that running under a debugger often perturbed them out of existence. 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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