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== Structure section needs a lot of work ==
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:I've made some attempts at cleaning up the large table and elaborating on some of the details. I also removed the fake "C structure" section entirely; it was not even close to being correct C code, and was not at all useful to this article. - [[User:Dmeranda|Dmeranda]] 21:09, 23 October 2007 (UTC)
== C programming language representation belongs in article ==
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: I appreciate trying to make this article more readable, but with this case I disagree. The ''reason'' is apparently because the JVM spec authors don't know C, or didn't care, or because they follow the code with dozens of pages of clarifying text which then go on to explain why the format is actually not like the pseudo-C code they just presented. This example code is not even remotely close to being correct C code (or even Java code), and trying to present it as such is quite misleading. Most of the "types" are of undefined variable length (C types are always fixed length). No padding bytes are ever introduced (as C frequently does). A definite byte-order is mandated, which C can never indicate. The things which look like C arrays are stored absolutely nothing like real C arrays. Even the indicated "length" of the arrays is completely wrong in the constant_pool case due to the double-slot flaw. In fact, just about everything in the file format is so completely not like C that I wonder what in the world the writers of the specification were thinking. If a reader can't understand a table of simple file offsets and wants to pretend like they can use a C structure then they can read the spec; that's why it's listed in the references. This article is not trying to replace the entire quite-lengthy specification, it is only showing a ''flavor'' of the format. Copying large blocks of poorly-authored text verbatim from the referenced document (which shouldn't be done regardless of quality) and taken out of context (from all the clarifying text that follows) provides no extra value here, and only clutters the article and adds about a dozen ways the reader can be easily misled. - [[User:Dmeranda|Dmeranda]] 14:08, 8 November 2007 (UTC)
== Re structure and representation ==
One thing that isn't mentioned in the structure section, and is a
very convenient thing to know in practice, is the assurance from
JVMS chapter 4 that the concrete representations of various data
types in the class file are exactly the representations consumed
and produced by java.io.DataInputStream and java.io.DataOutputStream,
respectively.--[[Special:Contributions/128.210.4.213|128.210.4.213]] ([[User talk:128.210.4.213|talk]]) 22:47, 3 January 2008 (UTC)
== Re history ==
It might be very useful for the history section to summarize the
differences between class file versions in terms of which new
attributes become mandatory in 46.0, 49.0, and 50.0 (JSR202 JVMS
revised p. 126), and the opcodes to become illegal in 51.0 (p. 166).
I haven't said more because I am not sure how much would be
disclosable under the annoying EULA I had to accept before reading
JSR202, but if others think such a brief summary could be allowed,
I think it would be a valuable addition to the page. Also, footnote
1 on revised p. 96 supplies a relationship between class version
and JDK version for JDK versions 1.2 and onward.--[[Special:Contributions/128.210.4.213|128.210.4.213]] ([[User talk:128.210.4.213|talk]]) 22:47, 3 January 2008 (UTC)
:Is this sort of history noteworthy, or is it sufficient to just link to more detailed external specifications? Certainly I don't think this article should get to the level of mentioning specific JVM opcodes. As far as using non-free sources of information, remember that anything here must be able to meet both the [[WP:V]] and [[WP:C]] rules. Many people including myself will never cross the EULA barrier, so it is hard to offer advise on what's allowed or not in this case. Perhaps you can get better answers from [[Wikipedia:Copyright assistance]]. - [[User:Dmeranda|Dmeranda]] ([[User talk:Dmeranda|talk]]) 15:54, 4 January 2008 (UTC)
::What I mostly thought would be useful was a high-level, one-liner sort of overview of what has happened between class-format/JVM revisions. Of course from one JDK version to the next there are always hundreds of changes to the Java library APIs and often changes to the source syntax as well; but changes to the virtual machine and the class file format are less common (a good thing, because they affect deployability of compiled code). When such changes are made, they tend to reveal interesting fundamental things about what new Java features were unimplementable on the older VM, and what minimal set of VM changes were required to support them. The information also helps in understanding which capabilities you are likely to lose when using -target to generate an older class format.
::I did find a nice EULA-free description of what happened in class format 50.0: [https://jdk.dev.java.net/verifier.html 50.0 type checking verifier].--[[Special:Contributions/128.210.4.214|128.210.4.214]] ([[User talk:128.210.4.214|talk]]) 19:12, 8 January 2008 (UTC)
== Major and Minor Version values in Class File header ==
What is the source for the description given for the Major version in the class file header?
I checked a number of JDK Java compilers, and found
* 45.3: javac 1.2.2
* 46.0: javac 1.4.2_12,1.4.2_14
* 49.0: javac 1.5.0_11,1.5.0_12
* 50.0: javac 1.6.0_06
So there is some difference to the description text for the major version given in this article, at least some JDK 1.4.x versions seem to be using major 46 instead of 48, and at least some 1.2.x versions use 45 instead of 46.
== boolean, byte, and short in constant pool ==
The article states, "Other integral types appearing in the high-level language, such as boolean, byte, and short must be represented as an integer constant."
While it's true that if you wanted to include a constant of one of these types into the constant pool, an integer entry
was needed, a standard compiler will never do so as there are instructions (bipush and sipush) for using small integer values directly without the need for a constant pool entry at all.
Since the Java bytecode makes no difference between these integral types, all int values fitting into the short value range are encoded this way. Only values outside this range or needed for initializing compile-time constants (e.g. static final fields) will end up in the constant pool. <span style="font-size: smaller;" class="autosigned">—Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/77.188.94.56|77.188.94.56]] ([[User talk:77.188.94.56|talk]]) 15:56, 22 February 2011 (UTC)</span><!-- Template:UnsignedIP --> <!--Autosigned by SineBot-->
: Indeed, byte, short, and smaller char constant values used in the code will never appear in the constant pool (and maybe this is worth mentioning in the article). But for other uses of constant pool entries, e.g. the compile-time values of static final variables, these types require an int entry. There is another difference between the general class file format and the byte code instructions: as the article states, there’s no padding in the class file format, but within byte code instructions there is an exception to this rule regarding the two switch instructions as their tables are aligned. [[Special:Contributions/77.12.173.11|77.12.173.11]] ([[User talk:77.12.173.11|talk]]) <span style="font-size: smaller;" class="autosigned"> — Preceding [[Wikipedia:Signatures|undated]] comment added 09:40, 16 April 2014 (UTC)</span><!--Template:Undated--> <!--Autosigned by SineBot-->
::without jdk why we cant run the java programm in comand promnt [[Special:Contributions/119.235.54.130|119.235.54.130]] ([[User talk:119.235.54.130|talk]]) 09:17, 3 April 2024 (UTC)
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