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Bytecode features not available in the Java language
- malft 11y agoDidn't see my favorite on the list: creating the bool 2. I believe there are a couple of methods in the JDK that do if (dangerousFlag == true) doCheck(); ... foo(dangerousFlag ? bar : baz);
- TheLoneWolfling 11y agoThat's in the list: > Overflow and implicitly assign byte, short, char and boolean values
- peeters 11y agoWait, I love learning new things about Java, but I feel in the dark here. What is your example demonstrating?
- onedognight 11y agoIf true is book(1) then the check bool(2) == true will fail but bool(2) ? a : b will act as if bool(2) were true and this inconsistency could lead to unintended execution paths.
- peeters 11y agoAh, I'm with you. Thanks.
- TheLoneWolfling 11y agoThe valid values of booleans / bytes / shorts / chars are only enforced in the Java compiler, not actually in bytecode. But the Java compiler assumes values will be in range - i.e. that a boolean will be zero or one, that sort of thing. And as such it makes some optimizations that are... questionable, if you're interfacing with bytecode that wasn't produced by the Java compiler. For example, if `flag` is supposedly a boolean, but you gave it the value `2` directly via bytecode, flag==true is false, as it gets compiled down to flag == 1, which is false, but flag?bar:baz will return bar. This can seriously break static analysis, among other things.
- whitten 11y agoIf the Java Virtual Machines don't always execute certain Java Byte Codes, how does a compiler to JVB for another language know that the generated code is run ?
- pests 11y agoWhich bytecodes do you mean? If you're talking about the jsr/ret thing they still have functionality but the compiler just doesn't generate those.
- krilnon 11y agoThere's an official specification for what and how a program that claims to be a JVM implementation should operate given Java bytecode. So someone writing a language that compiles to Java bytecode would typically either target the specification (and test on as many implementations as they felt necessary), or target a specific implementation like HotSpot and ignore the rest.
- kragen 11y agoWhat is the sound of one hand clapping?
- peeters 11y agoAs far as not catching checked exceptions goes, this is actually possible in the Java language as well since the language allows throwing a generic type parameter. It's pretty difficult to do by accident though, and does at least give a compile warning. public class Dangerous { public static void main (String ... args) { Dangerous.<RuntimeException>throwUnchecked(new IOException("Forgot to catch me!")); } static <T extends Exception> void throwUnchecked(Exception e) throws T{ throw (T) e; } }
- quotemstr 11y agoYou can define two methods that differ only by return type. That's how covariant returns work. Check the javap output and recoil in horror.
- peeters 11y agoTrue. One of them is a bridge method. Here's an example: class CovariantReturn extends CovariantReturnBase { public Integer produce() { return 5; } } class CovariantReturnBase { public Number produce() { return 4.5; } } And the relevant javap output for CovariantReturn: public java.lang.Integer produce(); flags: ACC_PUBLIC Code: ... public java.lang.Number produce(); flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC Code: ...
- jodah 11y agoI knew the SO answer must have been written by Raf before I even got to the author section. Check out his great library, ByteBuddy, if you want to try mucking with some of this stuff: http://bytebuddy.net http://bytebuddy.net