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0 pointsjawher13y ago0 comments

> The JVM, at its core, is not working with static types. The bytecode itself is free of static types, except for when you want to invoke a method, in which case you need a concrete name for the type for which you invoke that method ...

Not sure what gave you this impression, as the majority of Java bytecode instructions are typed. For example, the add instruction comes in typed variants: iadd (for ints), ladd (for longs), dadd (for doubles), fadd (floats), etc.

The same is true for most other instructions: the other arithmetic instructions (div, sub, etc.), the comparison instructions (*cmp), pushing constants on the stack, setting and loading local variables, returning values from methods, etc.

http://en.wikipedia.org/wiki/Java_bytecode_instruction_listi...

InvokeDynamic, as you point it out, was added to make implementing dynamic languages on the JVM easier, because the JVM was too statically typed at its core.

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bad_user13y ago

Arithmetic operations on numbers are not polymorphic, but polymorphism has nothing to do with static typing per se. You're being confused here by the special treatment the JVM gives to primitives, special treatment that was needed to avoid the boxing/unboxing costs, but that's a separate discussion and note that hidden boxing/unboxing costs can also happen in Scala, which treats numbers as Objects.

Disregarding primitives, the JVM doesn't give a crap about what types you push/pop the stack or what values you return.

invokeDynamic is nothing more than an invokeVirtual or maybe an invokeInterface, with the difference that the actual method lookup logic (specific to the Java language) is overridden by your own logic, otherwise it's subject to the same optimizations that the JVM is able to perform on virtual method calls, like code inlining:

http://cr.openjdk.java.net/~jrose/pres/200910-VMIL.pdf

> ... because the JVM was too statically typed at its core

Nice hand-waving of an argument, by throwing a useless Wikipedia link in there as some kind of appeal to authority.

I can do that too ... the core of the JVM (the HotSpot introduced in 1.4) is actually based on Strongtalk, a Smaltalk implementation that used optional-typing for type-safety, but not for performance:

http://strongtalk.org/ + http://en.wikipedia.org/wiki/HotSpot#History

jawherOP13y ago

> Nice hand-waving of an argument, by throwing a useless Wikipedia link in there as some kind of appeal to authority

No need to get agressive over this :) I disagreed with your first comment regarding the dynamic nature of the JVM, and replied trying to explain why.

I posted the wikipedia link not as kind of an "appeal to authority", but to give the readers a full listing of bytecode instructions, so that they can check what I was saying for themselves.

> Disregarding primitives, the JVM doesn't give a crap about what types you push/pop the stack or what values you return.

It depends how you see things: the JVM can't possibly provide instructions for every possible user type, so apart from primitives, the other object types are passed around as pointers or references, but whenever you try to do something other than storing/loading them on the stack, the type checking kicks in, ensuring that the reference being manipulated has the right types.

For instance, the putfield instruction doesn't just take the field name where the top of the stack is going to get stored. It also takes the type of the field as a parameter, to ensure that the types are compatible.

Constrast this to Python's bytecode, where the equivalent STORE_NAME (or the other variants) doesn't ask you to provide type informations.

But then again, we might be splitting hairs here: since this type checking is happening at runtime (when the JVM is running your code), you could indeed question calling it "static typing", which is usually performed at compile time (an is partially performed by the java compiler for example).

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