There's only one level of indirection. References are simply pointers. You can't see the raw bits of the pointer without Unsafe, but that's how the Sun JVM implements references. (The implementation calls them "ordinary object pointers".) GC rewrites these pointers when it moves objects; there is no separate lookup table of all objects.
You might have been mislead by the existence of Object.identityHashCode(). The identity hash code is not a memory address and not guaranteed unique. It's an arbitrary value stored in some bits of the object's mark word and copied around when the object moves. That's how it remains stable across GCs.
Or you might be thinking of "compressed oops". Those are still pointers, but encoded as (object_address - start_of_heap) >> 3 to save space.
I think there is a global lookup table for interned strings and symbols loaded from class files, but slots in that table are not themselves pointed to by references.
If a program compiles with no unchecked or raw warnings, the synthetic casts inserted by the compiler will never fail.
class Unsound {
static class Constrain<A, B extends A> {}
static class Bind<A> {
<B extends A>
A upcast(Constrain<A, B> constrain, B b) {
return b;
}
}
static <T, U> U coerce(T t) {
Constrain<U, ? super T> constrain = null;
Bind<U> bind = new Bind<U>();
return bind.upcast(constrain, t);
}
public static void main(String[] args) {
String zero = Unsound.<Integer, String>coerce(0);
}
}
I imagine C# was able to learn from what was going on with Java at the time and not have to suffer the same fate. Or maybe it was just less popular and could force the issue (I’ve never dealt with it so I don’t know the history well).
In Java, List<Cat> may or may not be the subclass of List<Animal>, but it is up to the List implementor. This way Scala/Kotlin/another JVM language is free to define their own variance model independent of the host language. C# did limit their language ecosystem with it quite a bit. (Afaik Scala for CLR stopped in part due to this).
https://docs.microsoft.com/en-us/dotnet/csharp/language-refe... https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...
You probably know this, but I'm replying in the hope it clarifies things to other readers. The way it's expressed in Java is List<? extends Animal>
Also, unfortunately arrays are covariant so Cat[] is a subclass of Animal[] (both in Java and C# actually), where your mentioned example indeed introduces a “poisoned” value, waiting for a classcastexception.
What makes you think this is the case? Pointers to objects are direct, and garbage collectors freely change pointer values whenever it decides to relocate objects. Some collectors might use temporary forwarding pointers, to support concurrent collection.
This is not how Java works. I stopped reading after that.
1. No object value types; and
2. Boxing and primitives.
On (1), all objects in Java are references, being a pointer to a pointer effectively. This indirection has a cost but has been incredibly useful historically because it enables moving allocation around the address space because none of the references change. They're effectively just pointers to a lookup table and it's the lookup table that changes.
As for the example of packing an array of Points into a contiguous block of memory to avoid the dereferencing, I'll be curious to see what the proposal is here. Now, for example, if you have a collection of Points (x,y), you can put in a 3DPoint (x,y,z) that extends Point. You can't do that if you've flattened storage, not without some overhead at least.
For years it's been the dream in Java to have true value Object types. You can do this in C++, which also allows you to allocate onto the stack. My position is that the complexity cost of this is massive. Copy constructors, move constructors, assignment operators, implicit constructors, implicit casts, etc. I mean you can have class A with instance a1 on the stack and a2 on the heap and pass &a1 and &a2 to a A* parameter and the callee has absolutely no idea of the lifetime of those or where they're allocated.
But still, it would be nice to pass around an SHA1 hash as a 20 byte value instead of a reference to an object that contains a reference to an array that contains 20 byte elements.
As for (2), this was a deliberate design choice in Java 5 (1.5) when generics were added. The decision was made, for better or for ill, to retain backwards compatibility through type erasure such that a List<T> is a List. This decreased the pain of upgrading and legacy code but created this ugly corner of primitive types.
C# 2.0 came along later and decided to go the other way such an IList<T> is not an IList (IIRC the types; I'm not a C# guy).
There are pros and cons.
The fact that certain values are guaranteed to satisfy reference equality is kind of weird and (IMHO) confusing ie new Integer(127).equals(new Integer(127)) is true but new Integer(128).equals(new Integer(128)) isn't necessarily true. I guarantee you a lot of people don't know that.