Sun to Add Variance to Java in 1.5?

Ahe writes "I have been working on a joint project between Sun Microsystems, The University of Aarhus, and The Alexandra Institute. As you might know, Sun has for long been commited to adding generics to Java. Our project was to extend this with variance annotations for more flexible typing of parameterized classes and arrays. Recently Sun has released the project result as a new prototype with variance. If you like variance, please vote for this bug."

Comments

Short description of variance

[Ahe]

The term variance covers co-variance, contra-variance, and bi-variance. In the 1.5 prototype you can use variance annotations on parameterized types.

+ means co-variant (think read-only).
- means contra-variant (think write-only).
* mean bi-variant (niether read nor write).

You are familiar with co-variance from arrays in Java.

When an array of Integer is a subtype of array of Number, arrays are said to be covariant in their element type. Consider:

Number[] ns = new Integer[10];

With generics, you can have a list of Numbers:

List<Number> nl = new List<Number>();

However, since generics are added without changing the JVM, nl can only refer to lists of exactly Number, e.g., this is wrong:

List<Number> nl = new List<Integer>();

So why is this wrong, consider arrays:

Number[] ns = new Integer[10];
ns[0] = new Double(0.0); // run-time error

Since ns refers to an array of Integer, we cannot put Double in to it. The mechanism that catches this is called store-check.

Since the JVM is not modified, we cannot implement a store-check for generic classes. Variance annotations allows us more flexibility, however.

Consider this method:

void copy(Collection<+Number> src, Collection<-Number> dst) {
for (Number n : src) { dst.add(n); }
}

Here we use variance annotations to state that we will only read from src and only write to dst. In this way we are allow to make a call like this:

List<Integer> il = new List<Integer>();
List<Object> ol = new List<Object>();
copy(il, ol);

Sometimes we are not interested in reading or writing elements, so we can say:

List<*> list_of_unknown_type = ...;

Then we can only use methods like size() on list_of_unknown_type.

Re:Definitely a bug!

[elflord]

Generics are at best a trade-off: compile-time type safety in exchange for less readable code.

Given the choice between explicitly using a parameter, and using a nebulous "Object", I'd say that the template code is easier to read, because the parameter conveys more meaning than the word "object" (which offers no clue about what "type" is being used)

Where C++ has no safety without generics, Java has runtime safety.

Wrong. See dynamic_cast.

As for readability, C++ has two major benefits over Java: a preprocessor and typedefs

Shows how much you know -- you've named what are arguably two of the most botched legacy features of C++.

Without at least one of these features, generics make code almost unreadable.

This is not true. What makes code difficult to comprehend is complexity (allocaters, classes with several parametrised types, etc). Fortunately, C++ offers features that make complexity manageable (though typedefs don't deal with templates as well as they should)

(I say mostly, because using STL with threads in cross-platform development is a recipe for disaster

No, naively assuming that every STL implementation on every platform is thread safe is a recipe for disaster. Well, duh! The same is true even for straight C code -- you need to use thread safe versions of the library functions.