Question

I’m trying to write a class that has a generic member variable but is not, itself, generic. Specifically, I want to say that I have an List of values of “some type that implements comparable to itself”, so that I can call sort on that list… I hope that makes sense.

The end result of what I’m trying to do is to create a class such that I can create an instance of said class with an array of (any given type) and have it generate a string representation for that list. In the real code, I also pass in the class of the types I’m passing in:

String s = new MyClass(Integer.class, 1,2,3).asString();
assertEquals("1 or 2 or 3", s);
String s = new MyClass(String.class, "c", "b", "a").asString();
assertEquals("\"a\" or \"b\" or \"c\"", s);

Originally I didn’t even want to pass in the class, I just wanted to pass in the values and have the code examine the resulting array to pick out the class of the values… but that was giving me troubles too.

The following is the code I have, but I can’t come up with the right mojo to put for the variable type.

public class MyClass {
    // This doesn't work as T isn't defined
    final List<T extends Comparable<? super T>> values;

    public <T extends Comparable<? super T>> MyClass (T... values) {
        this.values = new ArrayList<T>();
        for(T item : values) {
            this.values.add(item);
        }
    }

    public <T extends Comparable<? super T>> List<T> getSortedLst() {
        Collections.sort(this.values);
        return this.values;
    }
}

error on variable declaration line:

Syntax error on token "extends", , expected

Any help would be very much appreciated.

Edit: updated code to use List instead of array, because I’m not sure it can be done with arrays.

@Mark: From everything I’ve read, I really want to say “T is a type that is comparable to itself”, not just “T is a type that is comparable”. That being said, the following code doesn’t work either:

public class MyClass {
    // This doesn't work
    final List<? extends Comparable> values;

    public <T extends Comparable> MyClass (T... values) {
        this.values = new ArrayList<T>();
        for(T item : values) {
            this.values.add(item);
        }
    }

    public <T extends Comparable> List<T> getSortedLst() {
        Collections.sort(this.values);
        return this.values;
    }
}

error on add line:

The method add(capture#2-of ? extends Comparable) in the type List<capture#2-of ? extends Comparable> is not applicable for the arguments (T)

error on sort line:

Type mismatch: cannot convert from List<capture#4-of ? extends Comparable> to List<T>

Conclusion:

What it comes down to, it appears, is that Java can’t quite handle what I want to do. The problem is because what I’m trying to say is:

I want a list of items that are
comparable against themselves, and I
create the whole list at once from the
data passed in at creation.

However, Java sees that I have that list and can’t nail down that all the information for my situation is available at compile time, since I could try to add things to the list later and, due to type erasure, it can’t guarantee that safety. It’s not really possible to communicate to Java the conditions involved in my situation without applying the generic type to the class.

Answer 1

I think that the simple answer is that you cannot do that. If the type of one of a classes attributes depends on a type parameter, that parameter has to be declared at the class level. And I don’t think that it “makes sense” any other way.

If T in your example is not a type parameter of the class, what is it? It cannot be the type parameter of the method, because that type is determined by how the method is called. (If the method is called in different static contexts with different inferred types for T, what is the notional type of T in the context of the attribute declaration?)

So to bring this back to what you are trying to do here, an instance of MyClass will hold elements of some type, and you want to be able to insert and remove elements in a statically typesafe fashion. But at the same time you don’t want to be able to say what that type is. So how is the compiler supposed to statically distinguish between a MyClass instance that holds (say) Integer objects and one that holds String objects?

I don’t even think you could implement this with explicit dynamic typechecks. (I think that type erasure means that the implementation of the getSortedList() method cannot find out what actual type is bound to its return type.)

No. The real solution is to make MyClass a generic class that declares the type parameter T; e.g.

public class MyClass <T extends Comparable<T>> {

and remove the declaration of the method-level type parameter T from the two methods.