Saturday, May 26, 2007

Removing Language Features?

As a language grows by the addition of features, it necessarily gets more complex. After all, you can't remove existing language features because existing programs use those features, but each additional feature adds complexity. Right?

Fellow Googler Matt Shulman asked me a question about the Closures for Java specification. He observed that much of the complexity arises because of support for checked exceptions in the spec. Things like throws type parameters, disjunctive types, and throws clauses on function interfaces would be unnecessary without checked exceptions. Matt asked me if we had considered if things would be simpler without all that. At first I misunderstood his question to be referring to just the Closures specification, so I answered that the facility wouldn't fit into the language as well without support for checked exceptions.

Matt clarified that he was asking not just about removing support for checked exceptions from the Closures spec, but from the entire programming language.

There has been an ongoing debate on the utility of checked exceptions. Many people are critical of Java's checked exceptions, characterizing them as a failed experiment in software engineering. In practice, checked exceptions can result in API complexity, and programs appear to be cluttered with exception handling code just to satisfy the compiler. Some people believe checked exceptions are a good language feature but are misused, even in the JDK. With the "experts" being such poor role models, how can we expect ordinary Java programmers to do better?

We did a Google search to see how many people have written in support of checked exceptions and how many people don't like them. The discussion seems to be lopsided against checked exceptions, but on the other hand that may be due to the fact that checked exceptions are the status quo.

This isn't a question I had thought much about. I believe the language could be simplified by treating all exception types as unchecked without breaking existing programs. This could also result in a simplification of future language extensions and APIs. But would the language and platform be better off without checked exceptions?

Sunday, May 20, 2007

A Limitation of Super Type Tokens

Watching Josh Bloch's presentation at JavaOne about new topics in the second edition of Effective Java makes me want to go out and get my own copy. Unfortunately, he's not scheduled to have the new edition in print until later this year.

There was a coincidental adjacency between two slides in Josh's talk that made me think a bit more about the idea of Super Type Tokens. The last slide of his discussion of generics gave a complete implementation of the mind-expanding Typesafe Heterogenous Containers (THC) pattern using Super Type Tokens:

import java.lang.reflect.*;

public abstract class TypeRef<T> {
    private final Type type;
    protected TypeRef() {
        ParameterizedType superclass = (ParameterizedType)
        type = superclass.getActualTypeArguments()[0];
    @Override public boolean equals (Object o) {
        return o instanceof TypeRef &&
    @Override public int hashCode() {
        return type.hashCode();

public class Favorites2 {
    private Map<TypeRef<?>, Object> favorites =
        new HashMap< TypeRef<?> , Object>();
    public <T> void setFavorite(TypeRef<T> type, T thing) {
        favorites.put(type, thing);
    public <T> T getFavorite(TypeRef<T> type) {
        return (T) favorites.get(type);
    public static void main(String[] args) {
        Favorites2 f = new Favorites2();
        List<String> stooges = Arrays.asList(
            "Larry", "Moe", "Curly");
        f.setFavorite(new TypeRef<List<String>>(){}, stooges);
        List<String> ls = f.getFavorite(
            new TypeRef<List<String>>(){});

But on the very next slide, the very first bullet of the summary of his presentation reminds us

  • Don't ignore compiler warnings.

This was referring to Josh's advice earlier in the presentation not to ignore or suppress unchecked compiler warnings without trying to understand them. Ideally, you should only suppress these warnings when you have good reason to believe that the code is type-safe, even though you might not be able to convince the compiler of that fact.

The method Favorites2.getFavorite, above, is annotated to suppress a warning from the compiler. Without that annotation, the compiler complains about the cast to the type T, a type parameter. Is this code demonstrably type safe? Is it possible to cause this cast to fail using code that is otherwise completely type safe? Unfortunately, the cast is not safe:

class Oops {
    static Favorites2 f = new Favorites2();

    static <T> List<T> favoriteList() {
        TypeRef<List<T>> ref = new TypeRef<List<T>>(){};
        List<T> result = f.getFavorite(ref);
        if (result == null) {
            result = new ArrayList<T>();
            f.setFavorite(ref, result);
        return result;

    public static void main(String[] args) {
        List<String> ls = favoriteList();
        List<Integer> li = favoriteList();
        for (String s : ls) System.out.println(s);

This program compiles without warning, but it exposes the loopole in the type system created by the cast to T in Favorites2.getFavorite. The compiler's warning does, after all, tell us about a weakness in the type safety of the program.

The issue is a subtle one: TypeRef treats two types as the same when the underlying java.lang.reflect.Type objects are equal. A given java.lang.reflect.Type object represents a particular static type appearing in the source, but if it is a type variable it can represent a different dynamic type from one point in the program's execution to another. The program Oops exploits that mismatch.

The Super Type Token pattern can be redeemed by disallowing the use of type variables anywhere in the Type object it stores. That can be enforced at runtime (but not at compile time) in the constructor.

Perhaps a better solution would be to reify generics (i.e., "erase erasure") in the language, making all this nonsense unnecessary.