[e-lang] Type safety of Java generics

[David Hopwood]

/*
In the Friday meeting, one of the things we discussed was the type safety
of Java 1.5 generics. I said that I would come up with an example in which:

 - the compiler does not output any warnings;
 - there are reflective operations, but none of those operations fail
   at run-time;
 - a non-reflective operation fails with a type error at run-time.

Here is the example.
*/

import java.util.ArrayList;
import java.lang.reflect.*;

public class UnsafeGenerics {
    public static void main(String[] args) {
        // After type erasure, this is really an ArrayList of Object.
        ArrayList<String> a = new ArrayList<String>();

        // The reflection API can put a non-String into the ArrayList.
        try {
            Method m = ArrayList.class.getMethod("add", Object.class);
            m.invoke(a, new Integer(42));
        } catch (Exception e) {
            System.err.println("reflective operation failed");
            throw new RuntimeException(e);
        }

        // This doesn't provoke a warning, but fails with a
        // ClassCastException at run-time.
        String s = a.get(0); // line 32
    }
}

>javac UnsafeGenerics.java

>java UnsafeGenerics
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer
cannot be cast to java.lang.String
        at UnsafeGenerics.main(UnsafeGenerics.java:32)

>java -version
java version "1.6.0_01"
Java(TM) SE Runtime Environment (build 1.6.0_01-b06)
Java HotSpot(TM) Client VM (build 1.6.0_01-b06, mixed mode, sharing)

>javac -version
javac 1.6.0_01


What is objectionable about this is that:

 - the reflection API silently allowed an object to get into a state that
   violates its declared type;
 - the compiler inserted an *implicit* cast that failed, without any warning.

Of course, in a real program the cast failure could be arbitrarily far
away (in time and code location) from the unsafe reflective operation.

-- 
David Hopwood <david.hopwood at industrial-designers.co.uk>