Artifact for Java and Scala's Type Systems are Unsound: The Existential Crisis of Null Pointers

We provide the three programs as listed in the paper (Amin and Tate, OOPSLA'16 forthcoming), and step-by-step instructions on how to compile and run them. We also offer an interactive playground.

• Unsound.java for Java 8
• Unsound9.java for Java 9
• unsound.scala for Scala

These short programs demonstrate the unsoundness of Java and Scala’s current type systems. In particular, these programs provide parametrically polymorphic functions that can turn any type into any type without casting. Fortunately, parametric polymorphism was not integrated into the Java Virtual Machine (JVM), so these examples do not demonstrate any unsoundness of the JVM

Getting Started Guide

You need JDK 8, JDK 9, Scala 2.11.8. You need to be able to use javac, java, scalac, scala. You need to be able to switch between JDK 8 and JDK 9. We give further pointers within the step by step instructions below.

Step-by-Step Instructions

Java

JDK 8 (Download)

Step 1: Check version

java -version

java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)

If necessary, switch to JDK 8: export JAVA_HOME=$(/usr/libexec/java_home -v 1.8) ; PATH=$JAVA_HOME/bin:$PATH

Step 2: Compile

javac Unsound.java

(compiles OK)

Step 3: Run

java Unsound

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  at Unsound.main(Unsound.java:15)

JDK 9 (Download)

Step 1: Check version

java -version

java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+121)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+121, mixed mode)

If necessary, switch to JDK 9: export JAVA_HOME=$(/usr/libexec/java_home -v 9) ; PATH=$JAVA_HOME/bin:$PATH

Step 2: Compile

javac Unsound9.java

(compiles OK)

Note that Unsound.java does not compile with JDK 9.

javac Unsound.java

Unsound.java:12: error: method upcast in class Bind<A> cannot be applied to given types;
    return bind.upcast(constrain, t);
               ^
  required: Constrain<U,B>,B
  found: Constrain<U,CAP#1>,T
  reason: inference variable B has incompatible bounds
    upper bounds: U
    lower bounds: T
  where U,T,B,A are type-variables:
    U extends Object declared in method <T,U>coerce(T)
    T extends Object declared in method <T,U>coerce(T)
    B extends U declared in method <B>upcast(Constrain<A,B>,B)
    A extends Object declared in class Bind
  where CAP#1 is a fresh type-variable:
    CAP#1 extends U super: T from capture of ? super T
1 error

Step 3: Run

java Unsound9

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer (in module: java.base) cannot be cast to java.lang.String (in module: java.base)
  at Unsound9.main(Unsound9.java:25)

Scala (Download)

Step 1: Check version

scala -version

Scala code runner version 2.11.8 -- Copyright 2002-2016, LAMP/EPFL

java -version

java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)

Scala does not support JDK 9 yet, so switch back to JDK 8 if necessary. You can also switch back to an earlier JDK, e.g. 6, to try the steps with more previous versions of Scala (download list). The earliest version we could get to work with JDK 8 is 2.10.2, and all the releases we sampled in between up to the current milestone 2.12.0-M4 reproduce the remaining steps. The example does not compile in Scala 2.9.3 and earlier due to restrictions on dependent method types. In unsound_legacy.scala, we give an example without dependent method types that exhibits the same issue using only features already available in Scala 2.5.0.

Step 2: Compile

scalac unsound.scala

(compiles OK)

Step 3: Run

scala unsound

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  at unsound$.main(unsound.scala:14)
  at unsound.main(unsound.scala)
  at ...