Heap pollution

In the Java programming language, heap pollution is a situation that arises when a variable of a parameterized type refers to an object that is not of that parameterized type.¹ This situation is normally detected during compilation and indicated with an unchecked warning.¹ Later, during runtime heap pollution will often cause a ClassCastException.²

Heap pollution in Java can occur when type arguments and variables are not reified at run-time. As a result, different parameterized types are implemented by the same class or interface at run time. All invocations of a given generic type declaration share a single run-time implementation. This results in the possibility of heap pollution.²

Under certain conditions, a variable of a parameterized type may refer to an object that is not of that parameterized type. The variable will always refer to an object that is an instance of a class that implements the parameterized type.

Heap Pollution in a non-varargs context

public class HeapPollutionDemo
{
    public static void main(String[] args)
    {
        Set s = new TreeSet<Integer>();
        Set<String> ss = s;              // unchecked warning
        s.add(new Integer(42));          // another unchecked warning
        Iterator<String> iter = ss.iterator();

        while (iter.hasNext())
        {
            String str = iter.next();    // ClassCastException thrown
            System.out.println(str);
        }
    }
}