: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.

{{cite web|title=The Java SE Tutorials|url=https://docs.oracle.com/javase/tutorial/java/generics/nonReifiableVarargsType.html#heap_pollution|publisher=Oracle|accessdate=16 July 2014}}

This situation is normally detected during compilation and indicated with an unchecked warning. Later, during runtime heap pollution will often cause a ClassCastException.

{{cite web|last1=Langer|first1=Angelika|title=Java Generics FAQs: Heap pollution|url=https://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html#Topic2|website=angelikalanger.com/|accessdate=15 July 2014}}

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.

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();

Set ss = s; // unchecked warning

s.add(new Integer(42)); // another unchecked warning

Iterator iter = ss.iterator();

while (iter.hasNext())

{

String str = iter.next(); // ClassCastException thrown

System.out.println(str);

}

}

}