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);
        }
    }
}

• Gosling, James; Joy, Bill; Steele, Guy; Bracha, Gilad; Buckley, Alex (2014). "4.12.2". The Java Language Specification, Java SE 8 Edition. Addison-Wesley. pp. 81–82. ISBN 978-0-13-390069-9. https://books.google.com/books?id=mh-KAwAAQBAJ&pg=PA81. 
• Friesen, Jeff (2011). Beginning Java 7. Expert's voice in Java. Apress. p. 211. ISBN 978-1430239093. https://books.google.com/books?id=CwSaQpCtfPkC&pg=PA211. 
• Reese, Richard; Reese, Jennifer (2012). Java 7 New Features Cookbook. Packt Publishing. pp. 38–40. ISBN 978-1-84968-562-7. http://dl.e-book-free.com/2013/07/java_7_new_features_cookbook.pdf. Retrieved 2014-07-16. 
• Stenzel, Kurt; Grandy, Holger; Reif, Wolfgang (2008). "Verification of Java Programs with Generics". Algebraic Methodology and Software Technology. Lecture Notes in Computer Science. 5140. pp. 315–329. doi:10.1007/978-3-540-79980-1. ISBN 978-3-540-79979-5. 

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