Heap pollution

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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]

A source of heap pollution in Java arises from the fact that 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. Indeed, 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, it is possible that a variable of a parameterized type refers 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

Further reading[edit]

  • 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.
  • Friesen, Jeff (2011). Beginning Java 7. Expert's voice in Java. Apress. p. 211. ISBN 1430239093.
  • Reese, Richard; Reese, Jennifer (2012). Java 7 New Features Cookbook (PDF). Packt Publishing. pp. 38–40. ISBN 978-1-84968-562-7.
  • 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.(subscription required)


  1. ^ a b "The Java SE Tutorials". Oracle. Retrieved 16 July 2014. CS1 maint: discouraged parameter (link)
  2. ^ a b Langer, Angelika. "Java Generics FAQs: Heap pollution". angelikalanger.com/. Retrieved 15 July 2014. CS1 maint: discouraged parameter (link)