Parameterized classes have expectations of the objects that they reference; they expect certain objects to match their paramaterized types. Heap pollution occurs when a parameterized class references an object that it expects to be of the parameterized type, but the object is of a different type. For more information on heap pollution, see the Java Language Specification, §4.12.2.1, "Heap Pollution," [JLS 2005]). For instance, consider the following code snippet.
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Extending legacy classes and making the overriding methods generic fails because this is disallowed by the Java Language Specification..
Mixing generically typed code with raw typed code is one common source of heap pollution. Prior to Java 5, all code used raw types. Allowing mixing enabled developers to preserve compatibility between non-generic legacy code and newer generic code. Using raw types with generic code causes most Java compilers to issue "unchecked" warnings but still compile the code. When generic and nongeneric types are used together correctly, these warnings can be ignored; at other times, these warnings can denote potentially unsafe operations.
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