Heap pollution occurs when a variable of a parameterized type references an object that is not of that parameterized type. (For more information on heap pollution, see The Java Language Specification (JLS), §4.12.2., "Variables of Reference Type" [JLS 2014].).
Mixing generically typed code with raw typed code is one common source of heap pollution. Generic types were unavailable prior to Java 5, so popular interfaces such as the Java Collection Framework relied on raw types. Mixing generically typed code with raw typed code allowed developers to preserve compatibility between nongeneric legacy code and newer generic code but also gave rise to heap pollution. Heap pollution can occur if the program performs some operation involving a raw type that would give rise to a compile-time unchecked warning.
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Even when heap pollution occurs, the variable is still guaranteed to refer to a subclass or subinterface of the declared type , but is not guaranteed to always refer to a subtype of its declared type. In this example, list does not refer to a subtype of its declared type (List<String>) but only to the subinterface of the declared type (List).
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The compiler prevents insertion of an Object object to the parameterized list because addToList() cannot be called with an argument whose type produces a mismatch. This code has consequently been changed to add a String to the list instead of an int to the list.
Compliant Solution (Legacy Code)
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The compiler still issues the unchecked warning, which may still be ignored. However, the code now fails when it attempts to add the Integer integer to the list, consequently preventing the program from proceeding with invalid data.
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Heap pollution can occur without using raw types such as java.util.List. This noncompliant code example builds a list of lists of strings before passing it to a modify() method. Because this method is variadic, it casts list into an array of lists of strings. But Java is incapable of representing the types of parameterized arrays. This limitation allows the modify() method to sneak a single integer into the list. Although the Java compiler emits several warnings, this program compiles and runs until it tries to extract the integer 42 from a List<String>.
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