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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 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 non-generic nongeneric legacy code and newer generic code but also gave rise to heap pollution.   Heap pollution can occur if the program performed performs some operation involving a raw type that would give rise to a compile-time unchecked warning.

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When generic and nongeneric types are used together correctly, these warnings can be ignored; at other times, these warnings can denote potentially unsafe operations. Mixing generic and raw types is allowed provided that heap pollution does not occur. For example, consider the following code snippet. In some cases, t it is possible that a compile-time unchecked warning will not be generated. According to the Java Language SpecificationJLS, §4.12.2.1, "Heap Pollution," [JLS 2005]:

Note that this does not imply that heap pollution only occurs if an unchecked warning actually occurred. It is possible to run a program where some of the binaries were compiled by a compiler for an older version of the Java programming language, or by a compiler that allows the unchecked warnings to suppressed [sicbe] suppressed. This practice is unhealthy at best.

Heap pollution can also occur if the program aliases an array variable of non-reifiable element type through an array variable of a supertype which that is either raw or non-genericnongeneric. 

Noncompliant Code Example

This noncompliant code example compiles but results in heap pollution.   The compiler produces an unchecked warning because a raw argument (the obj parameter in the addToList() method) is passed to the List.add() method. 

class ListUtility {
  private static void addToList(List list, Object obj) {
    list.add(obj); // uncheckedUnchecked warning
  }

  public static void main(String[] args) {
    List<String> list = new ArrayList<String> ();
    addToList(list, 42);
    System.out.println(list.get(0));  // throwsThrows ClassCastException
  }
}

Heap pollution is possible in this case because the parameterized type information is discarded prior to before execution.   The call to addToList(list, 42) succeeds in adding an integer to list list, although it is of type List<String>.   This Java runtime does not throw a ClassCastException until the value is read and has an invalid type (an int rather than a String). In other words, the code throws an exception some time after the execution of the operation that actually caused the error, complicating debugging.

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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Compliant Solution (Legacy Code)

While the The previous compliant solution eliminates use of raw collections, it may be infeasible to implement but implementing this solution when interoperating with legacy code may be infeasible.

Suppose that the addToList() method is legacy code that cannot be changed. The following compliant solution creates a checked view of the list by using the Collections.checkedList() method. This method returns a wrapper collection that performs runtime type checking in its implementation of the add() method before delegating to the backend back-end List<String>. The wrapper collection can be safely passed to the legacy addToList() method.

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class ListAdder {
  @SuppressWarnings("unchecked")
  private static void addToList(List list, Object obj) {
    list.add(obj);  // uncheckedUnchecked warning suppressed
  }

  private static <T> void printNum(T type) {
    if (!(type instanceof Integer || type instanceof Double)) {
      System.out.println("Cannot print in the supplied type");
    }
    List<T> list = new ArrayList<T>();
    addToList(list, 42);
    System.out.println(list.get(0));
  }

  public static void main(String[] args) {
    double d = 42;
    int i = 42;
    System.out.println(d);
    ListAdder.printNum(d);
    System.out.println(i);
    ListAdder.printNum(i);
  }
}

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This compliant solution generifies the addToList() method, eliminating any possible type violations.:

class ListAdder {
  private static <T> void addToList(List<T> list, T t) {
    list.add(t);     // No warning generated
  }

  private static <T> void printNum(T type) {
    if (type instanceof Integer) {
      List<Integer> list = new ArrayList<Integer>();
      addToList(list, 42);
      System.out.println(list.get(0));
    }
    else if (type instanceof Double) {
      List<Double> list = new ArrayList<Double>();

      addToList(list, 42.0);  // willWill not compile with 42 instead of 42.0

      System.out.println(list.get(0));
    } else {
      System.out.println("Cannot print in the supplied type");
    }
  }

  public static void main(String[] args) {
    double d = 42;
    int i = 42;
    System.out.println(d);
    ListAdder.printNum(d);
    System.out.println(i);
    ListAdder.printNum(i);
  }
}

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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 list 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. While Although the Java compiler emits several warnings, this program compiles and runs until it tries to extract the integer 42 from a List<String>.

class ListModifierExample {
  public static void modify(List<String>... list) {
    Object[] objectArray = list;
    objectArray[1] = Arrays.asList(42);   // pollutesPollutes list, no warning

    for (List<String> ls : list) {
      for (String string : ls) {          // ClassCastException on 42
        System.out.println(string);
      }
    }
  }
 
  public static void main(String[] args) {
    List<String> s = Arrays.asList("foo", "bar");
    List<String> s2 = Arrays.asList("baz", "quux");
    modify( s, s2);                       // uncheckedUnchecked varargs warning
  }
}

This program produces the following output:

foo
bar
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
	at ListModifierExample.modify(Java.java:13)
	at ListModifierExample.main(Java.java:25)
	at Java.main(Java.java:33)

Noncompliant Code Example (Array of

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Lists of

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Strings)

This noncompliant code example is similar, but it uses an explicit array of lists of strings as the single parameter to modify(). The program again dies with a ClassCastException from the integer 42 injected into a list of strings.

class ListModifierExample {
  public static void modify(List<String>[] list) {
    Object[] objectArray = list;          // Valid
    objectArray[1] = Arrays.asList(42);   // pollutesPollutes list, no warning

    for (List<String> ls : list) {
      for (String string : ls) {          // ClassCastException on 42
        System.out.println(string);
      }
    }
  }
 
  public static void main(String[] args) {
    List<String> s = Arrays.asList("foo", "bar");
    List<String> s2 = Arrays.asList("baz", "quux");
    List list[] = {s, s2};
    modify(list);                         // uncheckedUnchecked conversion warning
  }
}

Compliant Solution (List of

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Lists of

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Strings)

This compliant solution uses a list of lists of strings as the argument to modify(). This type safety enables the compiler to prevent the modify() method from injecting an integer into the list. In order to compile, the modify() method instead inserts a string, preventing heap pollution.

class ListModifierExample {
  public static void modify(List<List<String>> list) {
    list.set( 1, Arrays.asList("forty-two")); // noNo warning

    for (List<String> ls : list) {
      for (String string : ls) {            // ClassCastException on 42
        System.out.println(string);
      }
    }
  }
 
  public static void main(String[] args) {
    List<String> s = Arrays.asList("foo", "bar");
    List<String> s2 = Arrays.asList("baz", "quux");
    List<List<String>> list = new ArrayList<List<String>>();
    list.add(s);
    list.add(s2);
    modify(list);
  }
}

Note that to avoid warnings, we cannot use Arrays.asList() to build a list of list lists of strings , because that method is also variadic and would produce a warning about variadic arguments being parameterized class objects.

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Bibliography

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