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Parameterized classes have expectations of the objects that they reference; they expect certain objects to match their paramaterized types. Heap pollution occurs when a variable of a parameterized class type references an object that it expects to be of the parameterized type, but the object is of a different type. For is not of that parameterized type. (For more information on heap pollution, see the see The Java Language Specification (JLS), §4.12.2.1, "Heap Pollution,Variables of Reference Type" [JLS 20052015].). For instance

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.

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.

List llist = new ArrayList();
List<String> ls = llist; // Produces unchecked warning

It is insufficient to rely on unchecked warnings alone to detect violations of this ruleIn some cases, 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, "Variables of Reference Type" [JLS 20052015]:

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.

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.

According to the Java Language Specification, §4.8, "Raw Types," [JLS 2005]:

The use of raw types is allowed only as a concession to compatibility of legacy code. The use of raw types in code written after the introduction of genericity into the Java programming language is strongly discouraged. It is possible that future versions of the Java programming language will disallow the use of raw types.

Note that mixing generic and raw types is perfectly acceptable if heap pollution can be guaranteed not to occur.Heap pollution can also occur if the program aliases an array variable of non-reifiable element type through an array variable of a supertype that is either raw or nongeneric. 

Noncompliant Code Example

This noncompliant code example compiles but results in heap pollution. The compiler produces an unchecked warning because the raw type of the List.add() method is used a raw argument (the list obj parameter in the addToList() method) rather than the parameterized type. 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, 142);
    System.out.println(list.get(0));  // Throws ClassCastException
  }
}

When executed, this code throws an exception. This happens not because a List<String> receives an Integer but because the value returned by list.get(0) is an improper type (an Integer Heap pollution is possible in this case because the parameterized type information is discarded before execution. The call to addToList(list, 42) succeeds in adding an integer to 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).

Compliant Solution (Parameterized Collection)

This compliant solution enforces type safety by changing the addToList() method signature to enforce proper type checking.:

class ListUtility {
  private static void addToList(List<String> list, String str) {
    list.add(str);     // No warning generated
  }

  public static void main(String[] args) {
    List<String> list = new ArrayList<String> ();
    addToList(list, "142");
    System.out.println(list.get(0));
  }
}

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 instead of an int to the list instead of an Integer.

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 was is legacy code that could not 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.

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

  public static void main(String[] args) {
    List<String> list = new ArrayList<String> ();
    List<String> checkedList = Collections.checkedList(list, String.class);
    addToList(checkedList, 142);
    System.out.println(list.get(0));
  }
}

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.

...

This noncompliant code example compiles and runs cleanly because it suppresses the unchecked warning produced by the raw List.add() method. The printOneprintNum() method intends to print the value 142, either as an int or as a double depending on the type of the variable type.

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

  private static <T> void printOneprintNum(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, 142);
    System.out.println(list.get(0));
  }

  public static void main(String[] args) {
    double d = 142;
    int i = 142;
    System.out.println(d);
    ListAdder.printOneprintNum(d);
    System.out.println(i);
    ListAdder.printOneprintNum(i);
  }
}

However, despite list being correctly parameterized, this method prints 1 42 and never 142.0 because the int value 1 42 is always added to list without being type checked. This code produces the following output:

142.0
142
142
1
42

Compliant

...

Solution (Parameterized Collection)

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 printOneprintNum(T type) {
    if (type instanceof Integer) {
      List<Integer> list = new ArrayList<Integer>();
      addToList(list, 142);
      System.out.println(list.get(0));
    }
    else if (type instanceof Double) {
      List<Double> list = new ArrayList<Double>();

      // This will not compile if addToList(list, 142.0); is used
      addToList(list, 1.0); // Will 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 = 142;
    int i = 142;
    System.out.println(d);
    ListAdder.printOneprintNum(d);
    System.out.println(i);
    ListAdder.printOneprintNum(i);
  }
}

This code compiles cleanly and produces the correct output:

142.0
142.0
142
1
42

If the method addToList() is externally defined (such as in a library or as an upcall method) and cannot be changed, the same compliant method printOneprintNum() can be used, but no warnings result if addToList(1list, 42) is used instead of addToList(1list, 42.0). Great care must be taken to ensure type safety when generics are mixed with nongeneric code.

Noncompliant Code Example (Variadic Arguments)

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>.

class ListModifierExample {
  public static void modify(List<String>... list) {
    Object[] objectArray = list;
    objectArray[1] = Arrays.asList(42);   // Pollutes 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);                       // Unchecked 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 Lists of 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);   // Pollutes 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);                         // Unchecked conversion warning
  }
}

Compliant Solution (List of Lists of 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")); // 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<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 lists of strings because that method is also variadic and would produce a warning about variadic arguments being parameterized class objects.

Risk Assessment

Mixing generic and nongeneric code can produce unexpected results and exceptional conditions.

Automated Detection

Bibliography

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Image Added Image Added Image Removed      04. Object Orientation (OBJ)