Improved Compiler Warnings and Errors When Using Non-Reifiable Formal Parameters with Varargs Methods

This page covers the following topics:

Heap Pollution

Most parameterized types, such as ArrayList<Number> and List<String>, are non-reifiable types. A non-reifiable type is a type that is not completely available at runtime. At compile time, non-reifiable types undergo a process called type erasure during which the compiler removes information related to type parameters and type arguments. This ensures binary compatibility with Java libraries and applications that were created before generics. Because type erasure removes information from parameterized types at compile-time, these types are non-reifiable.

Heap pollution occurs when a variable of a parameterized type refers to an object that is not of that parameterized type. This situation can only occur if the program performed some operation that would give rise to an unchecked warning at compile-time. An unchecked warning is generated if, either at compile-time (within the limits of the compile-time type checking rules) or at runtime, the correctness of an operation involving a parameterized type (for example, a cast or method call) cannot be verified.

Consider the following example:

    List l = new ArrayList<Number>();
    List<String> ls = l;       // unchecked warning
    l.add(0, new Integer(42)); // another unchecked warning
    String s = ls.get(0);      // ClassCastException is thrown

During type erasure, the types ArrayList<Number> and List<String> become ArrayList and List, respectively.

The variable ls has the parameterized type List<String>. When the List referenced by l is assigned to ls, the compiler generates an unchecked warning; the compiler is unable to determine at compile time, and moreover knows that the JVM will not be able to determine at runtime, if l refers to a List<String> type; it does not. Consequently, heap pollution occurs.

As a result, at compile time, the compiler generates another unchecked warning at the add statement. The compiler is unable to determine if the variable l refers to a List<String> type or a List<Integer> type (and another heap pollution situation occurs). However, the compiler does not generate a warning or error at the get statement. This statement is valid; it is calling the List<String>.get method to retrieve a String object. Instead, at runtime, the get statement throws a ClassCastException.

In detail, a heap pollution situation occurs when the List object l, whose static type is List<Number>, is assigned to another List object, ls, that has a different static type, List<String>. However, the compiler still allows this assignment. It must allow this assignment to preserve backwards compatibility with versions of Java SE that do not support generics. Because of type erasure, List<Number> and List<String> both become List. Consequently, the compiler allows the assignment of the object l, which has a raw type of List, to the object ls.

Furthermore, a heap pollution situation occurs when the l.add method is called. The static type second formal parameter of the add method is String, but this method is called with an actual parameter of a different type, Integer. However, the compiler still allows this method call. Because of type erasure, the type of the second formal parameter of the add method (which is defined as List<E>.add(int,E)) becomes Object. Consequently, the compiler allows this method call because, after type erasure, the l.add method can add any object of type Object, including an object of Integer type, which is a subtype of Object.

Variable Arguments Methods and Non-Reifiable Formal Parameters

Consider the method ArrayBuilder.addToList in the following example. It is a variable arguments (also known as varargs) method that adds the objects of type T contained in the elements varargs formal parameter to the List listArg:

import java.util.*;

public class ArrayBuilder {

  public static <T> void addToList (List<T> listArg, T... elements) {
    for (T x : elements) {
      listArg.add(x);
    }
  }

  public static void faultyMethod(List<String>... l) {
    Object[] objectArray = l;  // Valid
    objectArray[0] = Arrays.asList(new Integer(42));
    String s = l[0].get(0);    // ClassCastException thrown here
  }

}

import java.util.*;

public class HeapPollutionExample {

  public static void main(String[] args) {

    List<String> stringListA = new ArrayList<String>();
    List<String> stringListB = new ArrayList<String>();

    ArrayBuilder.addToList(stringListA, "Seven", "Eight", "Nine");
    ArrayBuilder.addToList(stringListA, "Ten", "Eleven", "Twelve");
    List<List<String>> listOfStringLists = new ArrayList<List<String>>();
    ArrayBuilder.addToList(listOfStringLists, stringListA, stringListB);

    ArrayBuilder.faultyMethod(Arrays.asList("Hello!"), Arrays.asList("World!"));
  }
}

The Java SE 7 compiler generates the following warning for the definition of the method ArrayBuilder.addToList:

warning: [varargs] Possible heap pollution from parameterized vararg type T

When the compiler encounters a varargs method, it translates the varargs formal parameter into an array. However, the Java programming language does not permit the creation of arrays of parameterized types. In the method ArrayBuilder.addToList, the compiler translates the varargs formal parameter T... elements to the formal parameter T[] elements, an array. However, because of type erasure, the compiler converts the varargs formal parameter to Object[] elements. Consequently, there is a possibility of heap pollution. See the next section, Potential Vulnerabilities of Varargs Methods with Non-Reifiable Formal Parameters, for more information.

Note: The Java SE 5 and 6 compilers generate this warning when the ArrayBuilder.addToList is called; in this example, the warning is generated for the class HeapPollutionExample. These compilers do not generate the warning at the declaration site. However, the Java SE 7 generates the warning at both the declaration site and the call site (unless the warnings are preempted with annotations; see Suppressing Warnings from Varargs Methods with Non-Reifiable Formal Parameters for more information). The advantage of generating a warning when a compiler encounters a varargs method that has a non-reifiable varargs formal parameter at the declaration site as opposed to the call site is that there is only one declaration site; there are potentially many call sites.

Potential Vulnerabilities of Varargs Methods with Non-Reifiable Formal Parameters

The method ArrayBuilder.faultyMethod shows why the compiler warns you about these kinds of methods. The first statement of this method assigns the varargs formal parameter l to the Object array objectArgs:

    Object[] objectArray = l;

This statement can potentially introduce heap pollution. A value that does match the parameterized type of the varargs formal parameter l can be assigned to the variable objectArray, and thus can be assigned to l. However, the compiler does not generate an unchecked warning at this statement. The compiler has already generated a warning when it translated the varargs formal parameter List<String>... l to the formal parameter List[] l. This statement is valid; the variable l has the type List[], which is a subtype of Object[].

Consequently, the compiler does not issue a warning or error if you assign a List object of any type to any array component of the objectArray array as shown by this statement:

    objectArray[0] = Arrays.asList(new Integer(42));

This statement assigns to the first array component of the objectArray array with a List object that contains one object of type Integer.

Suppose you call the ArrayBuilder.makeArray method with the following statement:

    ArrayBuilder.faultyMethod(Arrays.asList("Hello!"), Arrays.asList("World!"));

At runtime, the JVM throws a ClassCastException at the following statement:

    String s = l[0].get(0);    // ClassCastException thrown here

The object stored in the first array component of the variable l has the type List<Integer>, but this statement is expecting an object of type List<String>.

Suppressing Warnings from Varargs Methods with Non-Reifiable Formal Parameters

If you declare a varargs method that has parameterized parameters, and you ensure that the body of the method does not throw a ClassCastException or other similar exception due to improper handling of the varargs formal parameter (as shown in the ArrayBuilder.faultyMethod method), you can suppress the warning that the compiler generates for these kinds of varargs methods by using one of the following options:

For example, the following version of the ArrayBuilder class has two additional methods, addToList2 and addToList3:

public class ArrayBuilder {

  public static <T> void addToList (List<T> listArg, T... elements) {
    for (T x : elements) {
      listArg.add(x);
    }
  }

  @SuppressWarnings({"unchecked", "varargs"})
  public static <T> void addToList2 (List<T> listArg, T... elements) {
    for (T x : elements) {
      listArg.add(x);
    }
  }

  @SafeVarargs
  public static <T> void addToList3 (List<T> listArg, T... elements) {
    for (T x : elements) {
      listArg.add(x);
    }
  }

  // ...

}
public class HeapPollutionExample {

  // ...

  public static void main(String[] args) {

    // ...

    ArrayBuilder.addToList(listOfStringLists, stringListA, stringListB);
    ArrayBuilder.addToList2(listOfStringLists, stringListA, stringListB);
    ArrayBuilder.addToList3(listOfStringLists, stringListA, stringListB);

    // ...

  }
}

The Java compiler generates the following warnings for this example:

Note: In Java SE 5 and 6, it is the responsibility of the programmer who calls a varargs method that has a non-reifiable varargs formal parameter to determine whether heap pollution would occur. However, if this programmer did not write such a method, he or she cannot easily determine this. In Java SE 7, it is the responsibility of the programmer who writes these kinds of varargs methods to ensure that they properly handle the varargs formal parameter and ensure heap pollution does not occur.

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