Type Erasure

Erasure of Generic Types

Generics were introduced to the Java language to provide tighter type checks at compile time and to support generic programming. To implement generics, the Java compiler applies type erasure to:

• Replace all type parameters in generic types with their bounds or Object if the type parameters are unbounded. The produced bytecode, therefore, contains only ordinary classes, interfaces, and methods.
• Insert type casts if necessary to preserve type safety.
• Generate bridge methods to preserve polymorphism in extended generic types.

Type erasure ensures that no new classes are created for parameterized types; consequently, generics incur no runtime overhead.

During the type erasure process, the Java compiler erases all type parameters and replaces each with its first bound if the type parameter is bounded, or Object if the type parameter is unbounded.

Consider the following generic class that represents a node in a singly linked list:

public class Node<T> {

    private T data;
    private Node<T> next;

    public Node(T data, Node<T> next) {
        this.data = data;
        this.next = next;
    }

    public T getData() { return data; }
    // ...
}

Because the type parameter T is unbounded, the Java compiler replaces it with Object:

public class Node {

    private Object data;
    private Node next;

    public Node(Object data, Node next) {
        this.data = data;
        this.next = next;
    }

    public Object getData() { return data; }
    // ...
}

In the following example, the generic Node class uses a bounded type parameter:

public class Node<T extends Comparable<T>> {

    private T data;
    private Node<T> next;

    public Node(T data, Node<T> next) {
        this.data = data;
        this.next = next;
    }

    public T getData() { return data; }
    // ...
}

The Java compiler replaces the bounded type parameter T with the first bound class, Comparable:

public class Node {

    private Comparable data;
    private Node next;

    public Node(Comparable data, Node next) {
        this.data = data;
        this.next = next;
    }

    public Comparable getData() { return data; }
    // ...
}

Erasure of Generic Methods

The Java compiler also erases type parameters in generic method arguments. Consider the following generic method:

// Counts the number of occurrences of elem in anArray.
//
public static <T> int count(T[] anArray, T elem) {
    int cnt = 0;
    for (T e : anArray)
        if (e.equals(elem))
            ++cnt;
        return cnt;
}

Because T is unbounded, the Java compiler replaces it with Object:

public static int count(Object[] anArray, Object elem) {
    int cnt = 0;
    for (Object e : anArray)
        if (e.equals(elem))
            ++cnt;
        return cnt;
}

Suppose the following classes are defined:

class Shape { /* ... */ }
class Circle extends Shape { /* ... */ }
class Rectangle extends Shape { /* ... */ }

You can write a generic method to draw different shapes:

public static <T extends Shape> void draw(T shape) { /* ... */ }

The Java compiler replaces T with Shape:

public static void draw(Shape shape) { /* ... */ }

Effects of Type Erasure and Bridge Methods

Sometimes type erasure causes a situation that you may not have anticipated. The following example shows how this can occur. The following example shows how a compiler sometimes creates a synthetic method, which is called a bridge method, as part of the type erasure process.

Given the following two classes:

public class Node<T> {

    public T data;

    public Node(T data) { this.data = data; }

    public void setData(T data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}

public class MyNode extends Node<Integer> {
    public MyNode(Integer data) { super(data); }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }
}

Consider the following code:

MyNode mn = new MyNode(5);
Node n = mn;            // A raw type - compiler throws an unchecked warning
n.setData("Hello");     // Causes a ClassCastException to be thrown.
Integer x = mn.data;    

After type erasure, this code becomes:

MyNode mn = new MyNode(5);
Node n = (MyNode)mn;         // A raw type - compiler throws an unchecked warning
n.setData("Hello");          // Causes a ClassCastException to be thrown.
Integer x = (String)mn.data; 

The next section explains why a ClassCastException is thrown at the n.setData("Hello"); statement.

Bridge Methods

When compiling a class or interface that extends a parameterized class or implements a parameterized interface, the compiler may need to create a synthetic method, which is called a bridge method, as part of the type erasure process. You normally do not need to worry about bridge methods, but you might be puzzled if one appears in a stack trace.

After type erasure, the Node and MyNode classes become:

public class Node {

    public Object data;

    public Node(Object data) { this.data = data; }

    public void setData(Object data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}

public class MyNode extends Node {

    public MyNode(Integer data) { super(data); }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }
}

After type erasure, the method signatures do not match; the Node.setData(T) method becomes Node.setData(Object). As a result, the MyNode.setData(Integer) method does not override the Node.setData(Object) method.

To solve this problem and preserve the polymorphism of generic types after type erasure, the Java compiler generates a bridge method to ensure that subtyping works as expected.

For the MyNode class, the compiler generates the following bridge method for setData():

class MyNode extends Node {

    // Bridge method generated by the compiler
    //
    public void setData(Object data) {
        setData((Integer) data);
    }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }

    // ...
}

The bridge method MyNode.setData(object) delegates to the original MyNode.setData(Integer) method. As a result, the n.setData("Hello"); statement calls the method MyNode.setData(Object), and a ClassCastException is thrown because "Hello" cannot be cast to Integer.

Non-Reifiable Types

We discussed the process where the compiler removes information related to type parameters and type arguments. Type erasure has consequences related to variable arguments (also known as varargs) methods whose varargs formal parameter has a non-reifiable type. See the section Arbitrary Number of Arguments in Passing Information to a Method or a Constructor for more information about varargs methods.

This page covers the following topics:

• Non-Reifiable Types
• Heap Pollution
• Potential Vulnerabilities of Varargs Methods with Non-Reifiable Formal Parameters
• Preventing Warnings from Varargs Methods with Non-Reifiable Formal Parameters

A reifiable type is a type whose type information is fully available at runtime. This includes primitives, non-generic types, raw types, and invocations of unbound wildcards.

Non-reifiable types are types where information has been removed at compile-time by type erasure — invocations of generic types that are not defined as unbounded wildcards. A non-reifiable type does not have all of its information available at runtime. Examples of non-reifiable types are List<String> and List<Number>; the JVM cannot tell the difference between these types at runtime. As shown in the Restrictions on Generics section, there are certain situations where non-reifiable types cannot be used: in an instanceof expression, for example, or as an element in an array.

Heap Pollution

Heap pollution occurs when a variable of a parameterized type refers to an object that is not of that parameterized type. This situation occurs if the program performed some operation that gives 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. For example, heap pollution occurs when mixing raw types and parameterized types, or when performing unchecked casts.

In normal situations, when all code is compiled at the same time, the compiler issues an unchecked warning to draw your attention to potential heap pollution. If you compile sections of your code separately, it is difficult to detect the potential risk of heap pollution. If you ensure that your code compiles without warnings, then no heap pollution can occur.

Potential Vulnerabilities of Varargs Methods with Non-Reifiable Formal Parameters

Generic methods that include vararg input parameters can cause heap pollution.

Consider the following ArrayBuilder class:

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(42);
    String s = l[0].get(0);       // ClassCastException thrown here
  }

}

The following example, HeapPollutionExample uses the ArrayBuiler class:

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

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

When compiled, the following warning is produced by the definition of the ArrayBuilder.addToList() method:

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.

The following statement 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(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 invoke ArrayBuilder.faultyMethod() with the following statement:

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

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

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

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>.

Prevent Warnings from Varargs Methods with Non-Reifiable Formal Parameters

If you declare a varargs method that has parameters of a parameterized type, 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, you can prevent the warning that the compiler generates for these kinds of varargs methods by adding the following annotation to static and non-constructor method declarations:

@SafeVarargs

The @SafeVarargs annotation is a documented part of the method's contract; this annotation asserts that the implementation of the method will not improperly handle the varargs formal parameter.

It is also possible, though less desirable, to suppress such warnings by adding the following to the method declaration:

@SuppressWarnings({"unchecked", "varargs"})

However, this approach does not suppress warnings generated from the method's call site. If you are unfamiliar with the @SuppressWarnings syntax, see the section Annotations.