Why does this method allow a double to be stored in ArrayList of type Integer?

Question

I have a method that is supposed to return an instance of a given collection class with the given entries. The implementation is shown below.

public static <E, C extends Collection<E>> C
         initalizeAndAddToCollection(Class<C> clazz, E... entries) {
    //Collection object
    Collection<E> collection;
    //try to invoke default constructor of C
    try {
        collection = clazz.getDeclaredConstructor().newInstance();
    } catch (InstantiationException | IllegalAccessException |
             InvocationTargetException | NoSuchMethodException e) {
        throw new RuntimeException();
    }
    //Add elements to collection
    for (E entry: entries)
        collection.add(entry);
    return (C) collection;
}

The problem is that the following code runs even though a double should not be able to be stored in a list of type Integer

 //Create and instantiate an ArrayList with element 1.0
 ArrayList<Integer> list = initalizeAndAddToCollection(ArrayList.class, 1.0);
 System.out.print(list.get(0));

Why does this code run, and how do I make it so that it results in either a compile or runtime error?

Edit: I noticed list.get(0).getClass() does generate an exception, but I am not sure why either (or why the previous code does not).

Answer 1

In Java, generics are a compile-time only feature because they're implemented via type erasure . So while you may think you're creating an ArrayList<Integer> at runtime with your newInstance call, you're really just creating an ArrayList .

In short, reflection breaks type safety for generics in Java.

Answer 2

This code gives a warning when I compile it. The warning says:

Note: MyTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

And in fact, when you do what it produces 4 warning messages:

MyTest.java:5: warning: [unchecked] Possible heap pollution from parameterized vararg type E
        initalizeAndAddToCollection(Class<C> clazz, E... entries) {
                                             ^
  where E,C are type-variables:
    E extends Object declared in method <E,C>initalizeAndAddToCollection(Class<C>,E...)
    C extends Collection<E> declared in method <E,C>initalizeAndAddToCollection(Class<C>,E...)


MyTest.java:18: warning: [unchecked] unchecked cast
        return (C) collection;
                   ^
  required: C
  found:    Collection<E>
  where E,C are type-variables:
    E extends Object declared in method <E,C>initalizeAndAddToCollection(Class<C>,E...)
    C extends Collection<E> declared in method <E,C>initalizeAndAddToCollection(Class<C>,E...)
MyTest.java:23: warning: [unchecked] unchecked method invocation: method initalizeAndAddToCollection in class MyTest is applied to given types
             initalizeAndAddToCollection(ArrayList.class, 1.0);
                                        ^
  required: Class<C>,E[]
  found: Class<ArrayList>,double
  where C,E are type-variables:
    C extends Collection<E> declared in method <E,C>initalizeAndAddToCollection(Class<C>,E...)
    E extends Object declared in method <E,C>initalizeAndAddToCollection(Class<C>,E...)
MyTest.java:23: warning: [unchecked] unchecked conversion
             initalizeAndAddToCollection(ArrayList.class, 1.0);
                                        ^
  required: ArrayList<Integer>
  found:    ArrayList
4 warnings

These warnings explain why this seemingly incorrect code is compiling. You are doing things that the compiler would tell you are incorrect.

---

Why does this code run.

Because it is not actually breaking runtime type-safety.

• Type erasure means that the collection is actually storing references to Object .

• When you then do System.out.print(list.get(0)); the parameter type for the print call is Object . That means no implicit cast to Integer is needed.

and how do I make it so that so that it results in either a compile or runtime error?

If you need a compile time error, tell the compiler to treat warnings as errors. (Or check for warnings in the compilation output.)

If you need a runtime error, you need to add some explicit runtime type checks probably in your initalizeAndAddToCollection method.