Why can an Object be cast as an Integer using generics in Java?

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I found a way to add Floats to an Integer array through generic types. I'm curious why Java doesn't catch this in a runtime error? I tried looking through the docs for any info about this, but I haven't had any luck.

For example, say I have two ArrayLists, one containing Integers, and one containing Floats, like so:

ArrayList<Integer> listInt = new ArrayList<>();
ArrayList<Float> listFlt = new ArrayList<>();
  
listInt.add(6);
listInt.add(71);
  
listFlt.add(4.92f);
listFlt.add(20.5f);

Using generics, I can copy the Floats from listFlt into listInt through a method like this:

public static <T extends Number, S extends Number> void copy(ArrayList<T> src, ArrayList<S> dest) {
   for (int i = 0; i < src.size(); i++) {
       Object x = src.get(i);
       dest.set(i, (S)x);
   }
}

Calling it with:

ClassName.<Float, Integer>copy(listFlt, listInt);

Now, when I print listInt, it displays:

4.92
20.5

How is it possible to cast an Object as an Integer when generics are involved? And how can listInt store Floats? Thanks!

floating-point integer casting generics java
1 Answers

Using generics, I can copy the Floats from listFlt into listInt through a method like this

Note that inside this method, you are casting to the type parameter S:

dest.set(i, (S)x);

This will cause an unchecked cast warning when you compile it (which you should not ignore!).

It's allowed, but because of type erasure, the JVM will not be able to check at runtime if the cast is allowed.

This is an example of heap pollution: because of the rules of the Java language and type erasure, you get into a situation where you can add objects into an ArrayList that are of the wrong type (in this case: you are adding Float objects to a list that is supposed to contain Integer objects).

When you just print the list, there is no problem, because in that case Java only needs to call toString() on the elements of the list, and that method exists for all objects.

But when you try to get an Integer out of the list, you will get a ClassCastException:

ClassName.<Float, Integer>copy(listFlt, listInt);

Integer i = listInt.get(0); // ClassCastException: Float cannot be cast to Integer

It's an unfortunate consequence of a combination of the rules of Java, which are partly the way they are because of backward compatibility, and the way that Java works with generics (with type erasure).

Point to remember: Do not ignore "unchecked cast" warnings; they are a hint that there is possible heap pollution going on in your code.

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