Would Automatic Casts be useful?

Question

Is there any downside or problem potential to change the Java compiler to automatically cast? In the example below the result of list.get(0) would automatically be casted to the type of the variable hi.

List list = new ArrayList();
list.add("hi");
String hi = list.get(0);

I know that generics allow you to reduce casting but they do so at the expense of making declaration more difficult. To me, the benefit of generics is that they allow you to have the complier enforce more rules -- not they they reduce casting (but I haven't used them much so I am somewhat uninformed). This proposal would only reduce the amount of code to type, not move it to another place. Also there are instances where generics can't be used because a collection can have different objectis. If that "looks too surprising" based on current usage maybe there could be a syntax tweak to use it.

From: http://jamesjava.blogspot.com/2007/01/automatic-casting.html

Answer 1

Casting is an explicit instruction to the Java compiler to ignore type safety so allowing automatic casts would remove one of the features purposely designed into the language.

I personally like compiler warnings and errors, since it's much harder to find this type of problem at run time (assuming the compiler somehow managed to force one object type to another).

Answer 2

I would avoid it, because I have a feeling it will come back and bite you later on. Generics are worth the hassle, in my opinion, because they save lots of headache down the road. Also, it's not too hard to use a "container" object for collections that hold different object types.

Answer 3

Seems to me like having type checks is a good thing. An automatic cast would remove a potentially useful compile-time error.

If you worry about the number of keystrokes, you could use an IDE like Eclipse, were the type cast can be inserted with a double-click.

Answer 4

Yes, it would move errors which are currently found at compile time to being found at runtime. While this is not considered to be a huge drawback by some, those people are using Python, Ruby or Perl and not Java ;-).

Answer 5

The biggest benefit of using generics in your example is that it changes what would be a run time error into a compile time error.

List list = new ArrayList();
list.add(new Integer(42));
String hi = (String) list.get(0); // run time error

List<String> list = new ArrayList<String>();
list.add(new Integer(42)); // compile time error
String hi = list.get(0);

Because run time errors are seen by users and compile time errors are seen by programmers, compile time errors are much, much better.

To answer your question, there would not be any drastic failures if casts were automatically made (and a ClassCastException was still thrown at runtime in your example). The benefit of requiring an explicit cast is that it requires the coder to think about what he's doing, and to realize that a cast is taking place.

Also there are instances where generics can't be used because a collection can have different object

You can still add different objects to a generic collection using wildcards.

Answer 6

As others have said, automatic type-casting would eliminate type safety. Generics have multiple benefits and really aren't any harder to declare. With Eclipse, you can can just declare the generic variable and then use auto-complete on the instantiation and it will automatically fill in the type you used. Plus you (or perhaps more importantly, someone else) have the advantage of actually knowing at a glance exactly what the list or whatever contains.