Why do some claim that Java's implementation of generics is bad?

Question

I\'ve occasionally heard that with generics, Java didn\'t get it right. (nearest reference, here)

Pardon my inexperience, but what would have made them better?

Answer 1

Ignoring the whole type erasure mess, generics as specified just don't work.

This compiles:

List<Integer> x = Collections.emptyList();

But this is a syntax error:

foo(Collections.emptyList());

Where foo is defined as:

void foo(List<Integer> x) { /* method body not important */ }

So whether an expression type checks depends on whether it is being assigned to a local variable or an actual parameter of a method call. How crazy is that?

Answer 2

The biggest problem is that Java generics are a compile-time only thing, and you can subvert it at run-time. C# is praised because it does more run-time checking. There is some really good discussion in this post, and it links to other discussions.

Answer 3

I wish this was a wiki so I could add to other people... but...

Problems:

• Type Erasure (no runtime availability)
• No support for primative types
• Incompatability with Annotations (they were both added in 1.5 I'm still not sure why annotations don't allow generics aside from rushing the features)
• Incompatability with Arrays. (Sometimes I really want to do somthing like Class<? extends MyObject>[], but I'm not allowed)
• Wierd wildcard syntax and behavior
• The fact that generic support is inconsistant across Java classes. They added it to most of the collections methods, but every once in a while, you run into an instance where its not there.

Answer 4

Bad:

• Type information is lost at compile time, so at execution time you can't tell what type it's "meant" to be
• Can't be used for value types (this is a biggie - in .NET a List<byte> really is backed by a byte[] for example, and no boxing is required)
• Syntax for calling generic methods sucks (IMO)
• Syntax for constraints can get confusing
• Wildcarding is generally confusing
• Various restrictions due to the above - casting etc

Good:

• Wildcarding allows covariance/contravariance to be specified at calling side, which is very neat in many situations
• It's better than nothing!

Answer 5

I'm going to throw out a really controversial opinion. Generics complicate the language and complicate the code. For example, let's say that I have a map that maps a string to a list of strings. In the old days, I could declare this simply as

Map someMap;

Now, I have to declare it as

Map<String, List<String>> someMap;

And every time I pass it into some method, I have to repeat that big long declaration all over again. In my opinion, all that extra typing distracts the developer and takes him out of "the zone". Also, when code is filled with lots of cruft, sometimes it's hard to come back to it later and quickly sift through all the cruft to find the important logic.

Java already has a bad reputation for being one of the most verbose languages in common use, and generics just add to that problem.

And what do you really buy for all that extra verbosity? How many times have you really had problems where someone put an Integer into a collection that's supposed to hold Strings, or where someone tried to pull a String out of a collection of Integers? In my 10 years of experience working at building commercial Java applications, this has just never been a big source of errors. So, I'm not really sure what you're getting for the extra verbosity. It really just strikes me as extra bureaucratic baggage.

Now I'm going to get really controversial. What I see as the biggest problem with collections in Java 1.4 is the necessity to typecast everywhere. I view those typecasts as extra, verbose cruft that have many of the same problems as generics. So, for example, I can't just do

List someList = someMap.get("some key");

I have to do

List someList = (List) someMap.get("some key");

The reason, of course, is that get() returns an Object which is a supertype of List. So the assignment can't be made without a typecast. Again, think about how much that rule really buys you. From my experience, not much.

I think Java would have been way better off if 1) it had not added generics but 2) instead had allowed implicit casting from a supertype to a subtype. Let incorrect casts be caught at runtime. Then I could have had the simplicity of defining

Map someMap;

and later doing

List someList = someMap.get("some key");

all the cruft would be gone, and I really don't think I'd be introducing a big new source of bugs into my code.

Answer 6

Java generics are compile-time only and are compiled into non-generic code. In C#, the actual compiled MSIL is generic. This has huge implications for performance because Java still casts during runtime. See here for more.