What is cool about generics, why use them?

Question

I thought I'd offer this softball to whomever would like to hit it out of the park. What are generics, what are the advantages of generics, why, where, how should I use them? Please, keep it fairly basic. Thanks.

Answer 1

I once gave a talk on this topic. You can find my slides, code, and audio recording at http://www.adventuresinsoftware.com/generics/.

Answer 2

A couple of things to add/expand on (speaking from the .NET point of view):

Generic types allow you to create role-based classes and interfaces. This has been said already in more basic terms, but I find you start to design your code with classes which are implemented in a type-agnostic way - which results in highly reusable code.

Generic arguments on methods can do the same thing, but they also help apply the "Tell Don't Ask" principle to casting, i.e. "give me what I want, and if you can't, you tell me why".

Answer 3

If you were to search the Java bug database just before 1.5 was released, you'd find seven times more bugs with NullPointerException than ClassCastException. So it doesn't seem that it is a great feature to find bugs, or at least bugs that persist after a little smoke testing.

For me the huge advantage of generics is that they document in code important type information. If I didn't want that type information documented in code, then I'd use a dynamically typed language, or at least a language with more implicit type inference.

Keeping an object's collections to itself isn't a bad style (but then the common style is to effectively ignore encapsulation). It rather depends upon what you are doing. Passing collections to "algorithms" is slightly easier to check (at or before compile-time) with generics.

Answer 4

the jvm casts anyway... it implicitly creates code which treats the generic type as "Object" and creates casts to the desired instantiation. Java generics are just syntactic sugar.

Answer 5

In summary, generics allow you to specify more precisily what you intend to do (stronger typing).

This has several benefits for you:

• Because the compiler knows more about what you want to do, it allows you to omit a lot of type-casting because it already knows that the type will be compatible.

• This also gets you earlier feedback about the correctnes of your program. Things that previously would have failed at runtime (e.g. because an object couldn't be casted in the desired type), now fail at compile-time and you can fix the mistake before your testing-department files a cryptical bug report.

• The compiler can do more optimizations, like avoiding boxing, etc.

Answer 6

• Allows you to write code/use library methods which are type-safe, i.e. a List<string> is guaranteed to be a list of strings.
• As a result of generics being used the compiler can perform compile-time checks on code for type safety, i.e. are you trying to put an int into that list of strings? Using an ArrayList would cause that to be a less transparent runtime error.
• Faster than using objects as it either avoids boxing/unboxing (where .net has to convert value types to reference types or vice-versa) or casting from objects to the required reference type.
• Allows you to write code which is applicable to many types with the same underlying behaviour, i.e. a Dictionary<string, int> uses the same underlying code as a Dictionary<DateTime, double>; using generics, the framework team only had to write one piece of code to achieve both results with the aforementioned advantages too.