C# in Depth

110 CHAPTER 3 Parameterized typing with generics3.6.5 Comparison with Java genericsWhere C++ includes more of the template in the generated code than C# does, Javaincludes less. In fact, the Java runtime doesn’t know about generics at all. The Javabytecode (roughly equivalent terminology to IL) for a generic type includes someextra metadata to say that it’s generic, but after compilation the calling code doesn’thave much to indicate that generics were involved at all—and certainly an instance ofa generic type only knows about the nongeneric side of itself. For example, aninstance of HashSet doesn’t know whether it was created as a HashSet ora HashSet. The compiler effectively just adds casts where necessary and performsmore sanity checking. Here’s an example—first the generic Java code:ArrayList strings = new ArrayList();strings.add("hello");String entry = strings.get(0);strings.add(new Object());and now the equivalent nongeneric code:ArrayList strings = new ArrayList();strings.add("hello");String entry = (String) strings.get(0);strings.add(new Object());They would generate the same Java bytecode, except for the last line—which is validin the nongeneric case but caught by the compiler as an error in the generic version.You can use a generic type as a “raw” type, which is equivalent to usingjava.lang.Object for each of the type arguments. This rewriting—and loss of information—iscalled type erasure. Java doesn’t have user-defined value types, but you can’teven use the built-in ones as type arguments. Instead, you have to use the boxed version—ArrayListfor a list of integers, for example.You may be forgiven for thinking this is all a bit disappointing compared withgenerics in C#, but there are some nice features of Java generics too:■■■■The runtime doesn’t know anything about generics, so you can use code compiledusing generics on an older version, as long as you don’t use any classes ormethods that aren’t present on the old version. Versioning in .NET is muchstricter in general—you have to compile using the oldest environment you wantto run on. That’s safer, but less flexible.You don’t need to learn a new set of classes to use Java generics—where a nongenericdeveloper would use ArrayList, a generic developer just uses Array-List. Existing classes can reasonably easily be “upgraded” to generic versions.The previous feature has been utilized quite effectively with the reflection system—java.lang.Class(the equivalent of System.Type) is generic, whichallows compile-time type safety to be extended to cover many situations involvingreflection. In some other situations it’s a pain, however.Java has support for covariance and contravariance using wildcards. Forinstance, ArrayList