Why is Java's Class generic?

Question

Why is Java's Class<T> generic?

score 34 · Accepted Answer · edited Jan 31 '13 at 16:34

34

So that generic typed methods can be used -

Class<Foo> klass = Foo.class;
Foo f = klass.newInstance();
Foo f = klass.cast(Object);

edited Jan 31 '13 at 16:34

Caesar Ralf

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answered Jul 07 '09 at 18:40

Steve B.

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Warning to anyone believing this answer to be correct because it was accepted: This cannot be correct because the code does not compile. `klass` is uninitialized before it is used. – Dan Nissenbaum May 19 '11 at 05:28
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... (follow-up): Notice that code that does compile - namely, `Class klass = Foo.class;` - begs the original question, because `Foo.class` seemingly provides the necessary information already. However, the ability to use generic typed methods is the correct answer; it's just not clear from the given example here. – Dan Nissenbaum May 19 '11 at 05:36

Chris Arguin · Answer 2 · 2011-05-30T04:08:05.713

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Here is a reasonably good summary of the advantages: http://download.oracle.com/javase/tutorial/extra/generics/literals.html

edited May 30 '11 at 04:08

answered Jul 07 '09 at 18:39

Chris Arguin

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score 14 · Answer 3 · answered Jul 07 '09 at 18:47

There's a short mention of this in the Generics section of the 1.5 version of the language guide:

More surprisingly, class Class has been generified. Class literals now function as type tokens, providing both run-time and compile-time type information. This enables a style of static factories exemplified by the getAnnotation method in the new AnnotatedElement interface:

<T extends Annotation> T getAnnotation(Class<T> annotationType);

This is a generic method. It infers the value of its type parameter T from its argument, and returns an appropriate instance of T, as illustrated by the following snippet:

Author a = Othello.class.getAnnotation(Author.class);

Prior to generics, you would have had to cast the result to Author. Also you would have had no way to make the compiler check that the actual parameter represented a subclass of Annotation

score 8 · Answer 4 · answered Jul 31 '12 at 17:50

The real reason is given by Neil Gafter:

When we added generics to Java in JDK5, I changed the class java.lang.Class to become a generic type. For example, the type of String.class is now Class < String > . Gilad Bracha coined the term type tokens for this. My intent was to enable a particular style of API, which Joshua Bloch calls the THC, or Typesafe Heterogenous Container pattern.

Why is Java's Class generic?

4 Answers4