创建一个具有可变数量的泛型类型的类

Question

I would like to create a class that can handle an undefined number of generic type is that even possible with java ?

The idea behind this weird question is to create a tuple with a size fixed in his contructor where each element at position i is of the type at the pos i%numberoftype .

Answer 1

No, types in Java have a fixed number of type parameters.

The closest you could get is to have a (finite) number of related classes:

class Tuple1<A> { ... }

class Tuple2<A, B> { ... }

class Tuple3<A, B, C> { ... }

And then provide a common class with factory methods:

class Tuples {
  static <A> Tuple1<A> of(A a) { ... }
  static <A, B> Tuple2<A,B> of(A a, B b) { ... }
  static <A, B, C> Tuple1<A,B,C> of(A a, B b, C c) { ... }
}

Answer 2

As Joachim says, Java generic types have a fixed number of type parameters. A variable number is not supported.

AFAIK, the JLS doesn't state this explicitly, but the Java syntax for a generic class only allows declarations with a fixed number of parameters.

See JLS 8.1.2 gives the grammar rules as:

TypeParameters    ::= '<' TypeParameterList '>'
TypeParameterList ::= TypeParameter { ',' TypeParameter }

(I have tweaked meta-syntax to distinguish language symbols meta-symbols.)

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I am not aware of any main-stream programming language that supports generic / parameterized types with a variable number of type parameters. I suspect that type-checking such a language (if it existed) would be challenging. And, I doubt that there would be many legitimate use-cases for such a language feature. (But perhaps that's just my lack of imagination ...)