Question

I’m writing a template class which should take an array as its type.

public class Foo<T> {...}

How best can I enforce that “T” is an array type? (int[], Bar[], …) Preferably compile time if possible, obviously, but otherwise what is the best way at debug run time to throw an exception or something if T is not an array?

Answer 1

It is not possible to do it exactly how you have asked and have the generic type specify an array or primitive.

The syntax allows for things like:

public class Foo <az09$_ extends MyClassOrInterface & Serializable & Closeable> {...}

Where az09$_ is any valid identifier that may also be parameterized with an identically formatted generic type.

But az09$_ is restricted to being a java identifier, so you cannot do public class Foo<T[]> {...} any more than you can do public class Foo[] {...}.

Typically you would do what you are after in your usage of T, i.e:

public class Foo<T> {

    public T[] processIt(T... ts) {
        // do something
        return ts;
    } 

}