Java create generic instance with type parameter obtained at runtime

Question

I have a class like this:

public class Pojo<T, P> {}

I know at compile time the type of T but I don't know the type of P. I was wondering if something like this is possible:

Class<Integer> t = Integer.class;
field = new Pojo<Integer, t>();

If not, any alternative solution to the problem is valid ;)

EDIT: I'm trying to obtain statistics from POJO fields, i.e, I have a class AirRegister with fields station, o3, so2, altitude, latitude... and I want to obtain the mode for these fields. The user can send any type with any field type to the program so until runtime the field type is unknown.

Example:

public static void main(String[] args) {
    String field = args[0];
    Class<fieldType> type = AirRegister.class.getDeclaredField(field).getClass();
    ModeImpl<AirRegister, type> p = new ModeImpl<AirRegister, type>();
}

Generics were invented to help the compiler ensure proper type checking, at compile time (as opposed to making everything an Object and then doing run-time class checks). A generic with a class that isn't known until run time therefore isn't very useful as a generic. I can't propose an alternative solution unless I know what you're really trying to accomplish. For example, what does the inside of your Pojo really look like? What were you hoping would happen when you instantiated your Pojo? — ajb
– ajb, Commented May 6, 2016 at 6:45

Hoopje · Accepted Answer · 2016-05-06 06:53:55Z

2

It is not possible. Generics are a compile-time only feature. They are used by the compiler to type-check your source code, but at run-time they are gone.

If you need to implement run-time type checking, you might use Class instances to store the required type. E.g.:

class Pojo<T> {
    private Class<?> clazz;
    Pojo(Class<?> clazz) {
        this.clazz = clazz;
    }
    void doSomething(T arg1, Object arg2) {
        if (!clazz.isInstance(arg2)) {
            throw new ClassCastException();
        }
        ...
    }
}

answered May 6, 2016 at 6:53

Hoopje

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