Inferring java generics parameters at runtime using reflection

Question

I have a method with the following signature:

// Converts a json string to a list of objects
// Assumption: json is an array, and all items in the list are o         


        

Answer 1

The only way I know of to guarantee to have access to the type at run time is to make it a parameter to the object constructor thus:

class MyClass<T> {
    private final Class<T> clazz;
    public MyClass(Class<T> clazz) {
        this.clazz=clazz;
    }
}

You then have to pass the class when you instantiate the object:

MyClass<String> object = new MyClass<String>(String.class);

Obviously, in your case, you have what is effectively a static utility method and no object, so I think you're stuck with either the Class parameter or else some kind of template object.

Answer 2

Due to type erasure, you definitely can't "infer" what T is--it doesn't even exist at runtime. The closest you could come is inspect the values in defValue (if it has values) and get the class of the elements there.

Class<?> tType = defValue.get(0).getClass();

if (Boolean.class.isAssignableFrom(tType)) { //...  

Edit

With regards to your thinking of using reflection like getTypeArguments(), etc. Those only provide data for declared types, never actual types. So for example, if you got a handle to the Method object and called getTypeParameters(), you'd just get an array containing a type object representing T--not the actual type that T represents at some specific runtime invocation.