How to get value type of a map in Java? [duplicate]

Question

Possible Duplicate:
Get generic type of java.util.List

I have a Map and I want to get the type of T from an instance of that Map. How can I do that?

e.g. I want to do something like:

Map<String, Double> map = new HashMap<String, Double>();
...
String vtype = map.getValueType().getClass().getName(); //I want to get Double here

Of course there's no such 'getValueType()' function in the API.

Answer 1

If your Map is a declared field, you can do the following:

import java.lang.reflect.*;
import java.util.*;

public class Generic {
    private Map<String, Number> map = new HashMap<String, Number>();

    public static void main(String[] args) {
        try {
            ParameterizedType pt = (ParameterizedType)Generic.class.getDeclaredField("map").getGenericType();
            for(Type type : pt.getActualTypeArguments()) {
                System.out.println(type.toString());
            }
        } catch(NoSuchFieldException e) {
            e.printStackTrace();
        }
    }
}

The above code will print:

class java.lang.String
class java.lang.Number

However, if you simply have an instance of the Map, e.g. if it were passed to a method, you cannot get the information at that time:

public static void getType(Map<String, Number> erased) {
    // cannot get the generic type information directly off the "erased" parameter here
}

You could grab the method signature (again, using reflection like my example above) to determine the type of erased, but it doesn't really get you anywhere (see edit below).

Edit:

BalusC's comments below should be noted. You really shouldn't need to do this; you already declared the Map's type, so you're not getting any more information than you already have. See his answer here.