Checking at runtime if a class has a specific constructor that is using generics

Question

Hello all :) I'm trying to chose the right constructor in a class. Here is the code:

Constructor[] constructors = targetClass.getConstructors();
Constructor goodConstructor = null;
for (Constructor constructor : constructors) {
    Class[] parameterTypes = constructor.getParameterTypes();
    if (parameterTypes.length = 1 && parameterTypes[0].equals(Map.class)) {//here
        goodConstructor = constructor;
    }
}

I want to switch from Map.class to Map<String, String>.class. I vaguely remember that generics are for compile time only, so this is why the compiler is complaining. How can I check at runtime that the class has the right constructor?

Best regards

Answer 1

You want to use getGenericParameterTypes() instead:

public class FindConstructor {

    public static void main(String[] args) throws IOException {
        for (Constructor<?> constructor : MyClass.class.getConstructors()) {
            Type[] parameterTypes = constructor.getGenericParameterTypes();
            if (parameterTypes.length == 1 && parameterTypes[0] instanceof ParameterizedType) {
                ParameterizedType parameterizedArg = (ParameterizedType) parameterTypes[0];
                if (parameterizedArg.getRawType() != Map.class) {
                    continue;
                }

                if (parameterizedArg.getActualTypeArguments()[0] != String.class) {
                    continue;
                }

                if (parameterizedArg.getActualTypeArguments()[1] != String.class) {
                    continue;
                }
            }
            System.out.println("found constructor " + constructor);
        }
    }
}

class MyClass {
    public MyClass(Map<String, String> map) {
    }
}

Now if you change MyClass() to take a Map<String, Integer> it will no longer match.

This becomes much easier with Guava's TypeToken, which utilizes an anonymous class to create a parameterized Type that we can compare to.

Type mapStringString = new TypeToken<Map<String, String>>(){}.getType();
for (Constructor<?> constructor : MyClass.class.getConstructors()) {
    Type[] parameterTypes = constructor.getGenericParameterTypes();
    if (parameterTypes.length == 1 && parameterTypes[0].equals(mapStringString)) {
        System.out.println("found constructor " + constructor);
    }
}