Java – checks at run time whether a class has a specific constructor that uses generics

Hello:) I'm trying to choose the correct constructor in the class This 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 start from map Class switch to map < string, string > class. I vaguely remember that generics are only used at compile time, so that's why the compiler complains How do I check that the class has the correct constructor at run time?

Best wishes

Solution

You want to use getgenericparametertypes() instead of:

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 get map < string, integer > it will no longer match

It will be easier to use guava's typetoken, which uses anonymous classes to create parameterized types that we can compare

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);
    }
}