Java反射调用方法“ getDeclaredConstructors”

Question

I didn't really know how to put the title of the question. Basically i'm trying to do some tricky stuff with the java reflect api. Here's what im trying to do:

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Scanner;

public class Test {

public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException, InstantiationException {
    Scanner in = new Scanner(System.in);
    String className = in.nextLine();
    Class c = Class.forName(className);
    Object myObj = c.newInstance();
    Method m = c.getMethod("getDeclaredConstructors");
    Object retn = m.invoke(myObj);
    System.out.println(retn);
    in.close();
}
}

I'm testing this with the input: java.lang.String I always get as output:

Exception in thread "main" java.lang.NoSuchMethodException: java.lang.String.getDeclaredConstructors()
    at java.lang.Class.getMethod(Class.java:1778)
    at Test.main(Test.java:15)

I understand that the method "getDeclaredConstructors()" is from the class Class.

Can someone please tell me what am i doing wrong here? What i was expecting was a list of the declared constructors for the class name i gave as input, in this case: java.lang.String

EDIT: I should have detailed more my problem. I know that i could execute c.getDeclaredConstructors(), but thats not the case.

This is only a small part of the program i want to build. In the final version, the user will provide as input a name of a class, say "java.lang.String" and then it will provide generic methods that he wishes to apply to that class. So the user would enter "getDeclaredConstructors" and i would have to apply the method with that name to the class "java.lang.String".

Im only doing this to understand how exactly i can achive that.

Answer 1

getMethod() returns the methods of String (in this case), not of Class . String doesn't have such a method.

And why exactly would you want to call this method reflectively in the first place? It's a public API.

Answer 2

I would say, you had a right problem but tested with a wrong method. Methods of java.lang.Class are accessed through the object of other classes. For this one, you can try access a method from java.lang.Object, like below,

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Scanner;

public class Test {

public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException, InstantiationException {
Scanner in = new Scanner(System.in);
String className = in.nextLine();
Class c = Class.forName(className);
Object myObj = c.newInstance();
Method m = c.getMethod("hashCode");
Object retn = m.invoke(myObj);
System.out.println(retn);
in.close();
}
}

Hope this helps you.