java反射constructor.newInstance给出“错误数量的参数”

Question

How to fix my code below?

package mypackage;

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;

public class testReflection {
    public class You {
        public You(String s) {
        }

        public void f(String s, int i) {
            System.out.println(i + 100);
        }
    }

    public static void main(String[] args) throws NoSuchMethodException {
        Constructor constructor =
                You.class.getConstructor(testReflection.class, String.class);
        try {
            You y = (You)constructor.newInstance("xzy");//Exception!!!!
            System.out.println("ok");
            y.f("xyz",2);
        }catch(Exception e){
            e.printStackTrace();
        }
    }
}

The exception message is:

java.lang.IllegalArgumentException: wrong number of arguments
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at mypackage.testReflection.main

Answer 1

From the documentation of Constructor#newInstance :

If the constructor's declaring class is an inner class in a non-static context, the first argument to the constructor needs to be the enclosing instance; see section 15.9.3 of The Java™ Language Specification.

Because You is an inner class, you need an instance of its enclosing class, testReflection , to create an instance of You . To do this, you can use the following:

You y = (You) constructor.newInstance(new testReflection(), "xzy");

I also recommend changing your class name to TestReflection as that follows the proper naming conventions.

Answer 2

The hint is on this line (the constructor takes 2 parameters):

Constructor constructor =
            You.class.getConstructor(testReflection.class, String.class);

You need to send an instance of testReflection to newInstance() :

testReflection outerObject = new testReflection();
You y = (You)constructor.newInstance(outerObject,  "xzy");