IllegalArgumentException：参数数量错误

Question

I'm trying to run the following code:

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

public class Reflection {

    /**
     * @param args
     * @throws InvocationTargetException
     * @throws IllegalArgumentException
     * @throws IllegalAccessException
     */
    public static void main(String[] args) throws IllegalAccessException,
            InvocationTargetException, IllegalArgumentException {
        Class<Cls> cls = Cls.class;
        Method[] methods = cls.getMethods();
        for (Method m : methods) {
            m.invoke(cls);
        }
    }

}

class Cls {
    public static void method1() {
        System.out.println("Method1");
    }

    public static void method2() {
        System.out.println("Method2");
    }
}

I keep getting an IllegalArgumentException: wrong number of arguments, even though the two methods take no arguments.

I tried passing null to the invoke method, but that throws a NPE.

Answer 1

You are actually invoking the methods correctly, the problem is that the Class.getMethods() method returns ALL methods in the class, INCLUDING those inherited from super classes, such as Object in this case. The documentation states:

Returns an array containing Method objects reflecting all the public member methods of the class or interface represented by this Class object, including those declared by the class or interface and those inherited from superclasses and superinterfaces.

In this case you might end up calling Object.equals without any arguments, for example. The same goes for the Object.wait methods etc.

Answer 2

According to documentation :

If the number of formal parameters required by the underlying method is 0, the supplied args array may be of length 0 or null.

Have you tried this?

m.invoke(cls, null);