Question

I have been looking at Reflections lately in java and I have come up with a simple example

Reflection.java

 import java.lang.reflect.*;

  public class Reflection{
  }

  public static void main(String args[])
  {
     try {
       Class cls = Class.forName("Method");

        Method methlist[] 
          = cls.getDeclaredMethods();
        for (int i = 0; i < methlist.length;
           i++) {  
           Method m = methlist[i];
           System.out.println("name 
             = " + m.getName());
           System.out.println("decl class = " +
                          m.getDeclaringClass());
           Class pvec[] = m.getParameterTypes();
           for (int j = 0; j < pvec.length; j++)
              System.out.println("
               param #" + j + " " + pvec[j]);
           Class evec[] = m.getExceptionTypes();
           for (int j = 0; j < evec.length; j++)
              System.out.println("exc #" + j 
                + " " + evec[j]);
           System.out.println("return type = " +
                              m.getReturnType());
           System.out.println("-----");
        }
     }
     catch (Throwable e) {
        System.err.println(e);
     }
  }
 }

Method.java

 public class Method {
  private int f1(
   Object p, int x) throws NullPointerException
  {
     if (p == null)
        throw new NullPointerException();
     return x;
  }

From the above example i am able to return all the information from Method.java which looks like this

name = f1
decl class = class Method
param #0 class java.lang.Object
param #1 int
exc #0 class java.lang.NullPointerException
return type = int
-----
name = main
decl class = class method1
param #0 class [Ljava.lang.String;
return type = void
-----

But my question is, is there a way to test Method.java from Reflection.java with something like

int x = cls.getMethod("f1", Object = anObject, Integer.TYPE = 2 )
System.out.println(x);

And the console should print out 2 because f1 was passed an object?

Answer 1

You need to create an instance of Method and invoke the private method f1 with the two parameters. It will return an object. There are a couple of pitfalls in your code, but here’s a working solution (Note: I renamed your class Method to MyMethod to avoid a name clash:

Method f1Method = MyMethod.class.getDeclaredMethod("f1", Object.class, int.class);
f1Method.setAccessible(true);
int result = (Integer) f1Method.invoke(new MyMethod(), 
                           new Object[]{new Object(), 2});
System.out.println(result);

• The method is private so you need to get the declared method and make it accessible
• The method is not static so we need an instance of MyMethod to invoke the method
• We pass and receive a java primitive type. int.class is allowed. JavaDoc tells us, that if the value has a primitive type, it is first appropriately wrapped in an object. So we’ll receive an instance of Integer.