Class vs. interface casting

Question

This is a long read. But since I wrote test code and think its a ligit question:
After compiling, if I move a *.class file to a different directory, casting with a class (but not an interface) fails. Please look at my test code to understand what I am trying to say:
public class Main {
  public static void main(String[] args) {
    try {
      MyClassLoader mcL = new MyClassLoader();
      Class classA = mcL.loadClass("whatever");
      I inst = (I) classA.newInstance(); // <-- good
      // ClassA inst = (ClassA) classA.newInstance(); <-- bad
      inst.foo();
    } catch(Exception e) { e.printStackTrace() ; }
      public static class MyClassLoader extends ClassLoader {
        protected Class<?> findClass(String s) throws ClassNotFoundException, ClassFormatError {
          try {
            InputStream in = new FileInputStream(new File("/etc/ClassA.class"));
            int i = in.available();
            byte[] buf = new byte;
in.read(buf, 0, buf.length);
return defineClass(buf, 0, buf.length);
} catch(Exception e) {e.printStackTrace(); }
return null;
public class ClassA implements I {
public void foo() { println("ClassA::foo-->ok"); }
public interface I { public void foo(); }
C:\dev\dir *java
  Main.java
  ClassA.java
  I.java
C:\dev\javac *java
C:\dev\move ClassA.class \etc
C:\dev\java Main
using interface cast = no problem
using a class to cast = _fail_
Exception in thread "main" java.lang.NoClassDefFoundError: ClassA
     at Main.main(Main.java:18)
Caused by: java.lang.ClassNotFoundException: ClassA
     at java.net.URLClassLoader$1.run(Unknown Source)
     at java.security.AccessController.doPrivileged(Native Method)
     at java.net.URLClassLoader.findClass(Unknown Source)
     at java.lang.ClassLoader.loadClass(Unknown Source)
     at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
     at java.lang.ClassLoader.loadClass(Unknown Source)
     at java.lang.ClassLoader.loadClassInternal(Unknown Source)
     ... 1 more
I will only cast with interfaces, but wondering what is going on with why casting with a class fails.
_note_: to test that casting with classes works at all I wrote this code snippet:
Class c = Thread.class;
Class[] noArg = new Class[0];
Constructor constr =  c.getConstructor(noArg);
Thread t = (Thread) constr.newInstance(new Object[0]);
println("-->" + t.getId());
I hope this proves casting with classes is possible.So sometimes casting with a class is ok, sometimes not. I wonder why.
thanks.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

Answer 1

ejp wrote:
After compiling, if I move a *.class file to a different directoryYou can't do that. The .class file has to be in whatever directory is indicated by its package statement. Two classes with the same name in different packages are different, and a .class file in the wrong directory will cause NoClassDefFoundErrors.can i ask a follow-up?
To be painfully clear, when I hit this line:
ClassA inst = (ClassA) classA.newInstance();The type-checking tripped me up. I had a valid "ClassA" object. Object creation had been internalized to my class loader. However, when the jvm did a type-check, it needed to load the ClassA.class file, and it had been removed. So, my casting with a class failed only because of type-checking?