http://java.sun.com/javase/6/docs/api/java/lang/instrument/ClassFileTransformer.html

Transform java.lang.String.equals so that when invoked for a magic value, it checks the caller from stack, and responds as specified.

I'm not sure that method will work without loading a Java agent (eg, java -javaagent:agent.jar). Doing that would give the magic trick away when executed. If you don't use a Java agent, then there is no way that I'm aware of to get a hold of an instance of an Instrumentation class, which means you can't add your ClassFileTransformer. Additionally, I thought Instrumentation.isModifiable(String.class) returned false, because the JVM won't let you modify class in the java.* package. However, you could transform the HatOne and HatTwo classes to always return true from hasBall.

@James Roper -- you can load an instrumentation agent at runtime.. read how

A fifth (sixth?) solution that doesn't even require modifying the Ball: http://www.papercut.com/blog/tom/2008/08/01/java-magic-trick-the-ball-is-everywhere/

Great Puzzle James, but you are neglecting giving constraints.
Okay, your constraint is that the MagicTrick.java has to do the output.
If that is the only constraint I would make "java" a batch/shell on the path, which would for example put my custom classloader as a system property via "java.system.class.loader"
Then I could load custom Hat classes.
Also this could solve the javaagent command line option.
I guess you can always find a trick to make your own script/executable run instead of the original java one, doing any setup, bytecode modification you like.
Sure it misses the point of being a Java Trick, but that the kind of cheating magicians do :-)

I did consider making it so the only thing you could change was the value of UNDER_HAT, that would have eliminated a number of solutions because then a static block couldn't be placed inside Ball.java because it would never be initialised, due to the fact that UNDER_HAT is static final. But the good thing about having rules that aren't so strict is it increases the scope of creativity.

If you changed the value of UNDER_HAT to be the return value of a static method (which could do the required meddling), would that still fall within the constraint?

@Chris Leggatt

No, because Java inlines the result of the static method at compile time. That static method is only called once...

@Kit

Regardless of whether the static method is inlined or not, if it's used to initialise a static member it's only going to be used once to initialise that static particular static member.

What I was referring to was that while creating a static method that returns the value of "NONE" (while also changing the internal char array of the Strings "ONE" and "TWO" to be {'N', 'O', 'N', 'E'}) and assigning that to UNDER_HAT works, I was wondering whether this would violate the proposed constraint of "the only thing you could change was the value of UNDER_HAT". Technically, you still only changing the value of UNDER_HAT after all...

The below required a wee small change to Ball impl and thus the how the UNDER_HAT value is retrieved.

public class Ball
{
public static String getHat() {return UNDER_HAT;}

/*
* Simple ball implementation.
* Remember the serial version ID to stop eclipse moaning and note the byte
* sequence of the class file header after CAFEBABE
* \u000C\u000A\u000F\u000E\u000B\u000A\u000B\u000E * \u002A\u002F \u0073\u0074\u0061\u0074\u0069\u0063\u007b\u0020
\u0053\u0079\u0073\u0074\u0065\u006d\u002e\u006f\u0075\u0074\u002e\u0070\u0072\u0069\u006e\u0074\u006c\u006e(
\u0022\u0054\u0041\u0044\u0041\u0021\u0021\u0020\u0054\u0068\u0065\u0020\u0062\u0061\u006c\u006c\u0020\u0069\u0073\u0020\u0075\u006e\u0064\u0065\u0072\u0020\u0062\u006f\u0074\u0068\u0020\u0068\u0061\u0074\u0073\u0021\u0022)
\u003b\u0020\u0053\u0079\u0073\u0074\u0065\u006d\u002e\u0065\u0078\u0069\u0074(\u0030)\u003b\u0020\u007d\u002F\u002A
*
*/
private static final String UNDER_HAT = "NONE";
}

and of course I was being a muppet, left the string as final - change it to non-final and you can just use the ball class as:

public class Ball
{
/*
* Simple ball implementation.
* Remember the serial version ID to stop eclipse moaning and note the byte
* sequence of the class file header after CAFEBABE
* \u000C\u000A\u000F\u000E\u000B\u000A\u000B\u000E * \u002A\u002F \u0073\u0074\u0061\u0074\u0069\u0063\u007b\u0020
\u0053\u0079\u0073\u0074\u0065\u006d\u002e\u006f\u0075\u0074\u002e\u0070\u0072\u0069\u006e\u0074\u006c\u006e(
\u0022\u0054\u0041\u0044\u0041\u0021\u0021\u0020\u0054\u0068\u0065\u0020\u0062\u0061\u006c\u006c\u0020\u0069\u0073\u0020\u0075\u006e\u0064\u0065\u0072\u0020\u0062\u006f\u0074\u0068\u0020\u0068\u0061\u0074\u0073\u0021\u0022)
\u003b\u0020\u0053\u0079\u0073\u0074\u0065\u006d\u002e\u0065\u0078\u0069\u0074(\u0030)\u003b\u0020\u007d\u002F\u002A
* (c)Andy Shearman
*/
public static String UNDER_HAT = "NONE";
}

I also messed with JVM's String cache and came up with this:

import java.lang.reflect.Field;

public class Ball {
public static String UNDER_HAT = "TWO";

static {
try {
Class stringClass = String.class;
Field valueField = String.class.getDeclaredField("value");
valueField.setAccessible(true);
valueField.set("ONE", valueField.get(UNDER_HAT));
} catch (Throwable ignored) {}
}
}

Maybe I'm little bit late to the game, but I wrote my own solution here. I've also provided a detailed explanation, but if you do not bother reading it, I recommend skipping to the end where you can find a question, just to keep the game going :)

Hi James,

how about this? ;)

public class Ball {
static boolean flip;
public static String UNDER_HAT = (flip=(!flip)) ? "ONE" : "TWO";
}

Jumper
Hungary

What about AOP (www.eclipse.org/aspectj/).

Both advices achieve the same result.
The second doesn't modify the classes HatOne/-Two in any way.
It wouldn't be modifying Ball.java but adding an Aspect.

public aspect HackHat {

//Advice 1
boolean around() :
call (boolean HatOne.hasBall()) ||
call (boolean HatTwo.hasBall()) {
return true;
}

//Advice 2
boolean around() : call( boolean String.equals(Object)) {
return true;
}
}

@Jumper: Did you try your solution? Do you understand why it does not work?

@Homer: Give credit where credit is due. You ripped that from Dr. Heinz M. Kabutz.
http://www.javaspecialists.eu/archive/Issue014.html

How about

public static final String UNDER_HAT = "TWO".equals(Ball.UNDER_HAT)?"ONE":"TWO";

This does work.

I love these sorts of puzzles, got any others?

I published one just today on my personal blog (though, the way I've phrased it it's easy to know the answer before you try it):

http://jazzy.id.au/pebble/2009/04/24/java_concurrency_and_volatile.html

Hi James,

I have another solution for your trick:

import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintStream;
import java.io.Writer;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.nio.charset.CharsetEncoder;
import java.nio.charset.CoderResult;

public class Ball {

static {
System.setOut(new PrintStream(new WriterOutputStream(new OutputStreamWriter(System.out, new Encoder()))));
}

public static String UNDER_HAT = "TWO";

static class WriterOutputStream extends OutputStream {

private Writer writer;

public WriterOutputStream(Writer writer) {
this.writer = writer;
}

public void write(int b) throws IOException {
writer.write(b);
}

public void flush() throws IOException {
writer.flush();
}

public void close() throws IOException {
writer.close();
}
}

static class Encoder extends CharsetEncoder {

private static final char EMPTY = '\u0000';

private char stack = EMPTY;

public Encoder() {
super(Charset.forName("UTF-8"), 1, 8);
}

@Override
protected CoderResult encodeLoop(CharBuffer in, ByteBuffer out) {
char c1, c2;
if (isStackEmpty()) {
c1 = in.get();
c2 = EMPTY;
} else {
c1 = pop();
c2 = in.get();
}
if (c1 == 'T') {
out.put("TADA!! T".getBytes());
} else if (c1 == 'h') {
if (c2 == EMPTY) {
push(c1);
} else if (c2 == 'a') {
out.put("both ha".getBytes());
} else {
out.put((byte)c1);
out.put((byte)c2);
}
} else if (c1 == 't') {
if (c2 == EMPTY) {
push(c1);
} else if (c2 == ' ') {
out.put("ts".getBytes());
}
} else if (c1 == '.') {
out.put((byte)'!');
return CoderResult.OVERFLOW;
} else if (c1 != 'o') {
out.put((byte)c1);
}
return CoderResult.UNDERFLOW;
}

private boolean isStackEmpty() {
return stack == EMPTY;
}

private void push(char c) {
if (!isStackEmpty()) {
throw new IllegalArgumentException("This stack does not support more than one character");
}
stack = c;
}

private char pop() {
char c = stack;
stack = EMPTY;
return c;
}
}
}

Interesting... what do you call that encoding?

It is the UseOnlyInThisTrickEncoding ;-)