Java Puzzle

如果你認為自己係Java高手，恭喜你， show time !!!

RULES：
1). 唔準runtime改code, 改class, 改memory, 即係無得用我最愛的setAccessible()
2). 除左(1) 同題目既要求之外，你點寫都得，總之要做到
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Introduction

This is the first in a series of Java puzzles, that put your Java skills to the test, in a challenging and fun way!
A puzzle consists of some given Java code with a line in it that seems to be impossible to reach. It’s up to you to find the hole in it, abuse a subtle behavior of Java to make execution reach that line anyways.

There almost aren’t any rules; any cheating inside your code is allowed; it is the whole point of the puzzle. [Clarification: Cheating the environment is not]. You must run with the security manager enabled (java -Djava.security.manager), otherwise it would be too easy (with setAccessible for example). [Update: Use common sense, and see the exact rules if you're in doubt].

The puzzle

How can you fit 20 clowns into a Volkswagen? Two classes are given: an empty Clown class, and a Volkswagen class to which you can add clowns. When you try to add a Clown, it is checked that it isn’t already full. But if you just try hard enough, there’s always room for some extra clowns…

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package clowns; public class Clown { }

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package clowns; import java.util.HashSet; import java.util.Set; public class Volkswagen {     private static final int CAPACITY = 5;     private Set<Clown> clowns = new HashSet<Clown>();     public synchronized void add(Clown clown) {         if (clowns.size() >= CAPACITY) {             throw new IllegalStateException("I'm full");         } else {             clowns.add(clown);         }     }     public synchronized void done() {         if (clowns.size() == 20) {             // The goal is to reach this line             System.out.println("I'm a Volkswagen with 20 clowns!");         }     } }

Write a class that when executed pushes 20 clowns into the little car, and reaches the marked line. Here is one that won’t really work, just to get you started:

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package you; import clowns.Clown; import clowns.Volkswagen; public class You {     public static void main(String args[]) {         // TODO put 20 clowns into a Volkswagen         Volkswagen vw = new Volkswagen();         for (int i = 0; i < 20; i++) {             vw.add(new Clown());         }         vw.done();     } }

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PS: 這不是我出的題目，出題目既堅係神人
PSS: 真係好tricky
PSSS: 佢既下一條都好屈機

refs: http://wouter.coekaerts.be/2012/puzzle-clowns