Some nice Java puzzles

I came across these set of interesting java puzzles and for many of them I got trapped. See how do you fare ;) If you have a good explanation, don't hesitate to share it.

1. public class StringTest
   {
     public static void main(String[] args){
       foo();
       bar();
     }
     public static void foo(){
       String s = "str4";
       String s1 = "str" + s.length();
       System.out.println("(s==s1) = " + (s1==s));
     }
     public static void bar(){
       String s = "str4";
       String s1 = "str" + "4";
       System.out.println("(s==s1) = " + (s1==s));
    }
   }
   

   ---

   
   



2. public class Assignment {
     public static void main(String[] a){
       int count = 0;
       for (int i = 0; i < 3; i++)
         count += count++;
         System.out.println(count);
     }
   }
   

   ---

   
   



3. class StaticTest {
     static { initIfNecessary(); }
   
     private static int sum;
   
     public static int getSum() {
       initIfNecessary();
       return sum;
     }
     
     private static boolean initialized = false;
   
     private static synchronized void initIfNecessary() {
       if (!initialized) {
         for (int i = 0; i < 100; i++)
           sum += i;
         initialized = true;
       }
     }
     public static void main(String[] args) {
       System.out.println(getSum());
     }
   }
   

   ---

   
   



4. package click;
   public class CodeTalk {
     public void doIt() { printMessage(); }
     void printMessage() { System.out.println("Click"); }
   }
   ------------------
   package hack;
   import click.CodeTalk;
   public class TypeIt {
     private static class ClickIt extends CodeTalk {
       void printMessage() { System.out.println("Hack"); }
     }
     public static void main(String[] args) {
       new ClickIt().doIt();
     }
   }

   ---

   
   



5. public class Lazy {
     private static boolean initialized = false;
     static {
       Thread t = new Thread(new Runnable() {
         public void run() {
           initialized = true;
         }
       });
       t. start();
       try {
         t.join();
       } catch (InterruptedException e) {
         throw e.printStackTrace();
       }
     }
   
     public static void main(String[] args) {
       System.out.println(initialized);
     }
   }