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tomboxfan

About data structure
 
Hashtable is implemented using a linkedList array.
 
HashSet is implemented using a Hashtable.
 
TreeSet is a sorted collection. It's implemented using a red-black tree.
About red-black tree:
http://www.cs.auckland.ac.nz/software/AlgAnim/red_black.html
http://wangdei.iteye.com/blog/236157
 
 
Basic override

the synchronized keyword is not part of the method signature and thus maybe added during overriding.
 
for example. You can have
 
    public abstract void increment();
 
And in sub classes. You can have
 
    public synchronized void increment() {}
 
or
 
    public void increment() {}