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Collections

 
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What is a difference between Hashtable, HashMap and TreeMap? How do you decide which one to use?

Thank you
 
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Hashtable, HashMap and TreeMap are map implementations these collection classes should be used when you need to store data in key/value pairs.

Unlike the other two classes TreeMap is a Sorted map , sorts the element in the natural order. Hash Map & Hashtable are Unsorted & Unordered , Difference between Hashmap & hashtable is Hash map allows one null key & many null values but hash table does not . Also Hash table is synchronized
 
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Try reading the Collections trail at sun. That should answer your question.
 
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I also recommend the Collections chapter in Bruce Eckel's Thinking in Java. Reading it and doing the provided exercises will help a lot.
 
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Hashtable is passe.
 
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