TreeMap vs HashMap

After knowing Hashtable vs HashMap, now let us see the comparison of HashMap with TreeMap. Basically both are derived from Map interface and meant to store key/value pairs. But TreeMap inherits one more interface SortedMap and for this reason it attains the property of returning the elements in sorting order by default (irrespective of the addition of elements in any order).

By nature, all the classes derived from Map interface allow null values either as keys are values. Null keys are accepted only once because Map classes do not allow duplicate keys.
Similarities:

1. The methods of both are not synchronized. Both are not thread-safe. But we can obtain a synchronized version as follows.
   

   HashMap hm1 = new HashMap();
   Map m1 = Collections.synchronizedMap(hm1);
   

   Now the methods of m1 are synchronized but still the methods of hm1 are not synchronized. So the programmer still has got a choice of using synchronized or unsynchronized map. This is the height of intelligence of Java designers in developing collection framework classes. It is a much matured framework.
2. Both do not support duplicate keys. If added it overrides the earlier (but not error or exception).
3. Iterators of both are fail-fast.
4. Allows null values for both keys and values (null key is allowed only once because no duplicate keys).
5. HashMap is the direct implementation of Map whereas TreeMap comes with an intermittent SortedMap (see the above hierarchy).

Differences:

1. HashMap returns unordered values. TreeMap returns the elements in ascending order (known as natural order) of keys by default (the affect of deriving from SortedMap). TreeMap order can be customized using Comparator and Comparable interfaces.
2. The performance of HapMap is higher than TreeMap because TreeMap requires minimum execution of two method calls to create a tree structure and to print the elements in natural order. The performance of HashMap can still be enhanced by the programmer by choosing optimum initial capacity and load factor, at the time of HashMap object creation itself, as per his requirements.

What is synchronized?
The concept of synchronization comes in multithreading. In multithreaded applications, there is every chance that multiple threads accessing the same source of data at the same time. This results in data corruption and data inconsistency. Synchronization avoids this by allowing only one thread to access the resource of data at a time.
That is, when one thread is modifying the data of Hashtable, the other thread cannot modify. The other thread should wait until the first one completes its job. This is done by acquiring the lock on Hashtable by the first thread. The lock will not be released until the first completes its job.
What is fail-fast?
The concept of fail-fast comes with iterators. When an Iterator object is created and iteration is going on, the HashMap elements cannot be modified (like addition or deletion of elements cannot be done). This is explained programmatically in ConcurrentModificationException.
Programs on HashMap and TreeMap are available at HashMap Tutorial and TreeMap Tutorial.
About Hashing and Hashcode
Comparing two strings letter by letter in a for loop is a time taking process. To make faster, the JVM converts each string into an integer number called hashcode. Different strings with different sequence of characters have different hashcodes. Comparison with integer numbers gives maximum performance. Usage of hashcode numbers for comparison, searching of duplicate elements and identification is faster.
Hashing is process of converting a string or object into a 32-bit integer number. Two objects are said to be equal if their hashcodes are same. hashCode() is used in combination of equals() method. When compared, hashing is done automatically by the JVM. Hashing, in data structures, is done implicitly in the basic operations with add(), contains(), remove() and size() etc. Hashing is more useful to compare the sets of large content.