HashMap

A HashMap lets you look up values by exact key (always case-sensitive). It is very much like a Hashtable, except that it is faster and not thread-safe. There are some minor other differences:
• HashMaps work with Iterators where the older Hashtables work with Enumerations
• Hashtables work best with capacities that are prime numbers. HashMaps round capacities up to powers of two.

The Collection Framework provides two general-purpose Map implementation: HashMap and TreeMap. As with all the concrete implementations, which implement you use depends on your specific needs. For inserting, deleting and locating elements in a Map the HashMap offers best alternatively. If however you need to traverse the keys in a sorted order then TreeMap is better alternative. Depending upon your size of your collection, it may be faster to add elements to a HashMap then convert the Map to a TreeMap for sorted key traversal. Using a HashMap requires that the class of key added have a well-defined hashCode() implementation. With the TreeMap implementation elements added to the Map must be sortable.

To optimize HashMap usage you can tune the initial capacity and load factor. The TreeMap has no tuning options as the tree is always balanced

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the capacity is roughly doubled by calling the rehash method.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

Note that these implementation is not synchronized. If multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be "wrapped" using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

Map m = Collections.synchronizedMap(new HashMap(...));