HashMap : Java Glossary
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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:

Read the Hashtable entry. Nearly all of it also applies to HashMap,

The capacity of a table is not the number of elements you plan to add! If you specify a capacity of 128 and a load factor of .75, HashMap will allocate only 128 slots, not the 256 you need for efficient access. The load factor kicks in only when deciding when it is time to double the size of the table, in this case, just after you have added 96 elements, not in deciding its initial size. It is up to you to leave some breathing room.

Sample Use IdentityHashMap
AutoBoxing Enumerations Cake Eating
Manual Boxing Enumerations Learning More
Accumulate Links

Sample Use

In this case we use a String to look up a String in a HashMap. We use the modern JDK 1.5+ techniques of generics and for:each loops.

AutoBoxing Enumerations

} In this case we use a String to look up a number in a HashMap. We use the JDK 1.5+ autoboxing features and the JDK 1.5+ style for:each loop.

Old Style Manual Boxing Enumerations

In this case we use a String to look up a number in a HashMap. We use the JDK 1.1+ manual boxing and the old style for loop. We don’t use generics type checking.

Accumulate: A Real World Code Example

The Accumulate class uses a HashMap to accumulate values against a variety of categories. Source is included. You might use it to accumulate hours by category in a timesheet, or count how many times various words appeared in a document. Studying the source will give you some hints on what you can do with HashMaps. Here is how you use it:

IdentityHashMap

IdentityHashMap uses == instead of equals to compare objects for equality. It also uses the original Object. hashCode() via System.identityHashCode( Object x). This is useful when you want a HashMap that keeps track of a set of heterogeneous objects. Serialisation uses one to track which objects have already been sent to the stream and where in the stream they are.

Having Your Cake and Eating It Too

Let’s say you can’t decide between the speed of an ArrayList that looks up by an indexing int and TreeMap that looks up by surname and a HashMap that looks up by social insurance number.

The beauty of Java Collections is you can have have all three. Just maintain three different Collections on the same set of objects. The three Collections don’t have to contain the exact same elements.

HashMap will give you an Iterator or an array of the whole collection at any point, which may be sufficient so that you don’t need an auxiliary ArrayList.

Learning More

Sun’s Javadoc on the HashMap class : available:
Sun’s Javadoc on the Set interface returned by keySet and entrySet class : available:
Sun’s Javadoc on the IdentityHashMap class : available:
Sun’s Javadoc on java.util.Map : available:
Sun’s Javadoc on the HashSet class : available:
To understand HashMap, read up on Hashtable and HashCode.
autoboxing
binary search
HashCode
HashSet
Hashtable
Iterator
soft reference
TreeMap
weak reference
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