The ImmutableHashtable class declares the programmatic interface to objects that manage immutable associations of keys and values. ImmutableHashtable three primitive methods-size, get, and keys-provide the basis for all of the other methods in its interface. The size method returns the number of entries in the ImmutableHashtable. get returns the value associated with a given key. keys returns an object that lets you iterate through each of the keys in the ImmutableHashtable.
The other methods declared here operate by invoking one or more of these primitives. The non-primitive methods provide convenient ways of accessing multiple entries at once. A key-value pair within an ImmutableHashtable is called an entry. Each entry consists of one object that represents the key, and a second object which is that key's value. Within an ImmutableHashtable, the keys are unique. That is, no two keys in a single ImmutableHashtable are equal.
You establish an ImmutableHashtable's entries when it's created, and thereafter the entries can't be modified. A mutable hashtable allows the addition and deletion of entries at any time, automatically allocating memory as needed.