This page summarizes the enhancements made to the collections
framework in version 1.4 of the JavaTM 2 SDK.
The Collections utility class has several new methods:
rotate(List list, int distance) - Rotates all of the elements
in the list by the the specified distance. This operation is common in GUI
computations: moving one or more columns in a table can be accomplished
efficiently by applying this operation to a sublist. This operation is
also common in mathematical and scientific calculations.
swap(List list, int i, int j) - Swaps the elements at the
specified positions in the specified list. Essentially a convenience method,
though this implementation is faster than the naive implementation.
list(Enumeration e) - Returns an ArrayList containing the elements returned
by the specified enumeration. This convenience method provides
interoperability between legacy APIs that return enumerations and new APIs
that require collections.
New interface RandomAccess is a marker interface that allows List
implementations to indicate that they support fast (generally
constant time) random access. This allows generic algorithms to
alter their behavior to provide good performance when applied to either
random or sequential access lists.
New class LinkedHashMap provides
an insertion-ordered Map implementation that runs nearly as fast as
HashMap. Internally, it uses a hash table with a doubly linked list
running through it in insertion order. Also available is a corresponding
Set implementation, called LinkedHashSet.
New class IdentityHashMap is an
identity-based Map implementation based on a hash table. This class is useful
for topology-preserving object graph transformations (such as serialization or
deep-copying). To perform such transformations, you need to maintain an
identity-based "node table" that keeps track of which objects have already
been seen. Identity-based maps are also used to maintain
object-to-meta-information mappings in dynamic debuggers and similar systems.
Finally, identity-based maps are useful in thwarting "spoof attacks" resulting
from intentionally perverse equals methods. (IdentityHashMap never
invokes the equals method on its keys.) An added benefit of this
implementation is that it is fast.