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Java Collection Framework

We have tried you to make a walk through the Collection Framework. The Collection Framework provides a well-designed set if interface and classes for sorting and manipulating groups of data as a single unit, a collection.

The Collection Framework provides a standard programming interface to many of the most common abstractions, without burdening the programmer with too many procedures and interfaces.

The Collection Framework is made up of a set of interfaces for working with the groups of objects. The different interfaces describe the different types of groups. For the most part, once you understand the interfaces, you understand the framework. While you always need to create specific, implementations of the interfaces, access to the actual collection should be restricted to the use of the interface methods, thus allowing you to change the underlying data structure, without altering the rest of your code.

In the Collections Framework, the interfaces Map and Collection are distinct with no lineage in the hierarchy. The typical application of map is to provide access to values stored by keys.

When designing software with the Collection Framework, it is useful to remember the following hierarchical relationship of the four basic interfaces of the framework.

The historical collection classes are called such because they have been around since 1.0 release of the java class libraries. If you are moving from historical collection to the new framework classes, one of the primary differences is that all operations are synchronized with the new classes. While you can add synchronization to the new classes, you cannot remove from the old.

Explore the Interface and Classes of Java Collection Framework

Java Collection Framework - List Interface

This is an ordered collection (also known as a sequence). The List interface extends the Collection interface to define an ordered collection, permitting duplicates. The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2), and they typically allow multiple null elements if they allow null elements at all. It is not inconceivable that someone might wish to implement a list that prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but we expect this usage to be rare.

The List interface places additional stipulations, beyond those specified in the Collection interface, on the contracts of the iterator, add, remove, equals, and hashCode methods. Declarations for other inherited methods are also included here for convenience.

The List interface provides four methods for positional (indexed) access to list elements. Lists (like Java arrays) are zero based. Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a list is typically preferable to indexing through it if the caller does not know the implementation.

The List interface provides a special iterator, called a ListIterator, that allows element insertion and replacement, and bidirectional access in addition to the normal operations that the Iterator interface provides. A method is provided to obtain a list iterator that starts at a specified position in the list.

The List interface provides two methods to search for a specified object. From a performance standpoint, these methods should be used with caution. In many implementations they will perform costly linear searches.

The List interface provides two methods to efficiently insert and remove multiple elements at an arbitrary point in the list.

Note: While it is permissible for lists to contain themselves as elements, extreme caution is advised: the equals and hashCode methods are no longer well defined on a such a list.

Some list implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the list may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

Java Collection Framework- HashSet & TreeSet Classes

The Collection Framework provides two general purpose implementations often Set interface, HashSet and TreeSet. More often than not you will use a HashSet for storing your duplicate-free collection. For efficiency objects added to a HashSet need to implement the hashCode() method in a manner that properly distributes the hash codes. While most system classes override the default hashCode() implementation of the Object, when creating your own class to add to a HashSet remember to override hashCode().

The TreeSet implementations useful when you need to extract elements from a collection in a sorted manner. It is generally faster to add elements to the HasSet then convert the collection to a TreeeSet for sorted traversal.

To optimize HashSet space usage , you can tune initial capacity and load factor. TreeSet has no tuning options, as the tree is always balanced, ensuring log(n0 performance for insertions, deletions and queries.

Example

The program produces the following output. The duplicate entry is olypresent once and the second line outputs sorted

[one, two, three, four]
[four, one, three, two]

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:

Set s = Collections.synchronizedSet(new HashSet(...));

SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));