Java collections framework

The Java collections framework is a set of classes and interfaces that implement commonly reusable collection data structures.

Although referred to as a framework, it works in a manner of a library. The collections framework provides both interfaces that define various collections and classes that implement them.

Differences from Arrays
Collections and arrays are similar in that they both hold references to objects and they can be managed as a group. However, unlike arrays, Collections do not need to be assigned a certain capacity when instantiated. Collections can grow and shrink in size automatically when objects are added or removed. 

Collections cannot hold primitive data types such as int, long, or double. Instead, Collections can hold wrapper classes such as , , or .

Collections are generic and hence invariant, but arrays are covariant. This can be considered an advantage of generic objects such as  when compared to arrays, because under circumstances, using the generic  instead of an array prevents run time exceptions by instead throwing a compile-time exception to inform the developer to fix the code. For example, if a developer declares an  object, and assigns the  object to the value returned by a new  instance with a certain capacity, no compile-time exception will be thrown. If the developer attempts to add a  to this  object, the java program will throw an . On the other hand, if the developer instead declared a new instance of a  as , the Java compiler will (correctly) throw a compile-time exception to indicate that the code is written with incompatible and incorrect type, thus preventing any potential run-time exceptions.The developer can fix the code by instantianting  as an  object. If the code is using Java SE7 or later versions, the developer can instatiate  as an  object by using the diamond operator

Collections are generic and hence reified, but arrays are not reified.

History
Collection implementations in pre-JDK 1.2 versions of the Java platform included few data structure classes, but did not contain a collections framework. The standard methods for grouping Java objects were via the array, the Vector, and the Hashtable classes, which unfortunately were not easy to extend, and did not implement a standard member interface.

To address the need for reusable collection data structures, several independent frameworks were developed, the most used being Doug Lea's Collections package, and ObjectSpace Generic Collection Library (JGL), whose main goal was consistency with the C++ Standard Template Library (STL).

The collections framework was designed and developed primarily by Joshua Bloch, and was introduced in JDK 1.2. It reused many ideas and classes from Doug Lea's Collections package, which was deprecated as a result. Sun Microsystems chose not to use the ideas of JGL, because they wanted a compact framework, and consistency with C++ was not one of their goals.

Doug Lea later developed a concurrency package, comprising new Collection-related classes. An updated version of these concurrency utilities was included in JDK 5.0 as of JSR 166.

Architecture
Almost all collections in Java are derived from the  interface. Collection defines the basic parts of all collections. 

The interface has the  and  methods for adding to and removing from a Collection respectively. It also has the  method, which converts the Collection into an array of Objects in the Collection (with return type of Object[]). Finally, the 
 method checks if a specified element exists in the Collection. 

The Collection interface is a subinterface of , so any Collection may be the target of a for-each statement. (The Iterable interface provides the  method used by for-each statements.) All Collections have an  that goes through all of the elements in the Collection.

Collection is generic. Any Collection can store any . For example, any implementation of  contains  objects. No casting is required when using the  objects from an implementation of Collection<String>.  Note that the angled brackets  can hold a type argument that specifies which type the Collection holds.

Types of collection 
There are several generic types of Collection: Queues, maps, lists and sets.

Queues allow the programmer to insert items in a certain order and retrieve those items in the same order. An example is a waiting list. The base interfaces for queues are called Queue.

Dictionaries/Maps store references to objects with a lookup key to access the object's values. One example of a key is an identification card. The base interface for dictionaries/maps is called Map.

Lists are finite collections where it can store the same value multiple times.

Sets are unordered collections that can be iterated and contain each element at most once. The base interface for sets is called Set.

List interface
Lists are implemented in the collections framework via the interface. It defines a list as essentially a more flexible version of an array. Elements have a specific order, and duplicate elements are allowed. Elements can be placed in a specific position. They can also be searched for within the list.

List implementations
There are several concrete classes that implement List, including  and all of its corresponding subclasses, as well as .

AbstractList class
The direct subclasses of  class include ,  and .

 is an example of a skeletal implementation, which leverages and combines the advantages of interfaces and abstract classes by making it easy for the developer to develop their own implementation for the given interface.

ArrayList class
The  class implements the List as an array. Whenever functions specific to a List are required, the class moves the elements around within the array in order to do it.

LinkedList class
The  class stores the elements in nodes that each have a pointer to the previous and next nodes in the List. The List can be traversed by following the pointers, and elements can be added or removed simply by changing the pointers around to place the node in its proper place.

Vector class
The  class has  as its direct subclass. This is an example of a violation of the composition over inheritance principle in the Java platform libraries, since in computer science, a vector is generally not a stack. Composition would have been more appropriate in this scenario.

Stack class
The Stack class extends class  with five operations that allow a Vector to be treated as a Stack.
Stacks are created using . The Stack offers methods to put a new object on the Stack (method ) and to get objects from the Stack (method ). A Stack returns the object according to last-in-first-out (LIFO), e.g. the object which was placed latest on the Stack is returned first. java.util.Stack is a standard implementation of a stack provided by Java.

The Stack class represents a last-in-first-out (LIFO) stack of objects. The Stack class has five additional operations that allow a Vector to be treated as a Stack. The usual  and  operations are provided, as well as a method () to peek at the top item on the Stack, a method to test for whether the Stack is empty (), and a method to search the Stack for an item and discover how far it is from the top (). When a Stack is first created, it contains no items.

CopyOnWriteArrayList class
The  extends the  class, and does not extend any other classes.  allows for thread-safety without performing excessive synchronization. 

In some scenarios, synchronization is mandatory. For example, if a method modifies a static field, and the method must be called by multiple threads, then synchronization is mandatory and concurrency utilities such as  should not be used. 

However synchronization can incur a performance overhead. For scenarios where synchronization is not mandatory, then the  is a viable, thread-safe alternative to synchronization that leverages multi-core processors and results in higher CPU utilization.

Queue interfaces
The  interface defines the queue data structure, which stores elements in the order in which they are inserted. New additions go to the end of the line, and elements are removed from the front. It creates a first-in first-out system. This interface is implemented by java.util.LinkedList, , and .

Queue implementations

AbstractQueue class
The direct subclasses of  class include , , , ,
.
 and 
.

Note that  and  both extend  but do not extend any other abstact classes such as .

 is an example of a skeletal implementation.

PriorityQueue class
The java.util.PriorityQueue class implements java.util.Queue, but also alters it. PriorityQueue has an additional  method. Instead of elements being ordered in the order in which they are inserted, they are ordered by priority. The method used to determine priority is either the  method in the elements, or a method given in the constructor. The class creates this by using a heap to keep the items sorted.

ConcurrentLinkedQueue class
The java.util.concurrent.ConcurrentLinkedQueue class extends . ConcurrentLinkedQueue implements the  interface.

The ConcurrentLinkedQueue class is a thread-safe collection, since for any an element placed inside a , the Java Collection Library guarantees that the element is safely published by allowing any thread to get the element from the collection. An object is said to be safely published if the object's state is made visible to all other thread at the same point in time. Safe publication usually requires synchronization of the publishing and consuming threads.

BlockingQueue interface
The 
 interface extends Queue.

The  interface has the following direct sub-interfaces:  and .  works like a regular Queue, but additions to and removals from the BlockingQueue are blocking. If 
 is called on an empty BlockingQueue, it can be set to wait either a specified time or indefinitely for an item to appear in the BlockingQueue. Similarly, adding an item using the method  is subject to an optional capacity restriction on the BlockingQueue, and the method can wait for space to become available in the BlockingQueue before returning. BlockingQueue interface introduces a method  which removes and gets the head of the BlockingQueue, and waits until the BlockingQueue is no longer empty if required.

Double-ended queue (Deque) interfaces
The  interface
extends the  interface.  creates a double-ended queue. While a regular  only allows insertions at the rear and removals at the front, the  allows insertions or removals to take place both at the front and the back. A  is like a  that can be used forwards or backwards, or both at once. Additionally, both a forwards and a backwards iterator can be generated. The  interface is implemented by java.util.ArrayDeque and java.util.LinkedList.

Deque implementations

LinkedList class

LinkedList, of course, also implements the List interface and can also be used as one. But it also has the Queue methods. LinkedList implements the  interface, giving it more flexibility.

ArrayDeque class

ArrayDeque implements the Queue as an array. Similar to LinkedList, ArrayDeque also implements the  interface.

BlockingDeque interface
The  interface extends java.util.concurrent.BlockingQueue.  is similar to . It provides the same methods for insertion and removal with time limits for waiting for the insertion or removal to become possible. However, the interface also provides the flexibility of a Deque. Insertions and removals can take place at both ends. The blocking function is combined with the Deque function.

Set interfaces
Java's interface defines the Set. A Set can't have any duplicate elements in it. Additionally, the Set has no set order. As such, elements can't be found by index. Set is implemented by , , and .

Set interface implementations
There are several implementations of the Set interface, including  and its subclasses, and the final static inner class  (where  and  are formal type parameters).

AbstractSet 
 is a skeletal implementation for the  interface.

Direct subclasses of  include , , ,  and .

EnumSet class
The  class extends . The  class has no public constructors, and only contain static factory methods. 

 contains the static factory method . This method is an aggregation method. It takes in several parameters, takes into account of the type of the parameters, then returns an instance with the appropriate type. As of 2018, In Java SE8 OpenJDK implementation uses two implementations of  which are invisible to the client, which are  and . If the  no longer provided any performance benefits for small enum types, it could be removed from the library without negatively impacting the Java Collection Library.

 is a good replacement for the bit fields, which is a type of set, as described below.

Traditionally, whenever developers encountered elements of an enumerated type that needs to be placed in a set, the developer would use the int enum pattern in which every constant is assigned a different power of 2. This bit representation enables the developer to use the bitwise OR operation, so that the constants can be combined into a set, also known as a bit field. 
This bit field representation enables the developer to make efficient set-based operations and bitwise arithmetic such as intersection and unions.

However, there are many problems with bit field representation approach. A bit field is less readable than an int enum constant. Also, if the elements are represented by bit fields, it is impossible to iterate through all 
of these elements.

A recommended alternative approach is to use an , where an int enum is used instead of a bit field. This approach uses an  to represent the set of values that belong to the same  type. Since the  implements the  interface and no longer requires the use of bit-wise operations, this approach is more type-safe. Furthermore, there are many static factories that allow for object instantiation, such as the method  method.

After the introduction of the , the bit field representation approach is considered to be obsolete.

HashSet class
HashSet uses a hash table. More specifically, it uses a  to store the hashes and elements and to prevent duplicates.

LinkedHashSet class
The java.util.LinkedHashSet class extends  by creating a doubly linked list that links all of the elements by their insertion order. This ensures that the iteration order over the Set is predictable.

CopyOnWriteArraySet class
 is a concurrent replacement for a synchronized . It provides improved concurrency in many situations by removing the need to perform synchronization or making a copy of the object during iteration, similar to how  acts as the concurrent replacement for a synchronized .
On the other hand, similar to ,  should not be used when sychronization is mandatory.

SortedSet interface
The java.util.SortedSet interface extends the java.util.Set interface. Unlike a regular Set, the elements in a SortedSet are sorted, either by the element's  method, or a method provided to the constructor of the SortedSet. The first and last elements of the SortedSet can be retrieved using the  and  methods respectively, and subsets can be created via minimum and maximum values, as well as beginning or ending at the beginning or ending of the SortedSet. The java.util.TreeSet class implements the SortedSet interface.

NavigableSet interface
The  interface extends the java.util.SortedSet interface and has a few additional methods. The , , , and  methods find an element in the set that's close to the parameter. Additionally, a descending iterator over the items in the Set is provided. As with SortedSet, java.util.TreeSet implements NavigableSet.

TreeSet class
java.util.TreeSet uses a red–black tree implemented by a . The red–black tree ensures that there are no duplicates. Additionally, it allows TreeSet to implement .

ConcurrentSkipListSet class
 acts as a concurrent replacement for implementations of a synchronized . For example it replaces a  that has been wrapped by the  method.

Map interfaces
Maps are defined by the  interface in Java.

Map interface implementations
Maps are data structures that associate a key with an element. This lets the map be very flexible. If the key is the hash code of the element, the Map is essentially a Set. If it's just an increasing number, it becomes a list. 

Examples of  implementations include ,  , and .

AbstractMap class

 is an example of a skeletal implementation. 

The direct subclasses of  class include , , , ,  and .

EnumMap
 extends .  has comparable speed with an ordinal-indexed array. This is because  internally uses an array, with implementation details completely hidden from the developer.  Hence, the EnumMap gets the type safety of a  while the performance advantages of an array.

HashMap
 uses a hash table. The hashes of the keys are used to find the elements in various buckets. The  is a hash-based collection.

LinkedHashMap
 extends  by creating a doubly linked list between the elements, allowing them to be accessed in the order in which they were inserted into the map.  contains a protected removeEldestEntry method which is called by the put method whenever a new key is added to the Map. The Map removes its eldest entry whenever removeEldestEntry returns true. The removeEldestEntry method can be overridden.

TreeMap
, in contrast to  and , uses a red–black tree. The keys are used as the values for the nodes in the tree, and the nodes point to the elements in the Map.

ConcurrentHashMap
 is similar to  and is also a hash-based collection.  However, there are a number of differences, such as the differences in the locking strategy they use.

The  uses a completely different locking strategy to provide improved scalability and concurrency.  does not synchronize every method using the same lock. Instead,  use a mechanism known as lock striping. This mechanism provides a finer-grained locking mechanism. It also permits a higher degree of shared access.

ConcurrentSkipListMap class
 acts as a concurrent replacement for implementations of a synchronized .  is very similar to , since  replaces a  that has been wrapped by the  method.

Map subinterfaces

SortedMap interface
The  interface extends the java.util.Map interface. This interface defines a Map that's sorted by the keys provided. Using, once again, the compareTo() method or a method provided in the constructor to the SortedMap, the key-element pairs are sorted by the keys. The first and last keys in the Map can be called by using the  and  methods respectively. Additionally, submaps can be created from minimum and maximum keys by using the  method. SortedMap is implemented by java.util.TreeMap.

NavigableMap interface
The  interface extends java.util.SortedMap in various ways. Methods can be called that find the key or map entry that's closest to the given key in either direction. The map can also be reversed, and an iterator in reverse order can be generated from it. It's implemented by java.util.TreeMap.

ConcurrentMap interface

The  interface extends the java.util.Map interface. This interface a thread Safe  interface, introduced as of Java programming language's Java Collections Framework version 1.5.

Extensions to the Java collections framework
Java collections framework is extended by the Apache Commons Collections library, which adds collection types such as a bag and bidirectional map, as well as utilities for creating unions and intersections.

Google has released its own collections libraries as part of the guava libraries.

See also

 Collection
 Container
 Standard Template Library
 Java concurrency
 Java ConcurrentMap

Citation

References

Java (programming language)
Data structures libraries and frameworks