Java Collections Framework in Depth with Examples for Beginners

The Collection in Java is a framework that provides an architecture to store and manipulate the group of objects.

All the operations that you perform on data such as searching, sorting, insertion, manipulation, deletion, etc. can be achieved by Java Collections.

Collection Framework defines several classes and interfaces to represent a group of objects as a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data. Typically, they represent data items that form a natural group, such as a poker hand (a collection of cards), a mail folder (a collection of letters), or a telephone directory (a mapping of names to phone numbers).

What is a Collections Framework?

A collections framework is a unified architecture for representing and manipulating collections.

All collections frameworks contain the following:

• Interfaces: These are abstract data types that represent collections. Interfaces allow collections to be manipulated independently of the details of their representation. In object-oriented languages, interfaces generally form a hierarchy.
• Implementations: These are the concrete implementations of the collection interfaces. In essence, they are reusable data structures.
• Algorithms: These are the methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces. The algorithms are said to be polymorphic: that is, the same method can be used on many different implementations of the appropriate collection interface. In essence, algorithms are reusable functionality.

Benefits of the Java Collections Framework

The Java Collections Framework provides the following benefits:

1. Reduces programming effort by providing data structures and algorithms so you don't have to write them yourself.
2. Increases performance by providing high-performance implementations of data structures and algorithms. Because the various implementations of each interface are interchangeable, programs can be tuned by switching implementations.
3. Provides interoperability between unrelated APIs by establishing a common language to pass collections back and forth.
4. Reduces the effort required to learn APIs by requiring you to learn multiple ad hoc collection APIs.
5. Reduces the effort required to design and implement APIs by not requiring you to produce ad hoc collections APIs.
6. Fosters software reuse by providing a standard interface for collections and algorithms with which to manipulate them.

Hierarchy of Collection Framework

Let us see the hierarchy of Collection framework. The java.util package contains all the classes and interfaces for the Collection framework.

The collection interfaces are divided into two groups:

The most basic interface, java.util.Collection.

 The other collection interfaces are based on java.util.Map and are not true collections

Collection Interfaces

The collection interfaces are divided into two groups. The most basic interface, java.util.Collection, has the following descendants:

• java.util.Collection 
• java.util.List
• java.util.Set 
• java.util.SortedSet 
• java.util.NavigableSet 
• java.util.Queue 
• java.util.concurrent.BlockingQueue 
• java.util.concurrent.TransferQueue 
• java.util.Deque 
• java.util.concurrent.BlockingDeque 

The other collection interfaces are based on java.util.Map and are not true collections. However, these interfaces contain collection-view operations, which enable them to be manipulated as collections. Map has the following offspring:

• java.util.Map
• java.util.SortedMap
• java.util.NavigableMap 
• java.util.concurrent.ConcurrentMap 
• java.util.concurrent.ConcurrentNavigableMap

Collection Implementations

Classes that implement the collection interfaces typically have names in the form of . The general purpose implementations are summarized in the following table:

The AbstractCollection, AbstractSet, AbstractList, AbstractSequentialList and AbstractMapclasses provide basic implementations of the core collection interfaces, to minimize the effort required to implement them. The API documentation for these classes describes precisely how each method is implemented so the implementer knows which methods must be overridden, given the performance of the basic operations of a specific implementation.

Commonly used implementations

The Java Collections Framework provides several general-purpose implementations of the core interfaces:

• For the Set interface, HashSet is the most commonly used implementation.
• For the List interface, ArrayList is the most commonly used implementation.
• For the Map interface, HashMap is the most commonly used implementation.
• For the Queue interface, LinkedList is the most commonly used implementation.
• For the Deque interface, ArrayDeque is the most commonly used implementation.

Java Collections Tutorials/Guides

You can learn everything about Java collections framework here. Check out each article, tutorials, guides below gives you a hands-on experience.

   Interfaces

• Collections Framework - The Collection Interface
• Collections Framework - The Set Interface 
• Collections Framework - The SortedSet Interface 
• Collections Framework - The List Interface
• Collections Framework - The Queue Interface
• Collections Framework - The Deque Interface
• Collections Framework - The Map Interface 
• Collections Framework - The SortedMap Interface

List Implementations

General-Purpose List Implementations:

• Collections Framework - ArrayList Class // Popular
• Collections Framework - LinkedList Class // Popular

Special-Purpose List Implementations:

• Collections Framework - CopyOnWriteArrayList

Set Implementations

General-Purpose Set Implementations:

• Collections Framework - HashSet Class
• Collections Framework - LinkedHashSet Class
• Collections Framework - TreeSet Class

Special-Purpose Set Implementations:

• Collections Framework - CopyOnWriteArraySet
• Collections Framework - EnumSet

Map Implementations

General-purpose Map Implementations:

• Collections Framework - HashMap Class
• Collections Framework - LinkedHashMap Class
• Collections Framework - TreeMap class

Special-Purpose Map Implementations:

• Collections Framework - EnumMap
• Collections Framework - WeakHashMap
• Collections Framework - IdentityHashMap

Sorting

• Guide to Algorithms used in Java Library
• Guide to Comparable Interface
• Guide to Comparator Interface
• Sorting in Collections with Examples

Aggregate Operations(Streams)

• Collections Aggregate Operations
• Stream Reduction Operations

Collections Helper/Utility Classes

• java.util.Collections Class API Guide(Development)

          - Sorting

          - Shuffling

          - Routine Data Manipulation

          - Searching

          - Composition

          - Finding Extreme Values

• 18 Useful Collections Utility Methods (Development)
• Java CollectionUtils Class(Development)

Convert Collections Examples

• Conversion Between Array and Set in Java
• Conversion Between Array and List in Java
• Java Convert Map to Set Example
• Java Convert Map to List Example
• Java Convert Map to Array Example
• Convert a Map to an Array, List and Set in Java
• Java 8 Convert List to Map Example
• Java 8 - Merging Two Maps Example
• Java Convert Array to String [Snippet]

Collections Examples

• Different Ways to Iterate over List, Set and Map in Java
• Java Comparator Interface Example
• Java Comparable Interface Example
• Java IdentityHashMap Example
• Java WeakHashMap Example
• Java EnumMap Example
• Java CopyOnWriteArraySet Example
• Java EnumSet Class Example
• Guide to Java 8 forEach Method
• Different Ways to Iterate over a List in Java [Snippet]
• Different Ways to Iterate over a Set in Java [Snippet]
• Different Ways to Iterate over a Map in Java [Snippet]

Collections Differences

• Difference between LinkedList vs ArrayList in Java
• Difference between List and Set in Java Collection
• Difference between Stack and Queue Data Structure in Java
• Difference between HashMap and HashSet in Java
• Difference between TreeSet, LinkedHashSet, and HashSet in Java with Example
• Difference between EnumMap and HashMap in Java
• Difference between HashMap and HashSet in Java
• Difference between ArrayList and Vector in Java
• Difference between HashMap, LinkedHashMap, and TreeMap in Java
• Difference between PriorityQueue and TreeSet in Java?
• What is the difference between Enumeration and Iterator in Java?
• What is the difference between HashMap and Hashtable in Java?
• Difference between fail-fast Iterator vs fail-safe Iterator in Java 

Best Practices and Interviews

• Java Collection Framework Best Practice  (Best Practice)
• Difference between Array vs ArrayList in Java (interview)
• How the Size of the ArrayList Increases Dynamically? (interview)
• How To Remove Duplicate Elements From ArrayList In Java? (interview)

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