Collections Class in Java

Collections class in Java is one of the utility classes in Java Collections Framework. The java.util package contains the Collections class in Java. Java Collections class is used with the static methods that operate on the collections or return the collection. All the methods of this class throw the NullPointerException if the collection or object passed to the methods is null.  

Collection Class declaration

The syntax of the Collection class declaration is mentioned below:

public class Collections extends Object

Remember: Object is the parent class of all the classes.

 

Java Collection Class

Collection Framework contains both classes and interfaces. Although both seem the same but there are certain differences between Collection classes and the Collection framework. There are some classes in Java as mentioned below:

1. ArrayList

ArrayList is a class implemented using a list interface, in that provides the functionality of a dynamic array where the size of the array is not fixed.

Syntax:

ArrayList<_type_> var_name = new ArrayList<_type_>();

2. Vector

Vector is a Part of the collection class that implements a dynamic array that can grow or shrink its size as required.

Syntax:

public class Vector<E> extends AbstractList<E> implements List<E>, RandomAccess, 
Cloneable, Serializable

3. Stack

Stack is a part of Java collection class that models and implements a Stack data structure. It is based on the basic principle of last-in-first-out(LIFO) .

Syntax:

public class Stack<E> extends Vector<E>

4. LinkedList

LinkedList class is an implementation of the LinkedList data structure. It can store the elements that are not stored in contiguous locations and every element is a separate object with a different data part and different address part.

Syntax:

LinkedList name = new LinkedList();  

5. HashSet

HashSet is implemented using the Hashtable data structure. It offers constant time performance for the performing operations like add, remove, contains, and size.

Syntax:

public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable

6. LinkedHashSet

LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements.

Syntax:

public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, Serializable

7. TreeSet

TreeSet class is implementations of the SortedSet interface in Java that uses a Tree for storage. The ordering of the elements is maintained by a set using their natural ordering whether an explicit comparator is provided or not. 

Syntax:

TreeSet t = new TreeSet(Collection col);

8. PriorityQueue

The PriorityQueue is based on the priority heap. The elements of the priority queue are ordered according to the natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used. 

Syntax:

public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable

9. ArrayDeque

The ArrayDeque class in Java is an implementation of the Deque interface that uses a resizable array to store its elements. The ArrayDeque class provides constant-time performance for inserting and removing elements from both ends.

Syntax:

public class ArrayDeque<E> extends 
AbstractCollection<E> implements Deque<E>, Cloneable,Serializable

10. HashMap

HashMap Class is similar to HashTable but the data unsynchronized. It stores the data in (Key, Value) pairs, and you can access them by an index of another type.

Syntax:

public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable

11. EnumMap

EnumMap extends AbstractMap and implements the Map interface in Java.

Syntax:

public class EnumMap<K extends Enum<K>,​V> extends 
AbstractMap<K,​V> implements Serializable, Cloneable

12. AbstractMap

The AbstractMap class is a part of the Java Collection Framework. It implements the Map interface to provide a structure to it, by doing so it makes the further implementations easier.

Syntax:

public abstract class AbstractMap<K,V> extends Object, implements Map<K,V>

13. TreeMap

A TreeMap is implemented using a Red-Black tree.TreeMap provides an ordered collection of key-value pairs, where the keys are ordered based on their natural order or a custom Comparator passed to the constructor.

SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...)); 

Java Collections Class Fields

The collection class contains 3 fields as listed below which can be used to return immutable entities.

• EMPTY_LIST to get an immutable empty List
• EMPTY_SET to get an immutable empty Set
• EMPTY_MAP to get an immutable empty Map

Now let us do discuss methods that are present inside this class so that we can use these inbuilt functionalities later on in our program. Below are the methods have been listed below in a tabular format as shown below as follows:

Now, we are done with listing all the methods, so by now we have a faint hint with us in perceiving how important these methods are when thinking about a global programming perspective. The important and frequently used methods while writing optimized code, as you will see these methods somehow in nearly every Java optimized code because of havoc usage of Collections class in Java. So here, more likely than in any class, we will not just be implementing the method but also be discussing operations that can be performed so that one can have conceptual clarity and strong command while implementing the same. 

Java Collections Example

Examples of Collections Classes in Java are mentioned below:

• Adding Elements to the Collections 
• Sorting a Collection
• Searching in a Collection
• Copying Elements
• Disjoint Collection

1. Adding elements to the Collections class object 

The addAll() method of java.util.Collections class is used to add all the specified elements to the specified collection. Elements to be added may be specified individually or as an array.

Example 

Shoes Toys Fruits Bat Ball 

 2. Sorting a Collection

java.util.Collections.sort() is used to sort the elements present in the specified list of Collections in ascending order. java.util.Collections.reverseOrder() is used to sort in descending order. 

Example

Bat Fruits Mouse Shoes Toys 
Toys Shoes Mouse Fruits Bat 

3. Searching in a Collection

java.util.Collections.binarySearch() method returns the position of an object in a sorted list. To use this method, the list should be sorted in ascending order, otherwise, the result returned from the method will be wrong. If the element exists in the list, the method will return the position of the element in the sorted list, otherwise, the result returned by the method would be the – (insertion point where the element should have been present if exist)-1). 

Example

The index of Horse is 2
The index of Dog is -2

4. Copying Elements

The copy() method of java.util.Collections class is used to copy all the elements from one list into another. After the operation, the index of each copied element in the destination list will be identical to its index in the source list. The destination list must be at least as long as the source list. If it is longer, the remaining elements in the destination list are unaffected.

Example

The Original Destination list is 
Shoes Toys Horse Tiger 
The Destination List After copying is 
Bat Frog Lion Tiger 

5. Disjoint Collection

java.util.Collections.disjoint() is used to check whether two specified collections are disjoint or not. More formally, two collections are disjoint if they have no elements in common. It returns true if the two collections do not have any element in common. 

Example

true

Related Articles:

• java.util
• Java Collections Framework
• Static methods