Collections Class in Java

Collections class in Java is one of the utility classes in the Java Collections Framework. The java.util package contains the Collections class in Java. The 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 a NullPointerException if the collection or object passed to the methods is null.  

Example 1: Here, we will use ArrayList, which is a class from the Java Collections framework. It allows for storing elements in a list by maintaining the insertion order and also allows duplicates.

import java.util.ArrayList;

public class Geeks {
    public static void main(String[] args) {
      
        // Create an ArrayList to store elements
        ArrayList<String> al = new ArrayList<>();
        
        // Add elements to the list
        al.add("Apple");
        al.add("Banana");
        al.add("Apple"); // Duplicates are allowed

        System.out.println("" + al);
    }
}

[Apple, Banana, Apple]

Collection Class Declaration

The Declaration of collection class is listed below:

public class Collections extends Object

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

Common Java Collection Classes

Collection Framework contains both classes and interfaces. Although both seem the same but there are certain differences between Collection classes and the Collections framework.

To know the difference between Collection and Collections, refer this article: Collection vs. Collections

The Collection classes in Java are 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<_type_> var_name = new LinkedList<_type_>();

5. HashSet

HashSet is implemented using the Hashtable data structure. It helps us add, remove, or find items very quickly, no matter how many items there are.

Syntax:

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

Note: HashSet internally uses HashMap not Hashtable.

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<E> set = new TreeSet<>();

8. PriorityQueue

The PriorityQueue keeps the element in order based on their importance. We can use the normal order or can choose our own way to decide which item comes first.

Syntax:

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

9. ArrayDeque

ArrayDeque is a class that works with double ended queue. It uses a resizable array to store elements. It provides constant-time for adding and removing elements from both the 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 element in (Key, Value) pairs, and we can access the elemenets with the help of index.

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> implements Map<K,V>

13. TreeMap

A TreeMap is implemented using a Red-Black tree. It is a kind of map that keys the keys in order. The keys are sorted either by their usual order or by a custom rule we give when creating the map.

Syntax:

SortedMap<K, V> 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

Methods of Collections Class

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 have listed all the methods, It is clear how important they are in writing optimized Java code. The Collections class is widely used and its methods appear in almost every optimized Java program. Here, we will implement these methods also discuss their operations.

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:

// Adding Elements
// Using addAll() method

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Geeks {

    public static void main(String[] args) {
      
        List<String> l = new ArrayList<>();

        // Adding elements to the list
        l.add("Shoes");
        l.add("Toys");

        // Add one or more elements
        Collections.addAll(l, "Fruits", "Bat", "Ball");

        for (int i = 0; i < l.size(); i++) {
            System.out.print(l.get(i) + " ");
        }
    }
}

Shoes Toys Fruits Bat Ball 

2. Sorting a Collection

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

Example:

// Sorting a Collections using sort() method
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Geeks {

    public static void main(String[] args) {

        List<String> l = new ArrayList<>();

        // Adding elements to the list
        // using add() method
        l.add("Shoes");
        l.add("Toys");

        // Adding one or more 
        // element using addAll()
        Collections.addAll(l, "Fruits", "Bat", "Mouse");

        // Sorting according to default ordering
        // using sort() method
        Collections.sort(l);

        // Printing the elements
        for (int i = 0; i < l.size(); i++) {
            System.out.print(l.get(i) + " ");
        }

        System.out.println();

        // Sorting according to reverse ordering
        Collections.sort(l, Collections.reverseOrder());

        // Printing the reverse order
        for (int i = 0; i < l.size(); i++) {
            System.out.print(l.get(i) + " ");
        }
    }
}

Bat Fruits Mouse Shoes Toys 
Toys Shoes Mouse Fruits Bat 

3. Searching in a Collection

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, if the element does not exist in the list then this method will return a negative number that shows where the item would be inserted in the list - 1.

Example:

// Binary Search using Collections.binarySearch()
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Geeks {

    public static void main(String[] args) {

        List<String> l = new ArrayList<>();

        // Adding elements to object
        // using add() method
        l.add("Shoes");
        l.add("Toys");
        l.add("Horse");
        l.add("Ball");
        l.add("Grapes");

        // Sort the List
        Collections.sort(l);

        // BinarySearch on the List
        System.out.println(
            "The index of Horse is: "
            + Collections.binarySearch(l, "Horse"));

        // BinarySearch on the List
        System.out.println(
            "The index of Dog is: "
            + Collections.binarySearch(l, "Dog"));
    }
}

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

Note: The list must be sorted before using binarySearch to get the correct results.

4. Copying Elements

The copy() method of 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:

// Copying Elements using copy() method
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Geeks {

    public static void main(String[] args) {

        List<String> l1 = new ArrayList<>();

        // Add elements
        l1.add("Shoes");
        l1.add("Toys");
        l1.add("Horse");
        l1.add("Tiger");

        // Print the elements
        System.out.println(
            "The Original Destination list is: ");

        for (int i = 0; i < l1.size(); i++) {
            System.out.print(l1.get(i) + " ");
        }
        System.out.println();

        // Create source list
        List<String> l2 = new ArrayList<>();

        // Add elements
        l2.add("Bat");
        l2.add("Frog");
        l2.add("Lion");

        // Copy the elements from source to destination
        Collections.copy(l1, l2);

        // Printing the modified list
        System.out.println(
            "The Destination List After copying is: ");

        for (int i = 0; i < l1.size(); i++) {
            System.out.print(l1.get(i) + " ");
        }
    }
}

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

5. Disjoint Collection

Collections.disjoint() is used to check whether two specified collections have nothing in common. It returns true if the two collections do not have any element in common. 

Example:

// Working of Disjoint Function
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Geeks {

    public static void main(String[] args) {
        
        List<String> l1 = new ArrayList<>();

        // Add elements to l1
        l1.add("Shoes");
        l1.add("Toys");
        l1.add("Horse");
        l1.add("Tiger");

        List<String> l2 = new ArrayList<>();

        // Add elements to l2
        l2.add("Bat");
        l2.add("Frog");
        l2.add("Lion");

        // Check if disjoint or not
        System.out.println(
            Collections.disjoint(l1, l2));
    }
}

true