Collections in Java

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A collection in Java is a group of individual objects that are treated as a single unit. In Java, a separate framework named the "Collection Framework" was defined in JDK 1.2, which contains all the Java Collection Classes and interfaces. 

In Java, the Collection interface (java.util.Collection) and Map interface (java.util.Map) are the two main “root” interfaces of Java collection classes.

Needed for a Collection Framework

Before the Collection Framework (before JDK 1.2), Java used Arrays, Vectors, and Hashtables to group objects, but they lacked a common interface. Each had a separate implementation, making usage inconsistent and harder for developers to learn and maintain.

Let's understand this with an example of adding an element to a hashtable and a vector. 

Example:

import java.io.*;
import java.util.*;

class CollectionDemo {
    public static void main(String[] args)
    {
        // Creating instances of the array, vector and hashtable
        int arr[] = new int[] { 1, 2, 3, 4 };
        Vector<Integer> v = new Vector();
        Hashtable<Integer, String> h = new Hashtable();

        // Adding the elements into the vector
        v.addElement(1);
        v.addElement(2);

        // Adding the element into the hashtable
        h.put(1, "geeks");
        h.put(2, "4geeks");


        // Accessing the first element of the array, vector and hashtable
        System.out.println(arr[0]);
        System.out.println(v.elementAt(0));
        System.out.println(h.get(1));

    }
}

1
1
geeks

Advantages of the Java Collection Framework

Since the lack of a collection framework gave rise to the above set of disadvantages, the following are the advantages of the collection framework. 

1. Consistent API: Interfaces like List, Set, and Map have common methods across classes (ArrayList, LinkedList, etc.).
2. Less Coding Effort: Developers focus on usage, not designing data structures—supports OOP abstraction.
3. Better Performance: Offers fast, reliable implementations of data structures, improving speed and quality of code.

Hierarchy of the Collection Framework in Java

The Collection interface extends Iterable and serves as the root, defining common methods inherited by all collection classes.

Interfaces that Extend the Java Collections Interface

The collection framework contains multiple interfaces where every interface is used to store a specific type of data. The following are the interfaces present in the framework. 

1. Iterable Interface

Iterable interface is the root of the Collection Framework. It is extended by the Collection interface, making all collections inherently iterable. Its primary purpose is to provide an Iterator to traverse elements, defined by its single abstract method iterator().

Iterator iterator();

2. Collection Interface

Collection interface extends Iterable and serves as the foundation of the Collection Framework. It defines common methods like add(), remove(), and clear(), ensuring consistency and reusability across all collection implementations.

3. List Interface

List interface extends the Collection interface and represents an ordered collection that allows duplicate elements. It is implemented by classes like ArrayList, Vector, and Stack. Since all these classes implement List, a list object can be instantiated using any of them

For example: 

List <T> al = new ArrayList<> (); 
List <T> ll = new LinkedList<> (); 
List <T> v = new Vector<> (); 
Where T is the type of the object

The classes which implement the List interface are as follows:

ArrayList

ArrayList provides a dynamic array in Java that resizes automatically as elements are added or removed. Although slower than standard arrays, it is efficient for frequent modifications. It supports random access but cannot store primitive types directly—wrapper classes like Integer or Character are required

Let's understand the ArrayList with the following example:

import java.io.*;
import java.util.*;

class GFG {

    // Main Method
    public static void main(String[] args)
    {

        // Declaring the ArrayList with initial size n
        ArrayList<Integer> al = new ArrayList<Integer>();

        // Appending new elements at the end of the list
        for (int i = 1; i <= 5; i++)
            al.add(i);

        // Printing elements
        System.out.println(al);

        // Remove element at index 3
        al.remove(3);

        // Displaying the ArrayList after deletion
        System.out.println(al);

        // Printing elements one by one
        for (int i = 0; i < al.size(); i++)
            System.out.print(al.get(i) + " ");
    }
}

[1, 2, 3, 4, 5]
[1, 2, 3, 5]
1 2 3 5 

LinkedList:

LinkedList is a linear data structure where elements (nodes) are stored non-contiguously. Each node contains data and a reference to the next (and optionally previous) node, forming a chain of elements linked by pointers. 

Let's understand the LinkedList with the following example:

import java.io.*;
import java.util.*;

class GFG {

    // Main Method
    public static void main(String[] args)
    {

        // Declaring the LinkedList
        LinkedList<Integer> ll = new LinkedList<Integer>();

        // Appending new elements at
        // the end of the list
        for (int i = 1; i <= 5; i++)
            ll.add(i);

        // Printing elements
        System.out.println(ll);

        // Remove element at index 3
        ll.remove(3);

        // Displaying the List
        // after deletion
        System.out.println(ll);

        // Printing elements one by one
        for (int i = 0; i < ll.size(); i++)
            System.out.print(ll.get(i) + " ");
    }
}

[1, 2, 3, 4, 5]
[1, 2, 3, 5]
1 2 3 5 

Vector:

Vector provides a dynamic array in Java, similar to ArrayList, but with synchronized methods for thread safety. While slower due to synchronization overhead, it is useful in multi-threaded environments. Like ArrayList, it resizes automatically during element manipulation.

Let's understand the Vector with an example:

import java.io.*;
import java.util.*;

class GFG {

    // Main Method
    public static void main(String[] args)
    {

        // Declaring the Vector
        Vector<Integer> v = new Vector<Integer>();

        // Appending new elements at the end of the list
        for (int i = 1; i <= 5; i++)
            v.add(i);

        // Printing elements
        System.out.println(v);

        // Remove element at index 3
        v.remove(3);

        // Displaying the Vector after deletion
        System.out.println(v);

        // Printing elements one by one
        for (int i = 0; i < v.size(); i++)
            System.out.print(v.get(i) + " ");
    }
}

[1, 2, 3, 4, 5]
[1, 2, 3, 5]
1 2 3 5 

Stack

Stack class implements the LIFO (last-in-first-out) data structure. It supports core operations like push() and pop(), along with peek(), empty(), and search(). Stack is a subclass of Vector and inherits its properties.

Let's understand the stack with an example:

import java.util.*;
public class GFG {

    // Main Method
    public static void main(String args[])
    {
        Stack<String> stack = new Stack<String>();
        stack.push("Geeks");
        stack.push("For");
        stack.push("Geeks");
        stack.push("Geeks");

        // Iterator for the stack
        Iterator<String> itr = stack.iterator();

        // Printing the stack
        while (itr.hasNext()) {
            System.out.print(itr.next() + " ");
        }

        System.out.println();

        stack.pop();

        // Iterator for the stack
        itr = stack.iterator();

        // Printing the stack
        while (itr.hasNext()) {
            System.out.print(itr.next() + " ");
        }
    }
}

Geeks For Geeks Geeks 
Geeks For Geeks 

Note: Stack is a subclass of Vector and a legacy class. It is thread-safe which might be overhead in an environment where thread safety is not needed. An alternate to Stack is to use ArrayDequeue which is not thread-safe and has faster array implementation.

4. Queue Interface

The Queue interface follows the FIFO (First-In, First-Out) principle, where elements are processed in the order they are added—similar to a real-world queue (e.g., ticket booking). It is used when order matters. Classes like PriorityQueue and ArrayDeque implement this interface, allowing queue objects to be instantiated accordingly.

For example:

Queue <T> pq = new PriorityQueue<> (); 
Queue <T> ad = new ArrayDeque<> (); 
Where T is the type of the object. 

The most frequently used implementation of the queue interface is the PriorityQueue. 

Priority Queue

PriorityQueue processes elements based on their priority rather than insertion order. It uses a priority heap for internal storage. Elements are ordered either by their natural ordering or by a custom Comparator provided at construction.

Let's understand the priority queue with an example:

import java.util.*;

class GfG {

    // Main Method
    public static void main(String args[])
    {
        // Creating empty priority queue
        PriorityQueue<Integer> pQueue
            = new PriorityQueue<Integer>();

        // Adding items to the pQueue using add()
        pQueue.add(10);
        pQueue.add(20);
        pQueue.add(15);

        // Printing the top element of PriorityQueue
        System.out.println(pQueue.peek());

        // Printing the top element and removing it
        // from the PriorityQueue container
        System.out.println(pQueue.poll());

        // Printing the top element again
        System.out.println(pQueue.peek());
    }
}

10
10
15

5. Deque Interface

Deque interface extends Queue and allows insertion and removal of elements from both ends. It is implemented by classes like ArrayDeque, which can be used to instantiate a Deque object.

For example:

Deque<T> ad = new ArrayDeque<> (); 
Where T is the type of the object. 

The class which implements the deque interface is ArrayDeque. 

ArrayDeque

ArrayDeque class implements a resizable, double-ended queue that allows insertion and removal from both ends. It has no capacity restrictions and grows automatically as needed.

Let's understand ArrayDeque with an example:
 

import java.util.*;
public class ArrayDequeDemo {
    public static void main(String[] args)
    {
        // Initializing an deque
        ArrayDeque<Integer> de_que
            = new ArrayDeque<Integer>(10);

        // add() method to insert
        de_que.add(10);
        de_que.add(20);
        de_que.add(30);
        de_que.add(40);
        de_que.add(50);

        System.out.println(de_que);

        // clear() method
        de_que.clear();

        // addFirst() method to insert the
        // elements at the head
        de_que.addFirst(564);
        de_que.addFirst(291);

        // addLast() method to insert the
        // elements at the tail
        de_que.addLast(24);
        de_que.addLast(14);

        System.out.println(de_que);
    }
}

[10, 20, 30, 40, 50]
[291, 564, 24, 14]

6. Set Interface

Set interface represents an unordered collection that stores only unique elements (no duplicates). It's implemented by classes like HashSet, TreeSet, and LinkedHashSet, and can be instantiated using any of these
For example:

Set<T> hs = new HashSet<> (); 
Set<T> lhs = new LinkedHashSet<> (); 
Set<T> ts = new TreeSet<> (); 
Where T is the type of the object. 

The following are the classes that implement the Set interface:

HashSet

HashSet class implements a hash table and stores elements based on their hash codes. It does not guarantee insertion order and allows one null element.

Example:

import java.util.*;

public class HashSetDemo {

    // Main Method
    public static void main(String args[])
    {
        // Creating HashSet and
        // adding elements
        HashSet<String> hs = new HashSet<String>();

        hs.add("Geeks");
        hs.add("For");
        hs.add("Geeks");
        hs.add("Is");
        hs.add("Very helpful");

        // Traversing elements
        Iterator<String> itr = hs.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

Very helpful
Geeks
For
Is

LinkedHashSet

LinkedHashSet is very similar to a HashSet. The difference is that this uses a doubly linked list to store the data and retains the ordering of the elements. 

Let's understand the LinkedHashSet with an example: 

import java.util.*;

public class LinkedHashSetDemo {

    // Main Method
    public static void main(String args[])
    {
        // Creating LinkedHashSet and adding elements
        LinkedHashSet<String> lhs
            = new LinkedHashSet<String>();

        lhs.add("Geeks");
        lhs.add("For");
        lhs.add("Geeks");
        lhs.add("Is");
        lhs.add("Very helpful");

        // Traversing elements
        Iterator<String> itr = lhs.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

Geeks
For
Is
Very helpful

7. Sorted Set Interface

Sorted Set interface extends Set and maintains elements in sorted order. It includes additional methods for range views and ordering. It is implemented by the TreeSet class.

For example:

SortedSet<T> ts = new TreeSet<> (); 
Where T is the type of the object. 

The class which implements the sorted set interface is TreeSet. 
 
TreeSet

TreeSet uses a self-balancing tree (Red-Black Tree) to store elements in sorted order. It maintains natural ordering or uses a custom Comparator if provided during creation. Ordering must be consistent with equals to ensure proper Set behavior.

Let's understand TreeSet with an example:

import java.util.*;

public class TreeSetDemo {

    // Main Method
    public static void main(String args[])
    {
        // Creating TreeSet and
        // adding elements
        TreeSet<String> ts = new TreeSet<String>();

        ts.add("Geeks");
        ts.add("For");
        ts.add("Geeks");
        ts.add("Is");
        ts.add("Very helpful");

        // Traversing elements
        Iterator<String> itr = ts.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

For
Geeks
Is
Very helpful

Map Interface

Map is a data structure that supports the key-value pair for mapping the data. This interface doesn't support duplicate keys because the same key cannot have multiple mappings, however, it allows duplicate values in different keys. A map is useful if there is data and we wish to perform operations on the basis of the key. This map interface is implemented by various classes like HashMap, TreeMap, etc. Since all the subclasses implement the map, we can instantiate a map object with any of these classes. 

For example:

Map<T> hm = new HashMap<> (); 
Map<T> tm = new TreeMap<> ();

Where T is the type of the object.

The frequently used implementation of a Map interface is a HashMap. 

HashMap

HashMap is a basic implementation of the Map interface that stores data as key-value pairs. It uses hashing for fast access, converting keys into hash codes to index values efficiently. To retrieve a value, the corresponding key is required. Internally, HashSet is also backed by a HashMap.

Let's understand the HashMap with an example:

import java.util.*;
public class HashMapDemo {

    // Main Method
    public static void main(String args[])
    {
        // Creating HashMap and
        // adding elements
        HashMap<Integer, String> hm
            = new HashMap<Integer, String>();

        hm.put(1, "Geeks");
        hm.put(2, "For");
        hm.put(3, "Geeks");

        // Finding the value for a key
        System.out.println("Value for 1 is " + hm.get(1));

        // Traversing through the HashMap
        for (Map.Entry<Integer, String> e : hm.entrySet())
            System.out.println(e.getKey() + " "
                               + e.getValue());
    }
}

Value for 1 is Geeks
1 Geeks
2 For
3 Geeks

Methods of the Collection Interface

This interface contains various methods which can be directly used by all the collections which implement this interface. They are:

Related articles:

• List Interface
• Queue Interface
• Deque Interface
• Set Interface
• SortedSet Interface
• Map Interface

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