Map Interface

public interface Map<K,V>

A map contains values on the basis of key, i.e. key and value pair. Each key and value pair is known as an entry. A Map contains unique keys.

Map is useful, when we have to search, update and delete values based on keys.

An object that maps keys to values. A map cannot contain duplicate keys. Each key can map to at most one value.

This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.

The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings.

The Map interface present in java.util package.

The Map interface is not a subtype of the Collection interface.

Therefore it behaves a bit differently from the rest of the collection types. A map contains unique keys.

Map.Entry Interface

Entry is the sub interface of Map. So we will be accessed it by Map.Entry name. It returns a collection-view of the map, whose elements are of this class. It provides methods to get key and value.

References:https://www.javatpoint.com/java-map, https://docs.oracle.com/javase/8/docs/api/java/util/Map.html

Java Map Hierarchy

There are two interfaces for implementing Map in java: Map and SortedMap, and three classes: HashMap, LinkedHashMap, and TreeMap. The hierarchy of Java Map is given below:

A Map doesn’t allow duplicate keys, but you can have duplicate values. HashMap and LinkedHashMap allow null keys and values, but TreeMap doesn’t allow any null key or value.

A Map can’t be traversed, so you need to convert it into Set using keySet() or entrySet() method.(to be discussed)

Class	Description
HashMap	HashMap is the implementation of Map, but it doesn’t maintain any order.
LinkedHashMap	LinkedHashMap is the implementation of Map. It inherits HashMap class. It maintains insertion order.
TreeMap	TreeMap is the implementation of Map and SortedMap. It maintains ascending order.

For sample programs for map interface implementing class refer:

https://github.com/LathaMurugan-code/Collections

List interface

Arraylist

Arraylist class implements list interface.

To know the advantages of methods we can go through the sample program which shows the same.

Program

package collectionpackage;
import java.util.ArrayList;
public class ArraylistSample {
public static void main(String[] args) {
// TODO Auto-generated method stub
ArrayList a = new ArrayList();
a.add(23);
//adds integer object
a.add(0.5f);
a.add(“latha”);
System.out.println(“Index of latha- “+a.indexOf(“latha”));
a.add(2,true);
//add object at specified index
System.out.println(“At index 1- “+a.get(1));
//Returns value at specified index
System.out.println(a);
System.out.print(“Is object 23 exists? “);
System.out.println(a.contains(23));
ArrayList a2 = new ArrayList();
a2.addAll(a);
//adds all elements from a
System.out.println(“a2 ArrayList”+a2);
System.out.println(“Arraylist after remove operation”);
a.remove(2);
//remove the object at specified index
a.remove(“latha”);
System.out.println(a);
a.removeAll(a);
//removes whole collection class
System.out.println(a);
System.out.println(“Arraylist empty? “+a.isEmpty());
}
}

Output

Index of latha- 2
At index 1- 0.5
[23, 0.5, true, latha]
Is object 23 exists? true
a2 ArrayList[23, 0.5, true, latha]
Arraylist after remove operation
[23, 0.5]
[]
Arraylist empty? true

Java LinkedList class

Java LinkedList class uses a doubly linked list to store the elements. It provides a linked-list data structure. It inherits the AbstractList class and implements List and Deque interfaces.

ArrayList	LinkedList
1) ArrayList internally uses a dynamic array to store the elements.	LinkedList internally uses a doubly linked list to store the elements.
2) Manipulation with ArrayList is slow because it internally uses an array. If any element is removed from the array, all the bits are shifted in memory.	Manipulation with LinkedList is faster than ArrayList because it uses a doubly linked list, so no bit shifting is required in memory.
3) An ArrayList class can act as a list only because it implements List only.	LinkedList class can act as a list and queue both because it implements List and Deque interfaces.
4) ArrayList is better for storing and accessing data.	LinkedList is better for manipulating data.