Java – Map Interface – Ameerpet Technologies

Map:

Store values using keys
- Map = {key=value, key=value, key=value ….}

Keys must be unique in Map
Values can be duplicated.
For example, book names(unique) with prices(duplicates)
- Books = {C=300.0 , C++=300.0, Java=350.0, Python=330.0};

Methods are:

Method	Description
put(K key, V value)	store value using key.
V get(Object key)	return value of specified key.
boolean isEmpty()	Returns true if this map contains key-values.
void clear()	Removes all elements.
boolean containsKey(Object key)	Returns true if map contains specified key.
Set<K> keySet()	Returns a Set of keys contained in this map.
remove(Object key)	Removes key-value of specified key.
replace(K key, V value)	Replace the value of specified key with given value.
int size()	Returns the number of key-values in map.
Collection<V> values()	Returns Collection of values in this map.

Program to create HashMap and display:

import java.util.*;
class Code
{
            public static void main(String[] args)
            {
                        Map<Integer,String> map = new HashMap<Integer,String>();
                        map.put(10, “Ten”);
                        map.put(20, “Twenty”);
                        map.put(30, “Thirty”);
                        map.put(40, “Fourty”);
                        map.put(50, “Fifty”);
                        System.out.println(“Map : ” + map);
            }
}

Implementations of Map:

HashMap: doesn’t maintain insertion order. Allows only one null key
LinkedHashMap: maintains insertion order. Allows only one null key
TreeMap: maintains sorted order of keys. It doesn’t allow null key.
Hashtable: It is call legacy class. It maintains sorted order. It doesn’t allow null key.

import java.util.*;
class Code
{
            public static void main(String[] args) {
                        //Map<Integer,String> map = new LinkedHashMap<Integer,String>();
                        Map<Integer,String> map = new TreeMap<Integer,String>();
                        map.put(50, “Fifty”);
                        map.put(40, “Fourty”);
                        map.put(30, “Thirty”);
                        map.put(20, “Twenty”);
                        map.put(10, “Ten”);
                        System.out.println(“Map : ” + map);
            }
}

Set<K> keySet():

We cannot iterate the map object either by using Iterator or using for-each loop.
First we need to collect all keys of map using keySet() method.
We iterate keys set and get values by specifying each key.

import java.util.*;
class Code
{
           public static void main(String[] args) throws Exception {
                       Map<Integer, String> map = new HashMap<Integer, String>();
                       map.put(10, “Ten”);
                       map.put(20, “Twenty”);
                       map.put(30, “Thirty”);
                       map.put(40, “Fourty”);

                       System.out.println(“Map is : “);
                       Set<Integer> keys = map.keySet();
                       for(Integer key : keys){
                                   String value = map.get(key);
                                   System.out.println(key + ” = ” + value);
                       }
           }
}

Iterate Map through Iterator:

Collect the keys using keySet() method.
Create Iterator from the keys Set.
Iterate the object and get Values by specifying keys.

import java.util.*;
class Code
{
            public static void main(String[] args) throws Exception {
                        String[] books = {“C”, “C++”, “Java”, “Python”, “Android”};
                        double[] prices = {200.0, 300.0, 250.0, 200.0, 250.0};

                        Map<String, Double> map = new HashMap<String, Double>();
                        for(int i=0 ; i<=books.length-1 ; i++) {
                                    map.put(books[i], prices[i]);
                        }

                        System.out.println(“Map is : “);
                        Set<String> keys = map.keySet();
                        Iterator<String> itr = keys.iterator();
                        while(itr.hasNext()) {
                                    String key = itr.next();
                                    Double value = map.get(key);
                                    System.out.println(key + ” = ” + value);
                        }
            }
}

Copyright © 2023 Ameerpet Technologies | Powered by Ameerpet Technologies