Map interface

Map is used when you need  keyed elements. It means, Map stores your data with a key. This map concept is totally different from other collections like List, Set and Queue. There are four main implementations.

1. HashMap
2. LinkedHashMap
3. HashTable
4. TreeMap

Traverse on a Map

• Using Iterator

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class HashMapDemo {

   public static void main(String[] args) {
  
      Map<Integer, Integer> map = new HashMap<>();
      map.put(1, 10);
      map.put(8, 50);
      map.put(5, 10);
      map.put(10, 90);
  
      Iterator<Integer> iterator = map.keySet().iterator();
  
      while (iterator.hasNext()) {
          Integer key = iterator.next();
          Integer value = map.get(key);
   
          System.out.print(key + "-");
          System.out.print(value + " ");
      } 
   }
}

• Using enhanced for loop

for(Integer key : map.keySet()){
   Integer value = map.get(key);
   System.out.print(key + "-");
   System.out.print(value + " ");
}

Basic operations on Map interface

import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {

   public static void main(String[] args) {
  
       Map<Integer, Integer> map = new HashMap<>();
       map.put(1, 10);
       map.put(8, 50);
       map.put(5, 10);
       map.put(10, 90);
  
       System.out.println(map);
       System.out.println(map.isEmpty()); //check map is empty or not4
       System.out.println(map.size()); //get the size of the map
       System.out.println(map.keySet()); //get the keys
       System.out.println(map.values()); //get the values
       System.out.println(map.put(5, 100)); //replace value on given key
       System.out.println(map);
       map.clear(); //clear map
       System.out.println(map);
  
   }
}

You may look at the following code to understand how to use maps to find distinct words.

import java.util.HashMap;
import java.util.Map;

public class MapDemo {

 
   public static void main(String[] args) {
  
       Map<String, Integer> map = new HashMap<>();
       String statement = "If it is true this is also true and that is also ";
       String[] arr = statement.toLowerCase().split(" ");
  
       for(String s : arr){
           Integer i = map.get(s);
           map.put(s, (i==null) ? 1 : i+1);
       }
 
       System.out.println(map.size() + " distinct words : " + map);
    }
}

You may look at the following code to understand how to use maps to count characters in a String. 

import java.util.LinkedHashMap;
import java.util.Map;

public class StringOccurence {
    public static void main(String[] args) {
        String s = "easy java se";
        char[] chars = s.toCharArray();
 
        Map<Character, Integer> map = new LinkedHashMap<>();
        for(char c : chars){
            if(map.containsKey(c)){
                map.put(c, map.get(c)+1);
            }
            else{
                map.put(c, 1);
            }
        }
        System.out.println(map);
    }
}

HashMap vs LinkedHashMap vs HashTable vs TreeMap