Thursday, 9 March 2017

The Map Interface

The Map Interface


A Map is an object that maps keys to values. A map cannot contain duplicate keys. Each key can map to at most one value.
 The Map interface includes methods for basic operations 
·         put 
·         get 
·         remove 
·         containsKey 
·         containsValue 
·         size 
·         empty

For Bulk operations 
·         putAll  
·         clear

For collection views 
·         keyset 
·         entrySet 
·         values

The Java platform provided three general purpose Map implementations 
·         HashMap 
·         TreeMap 
·         LinkedHashMap
Their behavior and performance are similar to HashSet, TreeSet, and LinkedHashSet.

Map Interface Basic Operations
import java.util.*;

public class MapDemo {
    public static void main(String[] args) {
        Map<String, Integer> m = new HashMap<String, Integer>();
       
        // Initialize frequency table from command line
        for (String a : args) {
            Integer freq = m.get(a);
            m.put(a, (freq == null) ? 1 : freq + 1);
        }

        System.out.println(m.size() + " distinct words:");
        System.out.println(m);
    }
}
Execute the above program using command line passing command line arguments
java MapDemo i came i saw i conquered

We will get  the following output.
4 distinct words:
{came=1, saw=1, conquered=1, i=3}

Replace the HashMap implementation with TreeMap implementation in the above code
Map<String, Integer> m = new HashMap<String, Integer>();
Like
Map<String, Integer> m = new TreeMap<String, Integer>();
The output changes to
4 distinct words:
{came=1, conquered=1, i=3, saw=1}
In the output the words are now arranged in alphabetical order.
Map Interface Bulk Operations
·         The clear operation removes all the mappings from the Map.
·         The putAll operation is the Map similar of the Collection interface's addAll operation.
Collection Views
The Collection view methods allow a Map to be viewed as a Collection in three ways:
·         keySet  - the Set of keys contained in the Map.
·         values  -  The Collection of values contained in the Map.
·         entrySet — the Set of key-value pairs contained in the Map. The Map interface provides a small nested interface called Map.Entry, the type of the elements in this Set.
The Collection views provide the only means to iterate over a Map.

import java.util.*;
class HashMapDemo {
      public static void main(String args[]) {
//          Create a hash map
            HashMap hm = new HashMap();
            List l;
            //          Put elements to the map
            hm.put("John Doe", 3434.34);
            hm.put("Tom Smith", new Double(123.22));
            hm.put("Jane Baker", new Double(1378.00));
            hm.put("Todd Hall", new Double(99.22));
            hm.put("Ralph Smith", new Double(-19.08));
            hm.put("Ralph Smith", new Double(-9.08));
//          Get a set of the entries
            Set set = hm.entrySet();
     
            System.out.println(hm);
//          Get an iterator
            Iterator i = set.iterator();
//          Display elements
            while(i.hasNext()) {
                  //System.out.println(i.next());
                  Map.Entry me = (Map.Entry)i.next();
                  System.out.print(me.getKey() + ": ");
                  System.out.println(me.getValue());
                 
            }
            System.out.println();
//          Deposit 1000 into John Doe's account
            double balance = ((Double)hm.get("John Doe")).doubleValue();
            hm.put("John Doe", new Double(balance + 1000));
            System.out.println("John Doe's new balance: " +
                        hm.get("John Doe"));
      }
}




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