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"));       } }