Map Interface - Java Collections

A Map stores data in key and value association. Both key and values are objects. The key must be unique but the values can be duplicate. Although Maps are a part of Collection Framework, they can not actually be called as collections because of some properties that they posses. However we can obtain a collection-view of maps.

It provides various classes: HashMap, TreeMap, LinkedHashMap for map implementation. All these classes implements Map interface to provide Map properties to the collection.

Map Interface and its Subinterface

Map Interface Methods

These are commonly used methods defined by Map interface

• boolean containsKey(Object k): returns true if map contain k as key. Otherwise false.
• Object get(Object k) : returns values associated with the key k.
• Object put(Object k, Object v) : stores an entry in map.
• Object putAll(Map m) : put all entries from m in this map.
• Set keySet() : returns Set that contains the key in a map.
• Set entrySet() : returns Set that contains the entries in a map.

HashMap class

1. HashMap class extends AbstractMap and implements Map interface.
2. It uses a hashtable to store the map. This allows the execution time of get() and put() to remain same.
3. HashMap does not maintain order of its element.

HashMap has four constructor.

HashMap()
HashMap(Map< ? extends k, ? extends V> m)
HashMap(int capacity)
HashMap(int capacity, float fillratio)

HashMap Example

Lets take an example to create hashmap and store values in key and value pair. Notice to insert elements, we used put() method because map uses put to insert element, not add() method that we used in list interface.

import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    HashMap< String,Integer> hm = new HashMap< String,Integer>();
    hm.put("a",100);
    hm.put("b",200);
    hm.put("c",300);
    hm.put("d",400);

    Set<Map.Entry<String,Integer>> st = hm.entrySet();  //returns Set view
    for(Map.Entry<String,Integer> me:st)
    {
      System.out.print(me.getKey()+":");
      System.out.println(me.getValue());
    }
  }
}

a:100 b:200 c:300 d:400

TreeMap class

1. TreeMap class extends AbstractMap and implements NavigableMap interface.
2. It creates Map, stored in a tree structure.
3. A TreeMap provides an efficient means of storing key/value pair in efficient order.
4. It provides key/value pairs in sorted order and allows rapid retrieval.

Example:

In this example, we are creating treemap to store data. It uses tree to store data and data is always in sorted order. See the below example.

import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    TreeMap<String,Integer> tm = new TreeMap<String,Integer>();
    tm.put("a",100);
    tm.put("b",200);
    tm.put("c",300);
    tm.put("d",400);

    Set<Map.Entry<String,Integer>> st = tm.entrySet();
    for(Map.Entry<String,Integer> me:st)
    {
      System.out.print(me.getKey()+":");
      System.out.println(me.getValue());
    }
  }
}

a:100 b:200 c:300 d:400

LinkedHashMap class

1. LinkedHashMap extends HashMap class.
2. It maintains a linked list of entries in map in order in which they are inserted.
3. LinkedHashMap defines the following constructor

   LinkedHashMap()
   
   LinkedHashMap(Map< ? extends k, ? extends V> m)
   
   LinkedHashMap(int capacity)
   
   LinkedHashMap(int capacity, float fillratio)
   
   LinkedHashMap(int capacity, float fillratio, boolean order)

4. It adds one new method removeEldestEntry(). This method is called by put() and putAll() By default this method does nothing.

Example:

Here we are using linkedhashmap to store data. It stores data into insertion order and use linked-list internally. See the below example.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    LinkedHashMap<String,Integer> tm = new LinkedHashMap<String,Integer>();
    tm.put("a",100);
    tm.put("b",200);
    tm.put("c",300);
    tm.put("d",400);

    Set<Map.Entry<String,Integer>> st = tm.entrySet();
    for(Map.Entry<String,Integer> me:st)
    {
      System.out.print(me.getKey()+":");
      System.out.println(me.getValue());
    }
  }
}
  

a:100 b:200 c:300 d:400