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Java - SortedMap Interface

The SortedMap interface extends Map. It ensures that the entries are maintained in an ascending key order.

Several methods throw a NoSuchElementException when no items are in the invoking map. A ClassCastException is thrown when an object is incompatible with the elements in a map. A NullPointerException is thrown if an attempt is made to use a null object when null is not allowed in the map.

The methods declared by SortedMap are summarized in the following table −

Comparator comparator( )

Returns the invoking sorted map's comparator. If the natural ordering is used for the invoking map, null is returned.

Object firstKey( )

Returns the first key in the invoking map.

SortedMap headMap(Object end)

Returns a sorted map for those map entries with keys that are less than end.

Object lastKey( )

Returns the last key in the invoking map.

SortedMap subMap(Object start, Object end)

Returns a map containing those entries with keys that are greater than or equal to start and less than end.

SortedMap tailMap(Object start)

Returns a map containing those entries with keys that are greater than or equal to start.

The following diagram shows the hierarchy of SortedMap Interface in Java -

TreeMap class implements the SortedMap interface. We can use the TreeMap constructor to create a SortedMap instance.

Following is the syntax to create a sortemap instance:

// Create a sorted map
SortedMap<String, Double> map = new TreeMap<>();

Here we're creating a sorted map of String vs Double values. This map will store the keys based on alphanumeric order.

SortedMap provides the put() method, which can be used to add value to a sortedmap instance. Whenever a value is added to the map, the map automatically sorts itself based on the keys entered.

public V put(K key,V value)

Where the Key-Value pair represents the key and value associated with each other and are stored in the map. If this key is already associated with a value then that value is returned and the new value is associated with the key otherwise a null value is returned.

// Put elements to the map
map.put("Zara", Double.valueOf(3434.34));
map.put("Mahnaz", Double.valueOf(123.22));
map.put("Ayan", Double.valueOf(1378.00));
map.put("Daisy", Double.valueOf(99.22));
map.put("Qadir", Double.valueOf(-19.08));

Using the get(key) method, we can retrieve the value associated with a key.

public V get(Object key)

If the key is not present in the map, then it will return null otherwise it will return the associated value with the key provided.

Double value = map.get("Qadir");
System.out.print("Qadir: " + value);

We can update an existing value of a sortedmap by calling the put() method again with the same key. Being a sortedmap, the entries will be sorted again based on the sorting order of the newly entered key(s).

// Put elements to the map
map.put("Zara", Double.valueOf(3434.34));
map.put("Mahnaz", Double.valueOf(123.22));
map.put("Zara", Double.valueOf(1378.00));

SortedMap will consider the latest put() method call to update the entry with same key.

Using remove(key) method, we can remove the key, value associated with a key.

public V remove(Object key)

If key is not present in the map, then it will return null otherwise it will remove key-value association from the map and sort the map accordingly.

Double value = map.remove("Qadir");
System.out.print("Qadir removed with value: " + value);

SortedMap entries can be easily navigated. SortedMap provided a method entrySet() which provides all the entries in form of set.

public Set<Map.Entry<K,V>> entrySet()

Where Map.Entry contains the key-value pair to be iterated.

// Get a set of the entries
Set<Map.Entry<String, Double>> set = map.entrySet();

// Get an iterator
Iterator<Map.Entry<String, Double>> i = set.iterator();

// Display elements 
while(i.hasNext()) {
 Map.Entry<String, Double> me = i.next();
 System.out.println(me.getKey());
}

Following is an example showing how TreeMap can be used to get values of a SortedMap −

import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;

public class MapDemo {

   public static void main(String args[]) {
      // Create a hash map
      SortedMap<String, Double> map = new TreeMap<>();

      // Put elements to the map
      map.put("Zara", Double.valueOf(3434.34));
      map.put("Mahnaz", Double.valueOf(123.22));
      map.put("Ayan", Double.valueOf(1378.00));
      map.put("Daisy", Double.valueOf(99.22));
      map.put("Qadir", Double.valueOf(-19.08));
      
      // Get a set of the entries
      Set<Map.Entry<String, Double>> set = map.entrySet();
      
      // Get an iterator
      Iterator<Map.Entry<String, Double>> i = set.iterator();
     
      // Display elements 
      while(i.hasNext()) {
         Map.Entry<String, Double> me = i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
   }
}

Ayan: 1378.0
Daisy: 99.22
Mahnaz: 123.22
Qadir: -19.08
Zara: 3434.34

Following is an example showing how TreeMap can be used to set values of a SortedMap −

import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;

public class MapDemo {

   public static void main(String args[]) {
      // Create a hash map
      SortedMap<String, Double> map = new TreeMap<>();

      // Put elements to the map
      map.put("Zara", Double.valueOf(3434.34));
      map.put("Mahnaz", Double.valueOf(123.22));
      map.put("Ayan", Double.valueOf(1378.00));
      map.put("Daisy", Double.valueOf(99.22));
      map.put("Qadir", Double.valueOf(-19.08));
      
      // Get a set of the entries
      Set<Map.Entry<String, Double>> set = map.entrySet();
      
      // Get an iterator
      Iterator<Map.Entry<String, Double>> i = set.iterator();
     
      // Display elements 
      while(i.hasNext()) {
         Map.Entry<String, Double> me = i.next();
         me.setValue(me.getValue() * 10);
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
   }
}

Ayan: 13780.0
Daisy: 992.2
Mahnaz: 1232.2
Qadir: -190.79999999999998
Zara: 34343.4

Following is an example showing how a TreeMap can be used to get key of a sortedMap entry −

import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;

public class MapDemo {

   public static void main(String args[]) {
      // Create a hash map
      SortedMap<String, Double> map = new TreeMap<>();

      // Put elements to the map
      map.put("Zara", Double.valueOf(3434.34));
      map.put("Mahnaz", Double.valueOf(123.22));
      map.put("Ayan", Double.valueOf(1378.00));
      map.put("Daisy", Double.valueOf(99.22));
      map.put("Qadir", Double.valueOf(-19.08));
      
      // Get a set of the entries
      Set<Map.Entry<String, Double>> set = map.entrySet();
      
      // Get an iterator
      Iterator<Map.Entry<String, Double>> i = set.iterator();
     
      // Display elements 
      while(i.hasNext()) {
         Map.Entry<String, Double> me = i.next();
         System.out.println(me.getKey());
      }
   }
}

Ayan
Daisy
Mahnaz
Qadir
Zara

• SortedMap ensures that the map is always sorted in ascending order of the keys. Whenever a key-value pair is added to the sortedmap, it is re-sorted
• Being sorted, sortedmap is very efficient in searches. In the case of large read-only datasets, sortedmap is an ideal choice to be implemented.
• We can customize the sorting mechanism by using a comparator on the key type.

• As a sortedmap has to be sorted every time an entry is added or changed, it becomes a performance bottleneck where changes are very frequent. In such cases, sortedmap is not preferred.
• As sortedMap maintains sort ability based on key, a key should be comparable and thus we cannot use a custom key if it is not implementing the comparable interface.

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