SortedMap Interface in Java with Examples

SortedMap is an interface in the collection framework. This interface extends the Map interface and provides a total ordering of its elements (elements can be traversed in sorted order of keys). The class that implements this interface is TreeMap.

The SortedMap interface is a subinterface of the java.util.Map interface in Java. It extends the Map interface to provide a total ordering of its elements, based on the natural order of its keys.

The main difference between a SortedMap and a regular Map is that the elements in a SortedMap are stored in a sorted order, whereas in a Map the elements are stored in an arbitrary order. The sorting order is determined by the natural order of the keys, which must implement the java.lang.Comparable interface, or by a Comparator passed to the SortedMap’s constructor.

Here’s an example of how to use the SortedMap interface:

Value of A: 1
Key: A, Value: 1
Key: C, Value: 3

The main characteristic of a SortedMap is that it orders the keys by their natural ordering, or by a specified comparator. So consider using a TreeMap when you want a map that satisfies the following criteria: 

• null key or null value is not permitted.
• The keys are sorted either by natural ordering or by a specified comparator.

Type Parameters:

• K – the type of keys maintained by this map
• V – the type of mapped values

The parent interface of SortedMap is Map<K, V>.  

The subInterfaces of SortedMap are ConcurrentNavigableMap<K, V>, NavigableMap<K, V>.

SortedMap is implemented by ConcurrentSkipListMap, TreeMap.

Declaration:

public interface SortedMap<K, V> extends Map<K, V>
{
    Comparator comparator();
    SortedMap subMap(K fromKey, K toKey);
    SortedMap headMap(K toKey);
    SortedMap tailMap(K fromKey);
    K firstKey();
    K lastKey();
}

 Example:

Key : 1  value : geeksforgeeks
Key : 2  value : practice
Key : 3  value : quiz
Key : 4  value : contribute
Key : 5  value : code

Creating SortedMap Objects

Since SortedMap is an interface, objects cannot be created of the type SortedMap. We always need a class that extends this list in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the SortedMap. This type-safe map can be defined as:

// Obj1, Obj2 are the type of the object to be stored in SortedMap

SortedMap<Obj1, Obj2> set = new TreeMap<Obj1, Obj2> ();

Performing Various Operations on SortedMap

Since SortedMap is an interface, it can be used only with a class that implements this interface. TreeMap is the class that implements the SortedMap interface. Now, let’s see how to perform a few frequently used operations on the TreeMap.

1. Adding Elements: In order to add an element to the SortedMap, we can use the put() method. However, the insertion order is not retained in the TreeMap. Internally, for every element, the keys are compared and sorted in ascending order.

{1=Geeks, 2=For, 3=Geeks}
{1=Geeks, 2=For, 3=Geeks}

2. Changing Elements: After adding the elements if we wish to change the element, it can be done by again adding the element with the put() method. Since the elements in the SortedMap are indexed using the keys, the value of the key can be changed by simply inserting the updated value for the key for which we wish to change.

{1=Geeks, 2=Geeks, 3=Geeks}
{1=Geeks, 2=For, 3=Geeks}

3. Removing Element: In order to remove an element from the SortedMap, we can use the remove() method. This method takes the key value and removes the mapping for the key from this SortedMap if it is present in the map.

{1=Geeks, 2=Geeks, 3=Geeks, 4=For}
{1=Geeks, 2=Geeks, 3=Geeks}

4. Iterating through the SortedMap: There are multiple ways to iterate through the Map. The most famous way is to use an enhanced for loop and get the keys. The value of the key is found by using the getValue() method.

1 : Geeks
2 : For
3 : Geeks

The class which implements the SortedMap interface is TreeMap.

TreeMap class which is implemented in the collections framework is an implementation of the SortedMap Interface and SortedMap extends Map Interface. It behaves like a simple map with the exception that it stores keys in a sorted format. TreeMap uses a tree data structure for storage. Objects are stored in sorted, ascending order. But we can also store in descending order by passing a comparator. Let’s see how to create a SortedMap object using this class.

{South Africa=3, India=1, Australia=2}
Map after removing Australia:{South Africa=3, India=1}

Methods of SortedMap Interface

Methods inherited from interface java.util.Map

Advantages of SortedMap:

1. Sorted order: The SortedMap interface provides a sorted order of its elements, based on the natural order of its keys or a custom Comparator passed to the constructor. This makes it useful in situations where you need to retrieve elements in a specific order.
2. Predictable iteration order: Because the elements in a SortedMap are stored in a sorted order, you can predict the order in which they will be returned during iteration, making it easier to write algorithms that process the elements in a specific order.
3. Search performance: The SortedMap interface provides an efficient implementation of the Map interface, allowing you to retrieve elements in logarithmic time, making it useful in search algorithms where you need to retrieve elements quickly.

Disadvantages of SortedMap:

1. Slow for inserting elements: Inserting elements into a SortedMap can be slower than inserting elements into a regular Map, as the SortedMap needs to maintain the sorted order of its elements.
2. Key restriction: The keys in a SortedMap must implement the java.lang.Comparable interface or a custom Comparator must be provided. This can be a restriction if you need to use custom keys that do not implement this interface.

Reference books:

1. “Java Collections” by Maurice Naftalin and Philip Wadler. This book provides a comprehensive overview of the Java Collections framework, including the SortedMap interface.
2. “Java in a Nutshell” by David Flanagan. This book provides a quick reference for the core features of Java, including the SortedMap interface.
3. “Java Generics and Collections” by Maurice Naftalin and Philip Wadler. This book provides a comprehensive guide to generics and collections in Java, including the SortedMap interface.