In this article, we will discuss TreeMap class – the implementation class for NavigableMap interface in detail

1. Key points about TreeMap:

• TreeMap doesn’t allow duplicate keys
• Maintains ascending sorting-order, by-default
• For pre-defined classes like String and wrapper classes, etc.

2. TreeMap:

• TreeMap is the implementation class of NavigableMap interface (i.e.; TreeMap implements NavigableMap)
• Uses red-black-tree to store key-value pairs (i.e.; map entries)
• Insertion-order is NOT maintained
• It allows only unique keys to be inserted
• Stores key-value pairs in sorting order on the basis of keys only, not values
• TreeMap stores key-value pairs in sorting-order; it could be either default natural sorting-order or programmer defined customized sorting-order
• Keys inserted should be of comparable type, otherwise ClassCastException will be thrown
• At any time, TreeMap contains only unique keys and there is no restriction on corresponding values
• Allows NULL insertion but maximum of only one NULL key up-to Java 1.6 version
• From Java 1.7 version, even one NULL insertion is not possible
• TreeMap is non-synchronized
• Present in java.util package and extends java.util.AbstractMap implements java.util.NavigableMap interface
• Also, implements java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to TreeMap (provided by JVM at run time) like,
• java.lang.Cloneable: to create a duplicate object or to clone an object
• java.io.Serializable: to transfer objects across network

Source: Team BenchResources.Net

3. TreeMap constructors:

3.1 TreeMap tm = new TreeMap();

• creates an empty TreeMap object
• where key-value pairs will be inserted according to default natural sorting-order of keys

3.2 TreeMap tm = new TreeMap(Comparator c);

• creates an empty TreeMap object
• where key-value pairs will be inserted according to specified comparator object (i.e.; customized sorting-order)

3.3 TreeMap tm = new TreeMap(Map m);

• creates an equivalent TreeMap object for the specified map
• it is basically used for inter-conversion between map objects

3.4 TreeMap tm = new TreeMap(SortedMap m);

• create an equivalent TreeMap object for the specified SortedMap
• follows same ordering as per specified SortedMap

4. TreeMap examples:

TreeMapStringExample.java

package in.bench.resources.java.collection;

import java.util.TreeMap;

public class TreeMapIntegerStringExample {

	public static void main(String[] args) {

		// creating NavigableMap reference and TreeMap object
		TreeMap<Integer, String> tm = new TreeMap<Integer, String>();

		// adding key-value pairs to SortedMap/TreeMap object
		tm.put(3, "Yahoo");
		tm.put(4, "Amazon");
		tm.put(7, "Twiter");
		tm.put(1, "Google");
		tm.put(5, "Reddit");
		tm.put(6, "LinkedIn");
		tm.put(2, "Facebook");

		// printing according to natural ordering
		System.out.println("Elements sorted "
				+ "according to Natural ordering:\n");
		System.out.println(tm);
}
}

Output:

Elements sorted according to Natural ordering:

{1=Google, 2=Facebook, 3=Yahoo, 4=Amazon, 5=Reddit, 6=LinkedIn, 7=Twiter}

Note: All methods of TreeMap is non-synchronized

Q) How to make TreeMap synchronized ?

• but it can be easily converted into synchronized TreeMap
• Using utility method synchronizedMap(tm); of java.util.Collections class

Map map = Collections.synchronizedMap(tm);

Related Articles:

• Map interface
• Entry interface
• HashMap class
• LinkedHashMap class
• IdentityHashMap class
• WeakHashMap class
• SortedMap interface
• NavigableMap interface
• TreeMap class
• Hashtable class
• HashMap vs LinkedHashMap
• HashMap v/s LinkedHashMap v/s TreeMap
• HashMap v/s HashSet
• HashMap v/s Hashtable
• Properties class

References:

• https://docs.oracle.com/javase/tutorial/collections/intro/
• https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Map.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html
• https://docs.oracle.com/javase/7/docs/api/java/util/SortedMap.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/sorted-map.html
• https://docs.oracle.com/javase/7/docs/api/java/util/NavigableMap.html
• https://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html
• https://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html

Happy Coding !!
Happy Learning !!