Java - TreeMap Class

The TreeMap class extends the AbstractMap abstract class and it also implements the NavigableMap interface. By default, TreeMap stores key-value pairs in a sorted ascending order(based on the key).

The retrieval speed of an element out of a TreeMap is fast, even in a TreeMap with a large number of elements. Hence, TreeMap is the best candidate for a situation where one needs to store a large number of sorted elements in the form of key-value pairs and their fast access.

Important features of TreeMap

A null object cannot be stored as a key in a TreeMap.
A null object can be stored as a value in a TreeMap.
The ascending ordering of elements in a TreeMap can be changed by providing a custom Comparator to the constructor of TreeSet.
TreeMap is a good candidate in a situation where we need to retrieve element based on the closest match to a given value(more on this in the first example on this article).

A simple constructor of TreeMap

TreeMap()
This constructor creates an empty TreeMap to hold a particular type of key-value pair objects. Example -

TreeMap <String,Integer> tm = new TreeMap <String,Integer>();

This constructor creates a TreeMap to hold key-value pairs of String and Integer objects.

Some important methods of TreeMap class

Methods	Description
void clear()	Removes all the key/value pairs from the TreeMap.
int size()	Returns the total number of key-value pairs in a TreeMap.
boolean isEmpty()	Returns true if TreeMap has no key-value pair in it.
V get(Object key)	Returns the value associated with a specified key.
put(K key, V value)	Puts the specified key and its associated value in the TreeMap.
K higherKey(K key)	Returns a key just greater than the specified key.
K lowerKey(K key)	Returns a key just smaller than the specified key.
Set<Map.Entry<K,V>> entrySet()	Returns the Set containing Map.Entry objects.

TreeMap example

We are creating a TreeMap to store objects in the pair of key and value, where key is a String object and value is an Integer object. By default, the key-value pairs will be stored in an ascending order of its keys(based on the first character of the String). We will also obtain a value, by specifying its key.

//Java - Example of TreeMap


import java.util.*;


class TreeMapDemo
{
public static void main(String... ar)
{
TreeMap<String,Integer> tm = new TreeMap<String,Integer>();

tm.put("Tom", 14);
tm.put("Rick", 17);
tm.put("Ana", 15);
tm.put("Max", 20);
tm.put("Ian", 19);

System.out.println("TreeMap contents in an ascending order of its keys-" +tm);

System.out.println("Value at the key Ian is "+ tm.get("Ian"));
System.out.println("Value at the key Max is "+ tm.get("Max"));

System.out.println("Key greater than Ana is ")+ tm.higherKey("Ana");
System.out.println("Key smaller than Rick is ")+ tm.lowerKey("Rick");

}
}

Output is :

TreeMap contents in an ascending order of its keys-{Ana=15, Ian=19, Max=20 Rick=17, Tom=14}
Value at the key Ian is 19
Value at the key Max is 20
Key greater than Ana is Ian
Key smaller than Rick is Max

Program Analysis

Here, we have created a generic TreeMap to store objects in the pair of String(key) and Integer(value).
Method put() is used to put the values of key-value pair in a TreeMap, by specifying the key & its value.
Method get() is used to obtain a value associated with a particular key in a TreeMap.
On the first call, method get() has returned the value(19) associated with the key(Ian).
On the second call, method get() has returned Value(20) associated with the Key "Max".
Method higherKey() has returned the value(Ian), just higher than the key(Ana).
Method lowerKey() has returned the value(Max), just lower than the key(Rick).

Iterating over entries in a TreeMap using entrySet() method

In the next example, we are creating another TreeMap to store objects in the pair of key and value, where key is a String object and value is an Integer object. Next, will use the entrySet() method to get a Set view of a TreeMap and eventually we will iterate over each map entry(key-value pair) by using the for-each loop.

//Java - Iterating over entries in a TreeMap by using the entrySet() method


import java.util.*;


class TreeMapDemo
{
public static void main(String... ar)
{
TreeMap<String,Integer> tm = new TreeMap<String,Integer>();

tm.put("Tom", 14);
tm.put("Rick", 17);
tm.put("Ana", 15);
tm.put("Max", 20);
tm.put("Ian", 19);


System.out.println("Iterating TreeMap using Map.Entry and for-each loop");
Set<Map.Entry<String,Integer>> set = tm.entrySet();


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

}

}

Output is :

{Ana=15, Ian=19, Max=20 Rick=17, Tom=14}
Iterating TreeMap using Map.Entry and for-each loop
Ana:15
Ian:19
Max:20 
Rick:17
Tom:14

Program Analysis

We have created a generic TreeMap to store objects in the form of String and Integer objects.
The method entrySet() returns a Set of the entries in the Map, in the form of objects of type Map.Entry.
Inside for-each loop we have called methods of Map.Entry like getKey() and getValue() to iterate over each map entry which is in a pair of key and value.
The method getKey() of Map.Entry returns the key information of a particular may entry, while the method getValue() returns the value associated with this key.

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