A Brief Introduction to TreeMap in Java with Examples

Implementing a Map interface in Java is a very important task. For this purpose, we have TreeMap and HashMap. In this article our focus will be on TreeMap in Java in the Following Order:

• What is a TreeMap in Java?
• Features of TreeMaps
• Constructors in TreeMap
• Methods in TreeMap
• Example of TreeMap in Java

What is a TreeMap in Java?

A TreeMap in Java is used to implement Map interface and NavigableMap along with the Abstract Class. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used. This proves to be an efficient way of sorting and storing the key-value pairs.

The storing order maintained by the treemap must be consistent with equals just like any other sorted map, irrespective of the explicit comparators. The treemap implementation is not synchronized in the sense that if a map is accessed by multiple threads, concurrently and at least one of the threads modifies the map structurally, it must be synchronized externally.

Features of TreeMaps

• This class is a member of the Java Collections Framework.

• The class implements Map interfaces including NavigableMap, SortedMap and extends AbstractMap

• TreeMap in Java does not allow null keys (like Map) and thus a NullPointerException is thrown. However, multiple null values can be associated with different keys.

• All Map.Entry pairs returned by methods in this class and its views represent snapshots of mappings at the time they were produced.

• They do not support the Entry.setValue method.

Important Points to Remember

1. Apart from implementing the Map interface, Java TreeMap also implements NavigableMap and indirectly implements SortedMap interface. TreeMap also extends AbstractMap class.

2. TreeMap entries are sorted in the natural ordering of its keys. It also provides a constructor to provide Comparator to be used for ordering. So if you are using any class as key, make sure it’s implementing Comparable interface for natural ordering. Check out java collections interview questions to understand the importance of these methods.

3. Java TreeMap implementation provides guaranteed log(n) time cost for the containsKey, get, put and remove operations.

4. TreeMap is not synchronized and hence not thread-safe. For multithreaded environments, you can get a wrapped synchronized using Collections.synchronizedSortedMap method.

5. TreeMap methods to get keyset and values return Iterator that is fail-fast in nature, so any concurrent modification will throw ConcurrentModificationException.

6. TreeMap in java doesn’t allow null keys, however, you can have multiple null values associated with different keys.

Constructors in TreeMap

Methods in TreeMap

Example of TreeMap in Java

import java.util.TreeMap;
 
public class TreeMapMain {
 
	public static void main(String args[])
	{
		// TreeMap with Country as key and capital as value
		// TreeMap stores elements in natural ordering of keys.
		TreeMap<String,String> countryCapitalMap=new TreeMap<String,String>();
		countryCapitalMap.put("India","Delhi");
		countryCapitalMap.put("Japan","Tokyo");
		countryCapitalMap.put("France","Paris");
		countryCapitalMap.put("Russia","Moscow");
 
		System.out.println("-----------------------------");
		// Iterating TreeMap Using keySet() and for each loop
		System.out.println("Iterating TreeMap Using keySet() and for each loop");
		for (String countryKey:countryCapitalMap.keySet()) {
			System.out.println("Country:"+ countryKey +" and  Capital:"+countryCapitalMap.get(countryKey));
 
		}
		System.out.println("-----------------------------");
	}
 
}

Output:

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