Java Collection Framework Treemap

The TreeMap class is a implementation of Map interface based on red-black tree. It provides an efficient way of storing key-value pairs in sorted order.

It is similar to HashMap class except it is sorted in the ascending order of its keys. It is not suitable for thread-safe operations due to its unsynchronized nature.

It implements NavigableMap interface and extends AbstractMap class. Declaration of this class is given below.

TreeMap Declaration

public class TreeMap<K,V>extends AbstractMap<K,V>implements NavigableMap<K,V>, Cloneable, Serializable

Important Points:

• It contains only unique elements.
• It cannot have a null key but can have multiple null values.
• It is non synchronized.
• It maintains ascending order.

TreeMap Constructors

Example : Creating TreeMap

Lets take an example to understand the creation of treeMap which is initially empty.

  
class Demo
{
  public static void main(String args[])
  {
    // Creating TreeMap
    TreeMap<String,Integer> treeMap = new TreeMap<String,Integer>();
    // Displaying TreeMap
    System.out.println(treeMap);
  }
}
  

{}

TreeMap Methods

This table contains the methods of TreeMap that can be used to manipulate objects.

Example : Adding Elements to TreeMap

After creating TreeMap, lets add elements to it. We used put method to insert elements which takes two arguments: first is key and second is value.

  
    class Demo
{
  public static void main(String args[])
  {
    // Creating TreeMap
    TreeMap<String,Integer> treeMap = new TreeMap<String,Integer>();
    // Adding elements
    treeMap.put("a",100);
    treeMap.put("b",200);
    treeMap.put("c",300);
    treeMap.put("d",400);
    // Displaying TreeMap
    System.out.println(treeMap);
  }      
  

{a=100, b=200, c=300, d=400}

Get first and last key in LinkedHashMap

We can get first and last element of the map using the firstEntry() and lastEntry() method. It returns a pair of key and value.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating TreeMap
    TreeMap<String,Integer> treeMap = new TreeMap<String,Integer>();
    // Adding elements
    treeMap.put("a",100);
    treeMap.put("b",200);
    treeMap.put("c",300);
    treeMap.put("d",400);
    // Displaying TreeMap
    System.out.println(treeMap);
    // First Element
    System.out.println("First Element: "+treeMap.firstEntry());
    // Last Element
    System.out.println("Last Element: "+treeMap.lastEntry());
    
  }
}
  

{a=100, b=200, c=300, d=400} First Element: a=100 Last Element: d=400

Example: Find HeadMap and TailMap of the TreeMap

We can get head elements and tail elements of the TreeMap by using the headMap() and tailMap() methods. These methods return a map containing the elements.

  
import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    // Creating TreeMap
    TreeMap<String,Integer> treeMap = new TreeMap<String,Integer>();
    // Adding elements
    treeMap.put("a",100);
    treeMap.put("b",200);
    treeMap.put("c",300);
    treeMap.put("d",400);
    // Displaying TreeMap
    System.out.println(treeMap);
    // Head Map
    System.out.println("Head Map: "+treeMap.headMap("c"));
    // Tail Map
    System.out.println("Tail Map: "+treeMap.tailMap("c"));
    
  }
}
  

{a=100, b=200, c=300, d=400} Head Map: {a=100, b=200} Tail Map: {c=300, d=400}

Example: Iterating TreeMap

In this example, we are creating TreeMap to store data. It uses tree to store data and data is always in sorted order and iterating the map using loop. See the below example.

  
  import java.util.*;

class Demo
{
  public static void main(String args[])
  {
    TreeMap<String,Integer> tm = new TreeMap<String,Integer>();
    tm.put("a",100);
    tm.put("b",200);
    tm.put("c",300);
    tm.put("d",400);

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

a:100 b:200 c:300 d:400