How to create a TreeMap object in Java

Introduction

In this shot, we’ll learn about the TreeMap collection, a class present in the Collections framework of Java. We generally use it to instantiate objects for Map and NavigableMap interfaces. It is a key-value pair collection, and it is sorted based on its keys.

TreeMap does not allow us to insert a null key in the map.

Creation

There are three constructors or ways to create an object of the TreeMap class. Let’s discuss them one by one.

1. Using `TreeMap()`

We construct a TreeMap object using the new keyword and specify the type of the key-value for the TreeMap.

Code

Let’s look at the code below:

Explanation

Line 1: We import the required package.
Line 2: We make a Main class.
Line 6: We make the first constructor for TreeMap creation. We declare a TreeMap with Integer type keys and String type values.
Lines 8 to 12: We insert the entries in the map using the TreeMap.put() method.
Line 14: We display the TreeMap contents.

Let’s discuss the second approach now.

2. Using `TreeMap(Comparator c)`

It constructs a new and empty TreeMap object that needs an external specification to sort the elements of the TreeMap.

Code

Let’s look at the code below:

import java.util.*;
class Student
{
  int rollno;
  String name;
  public Student(int rollno, String name)
  {
    this.rollno = rollno;
    this.name = name;
  }
  public String toString()
  {
    return this.rollno + " " + this.name;
  }
}
class Sortbyroll implements Comparator<Student> 
{
  public int compare(Student a, Student b)
  {
    return a.rollno - b.rollno;
  }
}     
 
class Main 
{
  public static void main(String[] args)
  {
    TreeMap<Student, Integer> t2 = new TreeMap<Student, Integer>(new Sortbyroll());
    t2.put(new Student(11, "John"), 23);
    t2.put(new Student(91, "Jack"), 39);
    t2.put(new Student(21, "Dane"), 31);
    
    System.out.println("TreeMap using 2nd constructor is:"+t2);
  }
}

Explanation

Line 1: We import the required package.
Line 2: We make a Main class.
Lines 2 to 10: We create a class and initialize its data members with the help of the constructor.
Lines 11 to 14: We define a member function of the above class, which returns its data members.
Lines 17 to 23: We make another class that implements the Comparator interface and compares one of the data members of the above class. We will use this class to create a TreeMap, which sorts based on a custom comparator.
Line 29: We make the second constructor for TreeMap creation and sort it according to a comparator.
Lines 31 to 33: We insert the entries in the map using the TreeMap.put() method.
Line 35: We display the TreeMap content. We can see that the content is sorted based on the rollno.