Java TreeMap firstKey() Method

The TreeMap.firstKey() method in Java is used to retrieve the first (lowest) key in the map. We will also cover a real-world use case to show how TreeMap.firstKey() can be used effectively.

Table of Contents

1. Introduction
2. firstKey Method Syntax
3. Examples
   • Retrieving the First Key from a TreeMap
   • Handling an Empty TreeMap
4. Real-World Use Case
   • Example: Finding the First Student in a Class List
5. Conclusion

Introduction

The TreeMap.firstKey() method is a member of the TreeMap class in Java. It allows you to retrieve the first (lowest) key in the map. If the map is empty, the method throws a NoSuchElementException.

firstKey() Method Syntax

The syntax for the firstKey method is as follows:

public K firstKey()

• The method does not take any parameters.
• The method returns the first (lowest) key in the map.

Examples

Retrieving the First Key from a TreeMap

The firstKey method can be used to get the first key in the TreeMap.

Example

import java.util.TreeMap;

public class FirstKeyExample {
    public static void main(String[] args) {
        // Creating a TreeMap with String keys and Integer values
        TreeMap<String, Integer> treeMap = new TreeMap<>();

        // Adding entries to the TreeMap
        treeMap.put("Ravi", 25);
        treeMap.put("Priya", 30);
        treeMap.put("Vijay", 35);
        treeMap.put("Anita", 28);
        treeMap.put("Suresh", 40);

        // Retrieving the first key from the TreeMap
        String firstKey = treeMap.firstKey();

        // Printing the first key
        System.out.println("First key: " + firstKey);
    }
}

Output:

First key: Anita

Handling an Empty TreeMap

The firstKey method throws a NoSuchElementException if the TreeMap is empty.

Example

import java.util.TreeMap;

public class EmptyTreeMapExample {
    public static void main(String[] args) {
        // Creating an empty TreeMap
        TreeMap<String, Integer> treeMap = new TreeMap<>();

        try {
            // Attempting to retrieve the first key from an empty TreeMap
            String firstKey = treeMap.firstKey();
            System.out.println("First key in empty TreeMap: " + firstKey);
        } catch (java.util.NoSuchElementException e) {
            System.out.println("The TreeMap is empty.");
        }
    }
}

Output:

The TreeMap is empty.

Real-World Use Case

Example: Finding the First Student in a Class List

A common real-world use case for TreeMap.firstKey() is finding the first student in a class list where students are sorted by their names.

Example

import java.util.TreeMap;

public class StudentManager {
    public static void main(String[] args) {
        // Creating a TreeMap to manage student names and their IDs
        TreeMap<String, Integer> students = new TreeMap<>();

        // Adding student names and their IDs to the TreeMap
        students.put("Ravi", 101);
        students.put("Priya", 102);
        students.put("Vijay", 103);
        students.put("Anita", 104);
        students.put("Suresh", 105);

        // Retrieving the first student's name in the class list
        String firstStudent = students.firstKey();

        // Printing the first student's name
        System.out.println("First student in the class list: " + firstStudent);
    }
}

Output:

First student in the class list: Anita

In this example, TreeMap.firstKey() is used to find the first student’s name in a class list based on their names, making it easy to identify and work with the first student in the list.

Conclusion

The TreeMap.firstKey() method in Java provides a way to retrieve the first (lowest) key in the map. By understanding how to use this method, you can efficiently manage and navigate collections of key-value pairs in your Java applications. The method allows you to find and work with the lowest keys in the map, making it a versatile tool for data management in various scenarios.