Java HashMap putIfAbsent() Method

The HashMap.putIfAbsent() method in Java is used to insert a key-value pair into the HashMap only if the specified key is not already associated with a value.

Table of Contents

1. Introduction
2. putIfAbsent Method Syntax
3. Examples
   • Inserting Entries if Absent
   • Real-World Use Case: Adding New Employee Records
4. Conclusion

Introduction

The HashMap.putIfAbsent() method is a member of the HashMap class in Java. It allows you to insert a key-value pair into the map only if the key is not already present. This can be useful for avoiding overwriting existing values.

putIfAbsent() Method Syntax

The syntax for the putIfAbsent method is as follows:

public V putIfAbsent(K key, V value)

• The method takes two parameters:
  • key of type K, which represents the key to be inserted.
  • value of type V, which represents the value to be associated with the key.
• The method returns the previous value associated with the specified key, or null if there was no mapping for the key.

Examples

Inserting Entries if Absent

The putIfAbsent method can be used to insert a key-value pair into the HashMap only if the key is not already present.

Example

import java.util.HashMap;
import java.util.Map;

public class PutIfAbsentExample {
    public static void main(String[] args) {
        // Creating a Map with String keys and String values
        Map<String, String> countries = new HashMap<>();

        // Adding entries to the HashMap
        countries.put("India", "IN");
        countries.put("United States", "US");

        // Using putIfAbsent method
        countries.putIfAbsent("Canada", "CA");
        countries.putIfAbsent("India", "IND");  // This will not overwrite the existing value

        // Printing the updated HashMap
        System.out.println("Updated HashMap: " + countries);
    }
}

Output:

Updated HashMap: {India=IN, United States=US, Canada=CA}

Real-World Use Case: Adding New Employee Records

In a real-world scenario, you might use the putIfAbsent method to add new employee records to an employee database, ensuring that existing records are not overwritten.

Example

import java.util.HashMap;
import java.util.Map;

public class AddEmployeeRecords {
    public static void main(String[] args) {
        // Creating a Map with String keys (employee IDs) and String values (employee names)
        Map<String, String> employeeDatabase = new HashMap<>();

        // Adding entries to the HashMap
        employeeDatabase.put("E001", "Ravi Kumar");
        employeeDatabase.put("E002", "Priya Sharma");

        // Using putIfAbsent method to add new employee records
        employeeDatabase.putIfAbsent("E003", "Vijay Singh");
        employeeDatabase.putIfAbsent("E002", "Amit Patel");  // This will not overwrite the existing value

        // Printing the updated employee database
        System.out.println("Updated Employee Database: " + employeeDatabase);
    }
}

Output:

Updated Employee Database: {E001=Ravi Kumar, E002=Priya Sharma, E003=Vijay Singh}

Conclusion

The HashMap.putIfAbsent() method in Java provides a way to insert key-value pairs into a HashMap only if the key is not already present. By understanding how to use this method, you can avoid overwriting existing values and ensure data integrity. This method is useful in various scenarios, such as adding new records, managing collections of data, and implementing caching mechanisms.