How Private Constructor in Java Enhances Code Efficiency!
By Rohan Vats
Updated on Jun 27, 2025 | 16 min read | 59.02K+ views
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By Rohan Vats
Updated on Jun 27, 2025 | 16 min read | 59.02K+ views
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Did you know that Java is still widely used in 2025, with over 90% of Fortune 500 companies employing it for their software development needs. Learning Java’s core principles, like private constructors, is essential for building scalable and efficient enterprise-level applications. |
A private constructor in Java restricts class instantiation from external sources, ensuring full control over object creation. It is commonly used in Singleton design patterns to maintain a single class instance.
By making the constructor private, Java enforces strict control over object lifecycle and state management. In Java, private constructors are also crucial for utility classes, ensuring that only static methods are accessible and preventing unnecessary object creation.
In this blog, we will explore the fundamental concepts of private constructors in Java.
A private constructor in Java restricts object creation from outside the class, ensuring that the class is instantiated only under controlled conditions. This is especially useful in design patterns like Singleton, where only one instance of the class is allowed.
In machine learning, private constructors can be used in utility or configuration classes to ensure the proper management of resources, such as training data or model configurations.
If you want to learn Java concepts and design patterns, such as the Singleton and Builder patterns, the following upGrad courses can help you succeed.
When to Use Private Constructors?
Code Example: Private Constructor in a Utility Class
Create a utility class that provides static methods for mathematical operations like addition and subtraction. Ensure no objects of the utility class can be created.
java
// Utility class for mathematical operations
class MathUtils {
// Private constructor to prevent object creation
private MathUtils() {
throw new UnsupportedOperationException("Cannot instantiate MathUtils");
}
// Static method for addition
public static int add(int a, int b) {
return a + b;
}
// Static method for subtraction
public static int subtract(int a, int b) {
return a - b;
}
}
public class Main {
public static void main(String[] args) {
// Accessing static methods without creating an object
int sum = MathUtils.add(7, 3);
int difference = MathUtils.subtract(10, 4);
// Output the results
System.out.println("Sum: " + sum);
System.out.println("Difference: " + difference);
// Uncomment the following code to see the error
// MathUtils utils = new MathUtils();
}
}
Output
Sum: 10
Difference: 6
Steps to Run the Code
Copy the code into a Java IDE or text editor.
Save the file as Main.java (or use the appropriate class name if you change it).
Compile the file using javac Main.java.
Run the program using java Main.
Explanation
Also read: Scanner Class in Java: Types of Constructors & Methods, How to Use [With Examples]
To fully understand the purpose of private constructors in Java, let’s explore how they contribute to efficient and controlled object creation.
A private constructor in Java restricts class instantiation from external classes, allowing developers to enforce design constraints and object lifecycle control. It plays a key role in encapsulating object creation logic, especially in architectural patterns and utility class design.
Also read: 35 Essential Spring Boot Annotations for Faster Development
Let’s understand the key rules that govern how and when to use a private constructor in Java effectively.
Private constructor in Java are powerful tools for controlling how and when a class is instantiated. They follow specific rules to ensure restricted access and proper use in certain scenarios like design patterns and utility classes.
The Singleton pattern ensures that a class has only one instance throughout the application. It is particularly useful for managing shared resources like database connections, configuration settings, or logging.
A private constructor ensures that no external class can create additional instances. Instead, a static method is provided to return the sole instance. The class also typically includes lazy or eager initialization to ensure efficient resource use.
Code:
Create a Logger class to handle logging throughout the application, ensuring only one instance exists for centralized logging.
java
// Singleton Logger class implementation
class Logger {
private static Logger instance;
// Private constructor to prevent direct instantiation
private Logger() {
System.out.println("Logger Instance Created");
}
// Static method to return the single instance
public static Logger getInstance() {
if (instance == null) {
instance = new Logger();
}
return instance;
}
// Method to log messages
public void log(String message) {
System.out.println("Log: " + message);
}
}
public class Main {
public static void main(String[] args) {
// Accessing the Singleton Logger instance
Logger logger1 = Logger.getInstance();
Logger logger2 = Logger.getInstance();
// Logging messages
logger1.log("Starting the application...");
logger2.log("Loading resources...");
// Verifying that both references point to the same instance
System.out.println(logger1 == logger2); // true
}
}
Output
Logger Instance Created
Log: Starting the application...
Log: Loading resources...
true
Explanation
Utility classes provide reusable static methods or constants (e.g., Math, Collections). These classes do not require instantiation. A private constructor enforces this by preventing object creation, signaling that the class is purely functional.
Code:
Problem Statement:
Create a utility class to provide mathematical operations without allowing object instantiation.
java
// Utility class with static methods
class MathUtils {
// Private constructor to prevent instantiation
private MathUtils() {
throw new UnsupportedOperationException("Cannot instantiate MathUtils");
}
public static int add(int a, int b) {
return a + b;
}
public static int subtract(int a, int b) {
return a - b;
}
}
public class Main {
public static void main(String[] args) {
// Using static methods directly
System.out.println("Sum: " + MathUtils.add(5, 3));
System.out.println("Difference: " + MathUtils.subtract(10, 4));
// Uncommenting this line will cause a compile-time error
// MathUtils utils = new MathUtils();
}
}
Output:
Sum: 8
Difference: 6
Explanation:
The private constructor prevents object creation. Attempting to instantiate the class results in an exception, while static methods remain accessible.
In the Factory design pattern pattern, object creation is controlled via factory methods, which return instances of a class. The constructor is private to restrict direct instantiation. This allows flexibility in modifying object creation logic without changing external code.
Code:
Problem Statement:
Implement a factory method for creating products with predefined configurations.
java
class Product {
private String name;
// Private constructor to restrict object creation
private Product(String name) {
this.name = name;
}
// Factory methods
public static Product createLaptop() {
return new Product("Laptop");
}
public static Product createPhone() {
return new Product("Phone");
}
public String getName() {
return name;
}
}
public class Main {
public static void main(String[] args) {
// Creating objects using factory methods
Product laptop = Product.createLaptop();
Product phone = Product.createPhone();
System.out.println("Product 1: " + laptop.getName());
System.out.println("Product 2: " + phone.getName());
}
}
Output:
Product 1: Laptop
Product 2: Phone
Explanation:
The factory methods (createLaptop, createPhone) control how objects are created, and external classes cannot directly instantiate the Product class.
A private constructor in java prevents inheritance by ensuring that no subclass can call the parent class constructor. This is useful for creating immutable classes or enforcing specific behaviors in the class hierarchy.
Code:
Problem Statement:
Create a class that cannot be subclassed to ensure immutability.
java
final class ImmutableClass {
private ImmutableClass() {
System.out.println("This class cannot be subclassed");
}
public static ImmutableClass createInstance() {
return new ImmutableClass();
}
}
// Uncommenting this will cause a compile-time error
// class SubClass extends ImmutableClass {}
public class Main {
public static void main(String[] args) {
ImmutableClass obj = ImmutableClass.createInstance();
}
}
Output:
This class cannot be subclassed
Explanation:
The private constructor ensures that no other class can extend ImmutableClass, preserving its immutability and behavior.
Classes meant for storing constants or grouping static methods do not need instances. A private constructor prevents unnecessary instantiation, making the class clear in its intent.
Code:
Problem Statement:
Create a class to store application-wide constants without allowing object creation.
java
class AppConstants {
// Private constructor to prevent instantiation
private AppConstants() {
throw new UnsupportedOperationException("Cannot instantiate AppConstants");
}
public static final String APP_NAME = "MyApplication";
public static final int VERSION = 1;
}
public class Main {
public static void main(String[] args) {
System.out.println("App Name: " + AppConstants.APP_NAME);
System.out.println("Version: " + AppConstants.VERSION);
}
}
Output:
App Name: MyApplication
Version: 1
Explanation:
The private constructor ensures that the class serves its intended purpose of storing constants and cannot be instantiated.
Also read: Constructor Overloading in Java: Explanation, Benefits & Examples
Let’s now explore how implementing a private constructor in the Singleton pattern ensures controlled, single-instance object creation.
A private constructor in Java is essential when implementing the Singleton design pattern to ensure restricted, one-time object creation across the application. This pattern is common in backend configuration managers and frontend build tools where central control is required, like in JavaScript bundlers or HTML renderers.
Step 1: Declare a Static Instance Variable: Use a private static reference that holds the only instance, shared across the class globally.
Step 2: Define a Private Constructor: Prevent external instantiation and enforce controlled access, just like restricting direct access to CSS utility classes.
Step 3: Create a Public Static getInstance() Method: Check if the instance exists and return it. If not, create it using lazy initialization.
Step 4: Use the Singleton Object Across Modules: This object can coordinate configuration for logging, DB access, or UI themes like ReactJS or VueJS themes.
Step 5: Maintain Shared State Across Calls: Changes made to the singleton instance persist across calls, useful in server-side NodeJS or client-side browser apps.
Code Example:
class Logger {
private static Logger instance;
public String source;
private Logger() {
System.out.println("Logger created for UPI transaction logs.");
source = "NPCI-India";
}
public static Logger getInstance() {
if (instance == null) {
instance = new Logger();
}
return instance;
}
public void log(String message) {
System.out.println("[Source: " + source + "] Log: " + message);
}
}
public class Main {
public static void main(String[] args) {
Logger logger1 = Logger.getInstance();
Logger logger2 = Logger.getInstance();
logger1.log("Payment initiated via BHIM App.");
logger2.log("Transaction processed for ₹750 using QR code.");
System.out.println(logger1 == logger2);
}
}
Output:
Logger created for UPI transaction logs.
[Source: NPCI-India] Log: Payment initiated via BHIM App.
[Source: NPCI-India] Log: Transaction processed for ₹750 using QR code.
true
Code Explanation:
This example ensures consistent logging using a singleton design pattern with a private constructor in Java. Both logger references point to the same instance, maintaining state across method calls.
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To understand how private constructors ensure non-instantiability, let’s examine their role in utility classes.
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The private constructor in Java is essential in the Builder pattern, allowing controlled object creation through an inner builder class. It’s widely used in fields, such as form builders in Flask apps, ETL pipeline configurations in Apache Spark, or model configurations in Scala-based systems.
Here are some of the factors why you should consider a Private Constructor in the Builder Pattern:
Code Example:
public class Employee {
// Final fields to support immutability
private final String name;
private final int age;
private final String department;
// Private constructor prevents direct instantiation
private Employee(String name, int age, String department) {
this.name = name;
this.age = age;
this.department = department;
}
// Static inner Builder class
public static class Builder {
private String name;
private int age;
private String department;
public Builder setName(String name) {
this.name = name;
return this;
}
public Builder setAge(int age) {
this.age = age;
return this;
}
public Builder setDepartment(String department) {
this.department = department;
return this;
}
public Employee build() {
return new Employee(name, age, department);
}
}
@Override
public String toString() {
return "Employee{name='" + name + "', age=" + age + ", department='" + department + "'}";
}
public static void main(String[] args) {
// Create an Employee object with all fields
Employee emp1 = new Employee.Builder()
.setName("Rahul Mehra")
.setAge(32)
.setDepartment("Data Engineering")
.build();
// Create another Employee with partial fields
Employee emp2 = new Employee.Builder()
.setName("Ananya Sharma")
.setDepartment("HR")
.build();
System.out.println(emp1);
System.out.println(emp2);
}
}
Output:
Employee{name='Rahul Mehra', age=32, department='Data Engineering'}
Employee{name='Ananya Sharma', age=0, department='HR'}
Code Explanation:
The private constructor restricts object creation to the builder, ensuring controlled and structured instantiation. This pattern enhances flexibility and clarity, particularly in applications that involve multiple optional parameters.
Also read: 50 Java Projects With Source Code in 2025: From Beginner to Advanced
To better understand object creation, let’s explore how private constructors in the Builder pattern offer control and flexibility.
The private constructor in Java is crucial for the Builder pattern, ensuring controlled object creation through an inner builder class. This approach is common in cloud-based services like AWS Lambda or Azure, requiring flexible, immutable object construction.
Code Example:
public class Employee {
// Final fields ensure immutability
private final String name;
private final int age;
private final String department;
// Private constructor to restrict direct instantiation
private Employee(String name, int age, String department) {
this.name = name;
this.age = age;
this.department = department;
}
// Static inner Builder class
public static class Builder {
private String name;
private int age;
private String department;
public Builder setName(String name) {
this.name = name;
return this;
}
public Builder setAge(int age) {
this.age = age;
return this;
}
public Builder setDepartment(String department) {
this.department = department;
return this;
}
// Build method to return the Employee object
public Employee build() {
return new Employee(name, age, department);
}
}
@Override
public String toString() {
return "Employee{name='" + name + "', age=" + age + ", department='" + department + "'}";
}
public static void main(String[] args) {
// Create an Employee object with all fields
Employee emp1 = new Employee.Builder()
.setName("Ravi Kumar")
.setAge(28)
.setDepartment("Cloud Engineering")
.build();
// Create another Employee with partial fields
Employee emp2 = new Employee.Builder()
.setName("Priya Verma")
.setDepartment("Data Science")
.build();
System.out.println(emp1);
System.out.println(emp2);
}
}
Output:
Employee{name='Ravi Kumar', age=28, department='Cloud Engineering'}
Employee{name='Priya Verma', age=0, department='Data Science'}
Code Explanation:
The private constructor ensures that only the Builder can create instances, enabling flexible and consistent object construction. This pattern is particularly effective for building scalable cloud applications like AWS Lambda and Azure-based services.
A private constructor in Java is a critical tool for managing object instantiation and enforcing design patterns, such as the Singleton pattern. To use private constructors effectively, focus on understanding their role in controlling object lifecycle and system integrity.
Many developers struggle with managing scalable application design, especially when avoiding unwanted object creation becomes critical. upGrad’s additional courses can help you build the expertise needed to master these design techniques.
Curious which courses can enhance your Java skills and design pattern expertise? Contact upGrad for personalized counseling and valuable insights. For more details, you can also visit your nearest upGrad offline center.
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