Purpose of Private Constructor in Java

In this article, we will learn about the purpose of private constructor in Java. Constructor is a unique method used to initialize objects. Constructors are public or protected by default, which allows outside classes to create instances

Why Use a Private Constructor?

Private constructor is mainly applied to manage object creation. It does not allow other classes to instantiate the class, creating particular design patterns or restrictions.

Purpose of a Private Constructor

The private constructor is useful in case we want to restrict the object creation. For example ?

• Singleton pattern can be implemented using a private constructor.
• Utility classes prevent the instantiation of classes that contain only static methods.
  
• Factory methods control object creation by static methods instead of constructors.

Singleton Pattern Using a Private Constructor

The Singleton Pattern ensures that a class has only one instance and provides a global access point to that instance.

The following are the steps to create a singleton pattern using a private constructor ?

• Private Constructor (private Tester()): Prevents direct object creation using new Tester().
  
• Static Instance Variable (private static Tester instance): Holds the single instance of the class.
  
• Static Method (getInstance()): Returns the existing instance if already created and creates a new instance only if instance == null (first call).
  
• Checking Singleton Property: Two calls to getInstance() return the same object, so tester.equals(tester1) prints true.

Example

Below is an example of creating a singleton pattern using a private constructor in Java ?

public class Tester {
   private static Tester instance;
   private Tester(){}
 
   public static Tester getInstance(){
      if(instance == null){
         instance = new Tester();
      }
      return instance;
   }
 
   public static void main(String[] args) {
      Tester tester = Tester.getInstance();
      Tester tester1 = Tester.getInstance();
      System.out.println(tester.equals(tester1));
   }  
}

Output

true

Time complexity: O(1), constant time object retrieval or creation.
Space complexity: O(1), since only a single instance of Tester is stored.