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Private Constructor in Java: Use Cases Explained with Example

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9th Apr, 2021
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Private Constructor in Java: Use Cases Explained with Example

In Java programming, understanding the intricacies of class design and object creation is crucial for developers aiming to excel in their careers. Among these intricacies, the concept of a Private Constructor in Java emerges as a fundamental aspect that piques the curiosity of many aspiring to master the language. Through years of coding and project development, I’ve come to value the nuances and advantages of using private constructors in Java applications. This introduction is your doorway to grasping the what, why, and how of private constructors. From enhancing encapsulation to implementing singleton design patterns, I aim to share insights from real-world applications, clarifying how private constructors can be employed to forge more secure and well-organized code. This article is intended for professionals seeking to expand their Java knowledge, encompassing key topics such as the definition of a private constructor, its governing rules, and its practical applications, culminating in a thorough conclusion that highlights its importance in Java development.

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What is a Private Constructor?

In my journey as a software developer, I’ve often encountered scenarios where the concept of a Private Constructor in Java proved to be a game-changer. A private constructor is a particular type of constructor in Java designed to restrict the instantiation of a class from outside its own boundary. This means with a java class with private constructor, you cannot create an object of this class from any other class. 

Why use it, you might ask? Throughout my career, I’ve leveraged private constructors in several use cases, such as creating utility classes where static methods and fields need not be associated with any particular object. For instance, consider a utility class that provides mathematical operations. By making its constructor private, we ensure that no one can create an example of this class, maintaining its purpose as a pure utility provider without any instantiation overhead. Using a private constructor, we enforce this uniqueness, preventing any other class from creating another instance. 

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Rules for Private Constructors 

In my years of experience as a Java developer, I’ve encountered several scenarios where using a Private Constructor in Java was beneficial and essential. Let me share some rules and insights on this topic based on real-life applications and their impact.  

  • Singleton Pattern Implementation: The most common use case for a “Private Constructor in Java” is in the Singleton design pattern. Here, ensuring that a class has only one instance requires that the constructor be private. This control mechanism prevents any other class from instantiating it directly. 
  • Utility Classes: For classes that serve purely as collections of static methods and static variables, making the constructor private prevents instantiation. It’s a practical approach I’ve employed in numerous projects to signify that the class is a utility holder. 
  • Factory Method Pattern: In scenarios where classes are designed with factory methods responsible for object creation, private constructors restrict instantiation to within the class itself. This technique ensures that object creation is controlled and can be modified without affecting external code. 
  • Controlling Access: The private constructor in Java access rule is pivotal in controlling how other classes interact with the constructor. It effectively limits access to the constructor, thereby guiding the use of the class in a predefined manner. 
  • Extending Classes: It’s worth noting that a class with a private constructor cannot be subclassed. This rule is particularly useful when creating immutable classes or ensuring that the integrity of the class hierarchy is maintained without unexpected subclassing. 

Incorporating private constructors in Java, based on these rules, has significantly influenced the design and functionality of the applications I’ve worked on. 

Private Constructor Use-Cases

Private constructors in Java are accessed only from within the class. You cannot access a private constructor from any other class. If the object is yet not initialised, then you can write a public function to call the private instructor. If the object is already initialised, then you can only return the instance of that object. A private constructor in Java has the following use-cases:

  • You can use it with static members-only classes.
  • You can use it with static utility or constant classes.
  • You can use it to serve singleton classes.
  • You can use it to assign a name, for instance, creation by utilising factory methods.
  • You can use it to prevent subclassing.

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Singleton Class

The private constructor in Java is used to create a singleton class. A singleton class is a class in Java that limits the number of objects of the declared class to one. A private constructor in Java ensures that only one object is created at a time. It restricts the class instances within the declared class so that no class instance can be created outside the declared class. You can use the singleton class in networking and database connectivity concepts.

Example 1: The following example demonstrates how a private constructor in Java limits the number of objects in a singleton class.

import java.io.*;

class newTestClass {

static newTestClass object = null;

public int a = 20;

private newTestClass() {

}

//we have created a private constructor

static public newTestClass displayInstance()

/**

*displayInstance() is a public method that we will use to create instance.

*Call this method to access these instances from outside the class.

*/

{

if (object == null)

     object = new newTestClass();

     //singleton class object is created

     return object;

}

}

public class SingletonClass {

public static void main(String args[]) {

     newTestClass instance1 = newTestClass.displayInstance();

     //displaying the instance of the singleton class by calling the public method

    newTestClass instance2 = newTestClass.displayInstance();

    //displaying the instance of the singleton class by calling the public method

    instance1.a = instance1.a + 26;

    System.out.println(“Instance 1 created has the following value = ” + instance1.a);

    System.out.println(“Instance 2 created has the following value = ” + instance2.a);

}

}

Output

Instance 1 created has the following value = 46

Instance 2 created has the following value = 46

In the above program, the value of instance 2 automatically gets updated when we update the instance1 object value. This observation demonstrates that both instance1 and instance 2 refer to the same object.

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Example 2: The following example demonstrates how a private constructor in Java ensures that no more than one object is created at a time.

// Java program for a singleton class implementation

Import java.io.*;

public class newTestClass {

   private static newTestClass var=null;

   private newTestClass(){

   //Private Constructor can be accessed within the class

   }

   public static newTestClass testObject(){

           // This method ensures that only one object is created at a time

           if(var==null){

           var= new newTestClass();

           }

     return var;

   }

   public void display(){

           System.out.println(“This is an example of a Private constructor using a Singleton class. “);

   }

   public static void main(String args[]){

           newTestClass newObject= newTestClass.testObject();

           newObject.display();

   }

}

Output

This is an example of a Private constructor using a Singleton class.

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Example 2: Private Constructor in Java to limit the number of class instances.

This example demonstrates how you can limit the number of class instances by declaring a constructor private.

public class restrictInstanceCreationClass {

           public static void main(String[] args) {

                          RestrictInstance obj;

                          int a=1;

                          while(a<=20)

                          {

                                        obj = RestrictInstance.getLimInstance();

                                        a++;

                          }

           }

}

class RestrictInstance {

           public static int varCount = 0;

           /**

           *create private constructor that increases the count

           *of varCount variable after each instance creation

           */

           private RestrictInstance() {

                          varCount++;

                          System.out.println(“Instance number ” + varCount + ” is created.”);

           }

          

           public static synchronised RestrictInstance getLimInstance() {

                          if(varCount <5) {

                                        return new RestrictInstance();

                          }

                          System.out.println(“Maximum instance limit reached. You are not allowed to create anymore instances.”);

                          System.gc();

                          return null;

           }

           /**

           *delete the instance and decrease the count

           *of the varCount variable

           */

           public void finalise()

           {

                          System.out.println(“Instance is deleted.”);

                          varCount–;

           }}

Output

Instance number 1 is created.

Instance number 2 is created.

Instance number 3 is created.

Instance number 4 is created.

Instance number 5 is created.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Instance is deleted.

Instance is deleted.

Instance is deleted.

Instance is deleted.

Maximum instance limit reached. You are not allowed to create anymore instances.

Instance number 1 is created.

Instance number 2 is created.

Instance number 3 is created.

Instance number 4 is created.

Instance number 5 is created.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Maximum instance limit reached. You are not allowed to create anymore instances.

Instance is deleted.

Instance is deleted.

Instance is deleted.

Instance number 1 is created.

Instance number 2 is created.

Instance number 3 is created.

Instance number 4 is created.

Instance number 5 is created.

Conclusion

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The code used in the article is only for explanatory purposes. You can modify the statements given in the examples as per your requirements. In this blog, I have discussed how a private constructor in Java limits the number of objects in a singleton class.

You can try out the code to strengthen your Java constructor’s knowledge. If you want to gain an in-depth understanding of Java, check out the upGrad Executive PG Program in Full Stack Development course that is designed for working professionals to gain expertise in this area.

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Rohan Vats

Blog Author
Software Engineering Manager @ upGrad. Passionate about building large scale web apps with delightful experiences. In pursuit of transforming engineers into leaders.

Frequently Asked Questions (FAQs)

1What are constructors in programming?

Constructors in programming are the methods that are called automatically when an object is initialized. The constructor's purpose is to initialize the object. Constructors should always be public and they are declared without any return type. Constructors are used to initialize the object and to perform other tasks that can be done only once. Constructors are also used to perform initialization of a class, that is, to perform tasks that create the class.

2What are the types of constructors in Java?

There are five types of constructors in Java. They are: No-argument constructors: It has a default implementation, which is executed when the class is loaded into the memory. A class which has no no-argument constructor MUST be declared as final. Default constructors: This constructor is created when the class is written in Java and is used to initialize all the class's variables (attributes) to their default values (zero or null). Parameterized constructors: This constructor is used to initialize some variables from some values. Constructors with no return value: It is used to initialize some variables from some values. Constructors with return value: it is used to initialize some variables from some values and also returns with a value.

3What is a private constructor in Java?

Private constructors can be used to create an immutable class. Without using private constructors, we can create immutable classes by declaring the class final and creating all the attributes as final (e.g. private final String name;). A private constructor cannot be directly called by client objects of the class. They can only be called internally by methods of the class (even private). You can have a private constructor with one or more parameters. You cannot have return type of the constructor be primitive boolean. It can be either String or Class.