Introduction
A private constructor in Java is used in restricting object creation. It is a special instance constructor used in static member-only classes. If a constructor is declared as private, then its objects are only accessible from within the declared class. You cannot access its objects from outside the constructor class.  Â
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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. |
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Conclusion
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, we 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.
What 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.
What 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.
What 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.
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