Private Constructor in Java

A private constructor in Java is a constructor of a class specified with the "private" access modifier. A private constructor can only be accessed from within the class that declared it. This means that the class's objects cannot be created from outside the class.

Private constructors are often used for various design patterns and scenarios to control the instantiation of objects.

  public class MyTestClass {
    private MyTestClass() {
      // code
    }
  }

Importants points:

A private constructor is used in the singleton design pattern, utility classes, and factory methods in Java.

Example: Accessing Private Constructor Outside the Class

In Java, when we specify a private access modifier with the class properties (such as variables, methods, and constructors), that means we cannot access private properties outside the class. This is a restriction with the private access modifiers; if we try access outside the class, we will get a compile time error.

In the below example, there is a class named Test, and it has a private constructor. If we are trying to create the object of the Test class outside the class, then we will get a compile time error. Let's see the example below.

  
  package com.javabytechie.privateconstructor;
  class Test {
    // Private Constructor
    private Test() {
      System.out.println("Private Constructor Called...");
    }
  
    void display() {
      System.out.println("display() method called...");
    }
  }
  
  public class PrivateConstructorExample {
    public static void main(String[] args) {
      Test test = new Test();                   // Compile time Error
      test.display();
    }
  }

Exception in thread "main" java.lang.Error: Unresolved compilation problem: The constructor Test() is not visible at com.javabytechie.privateconstructor.PrivateConstructorExample.main(PrivateConstructorExample.java:14)

Example: Accessing Private Constructor Outside the Class Using Static Method

  
  package com.javabytechie.privateconstructor;
  class Test {
    private static Test testObj;
  
    // Private Constructor
    private Test() {
      System.out.println("Private Constructor Called...");
    }
  
    // Creating static method to access the object outside the class
    static Test getInstance() {
      if (testObj == null) {
        testObj = new Test();
      }
      return testObj;
    }
  
    void display() {
      System.out.println("display() method called...");
    }
  }
  
  public class PrivateConstructorExample2 {
    public static void main(String[] args) {
      Test test = Test.getInstance();
      test.display();
    }
  }

Example: Accessing Private Constructor in the same class

  
  package com.javabytechie.privateconstructor;
  public class PrivateConstructorExample3 {
    // Private Constructor
    private PrivateConstructorExample3() {
      System.out.println("Private Constructor Called...");
    }
    
    void display() {
      System.out.println("display() method called...");
    }
    
    public static void main(String[] args) {
      // Accessing private constructor inside the class
      PrivateConstructorExample3 objc = new PrivateConstructorExample3();
      objc.display();
    }
  }

In the above example, there is a private constructor in the class, creating an object inside the class and calling the display() method of the same class. This is valid, and the output will be generated without error.

A class cannot be extended if it has a private constructor

If a class has private constructors, then that class cannot be extended. If we try to extend, we will get a compile-time error. For understanding this scenario, below is an example that is invalid in Java; don't try to implement it.

  class A {
    // Private Constructor
    private A() {
    }
  }
  
  class B extends A {     // compile-time error
  }

Private Constructor in Java

Importants points:

Example: Accessing Private Constructor Outside the Class

Example: Accessing Private Constructor Outside the Class Using Static Method

Example: Accessing Private Constructor in the same class

A class cannot be extended if it has a private constructor

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