Java Program to implement private constructors

Example 1: Java program to create a private constructor

class Test {

  // create private constructor
  private Test () {
    System.out.println("This is a private constructor.");
  }

  // create a public static method
  public static void instanceMethod() {

    // create an instance of Test class
    Test obj = new Test();
  }

}

class Main {

  public static void main(String[] args) {

    // call the instanceMethod()
    Test.instanceMethod();
  }
}

Output

This is a private constructor.

In the above example, we have created a private constructor of the Test class. Hence, we cannot create an object of the Test class outside of the class.

This is why we have created a public static method named instanceMethod() inside the class that is used to create an object of the Test class. And from the Main class, we call the method using the class name.

Example 2: Java Singleton design using a private constructor

The Java Singleton design pattern ensures that there should be only one instance of a class. To achieve this we use the private constructor.

class Language {

  // create a public static variable of class type
  private static Language language;

  // private constructor
  private Language() {
    System.out.println("Inside Private Constructor");
  }

  // public static method
  public static Language getInstance() {

     // create object if it's not already created
     if(language == null) {
        language = new Language();
     }

      // returns the singleton object
      return language;
  }

  public void display() {
      System.out.println("Singleton Pattern is achieved");
  }
}

class Main {
  public static void main(String[] args) {
     Language db1;

     // call the getInstance method
     db1= Language.getInstance();

     db1.display();
  }
}

Output

Inside Private Constructor
Singleton Pattern is achieved

In the above example, we have created a class named Languages. The class contains,

• language - class type private variable
• Language() - private constructor
• getInstance() - public static class type method
• display() - public method

Since the constructor is private, we cannot create objects of Language from the outer class. Hence, we have created an object of the class inside the getInstance() method.

However, we have set the condition in such a way that only one object is created. And, the method returns the object.

Notice the line,

db1 = Language.getInstance();

Here,

• db1 is a variable of Language type
• Language.getInstance() - calls the method getInstance()

Since, getInstance() returns the object of the Language class, the db1 variable is assigned with the returned object.

Finally, we have called the display() method using the object.