Call private constructor in java

How to call Private Constructor in Java

The method java.lang.Class.getDeclaredConstructor() can be used to obtain the constructor object for the private constructor of the class. The parameter for this method is a Class object array that contains the formal parameter types of the constructor.

A program that demonstrates this is given as follows −

Example

package Test; import java.lang.reflect.*; public class Demo < String str; Double d; public Demo(String str, Double d) < this.str = str; this.d = d; >public static void main(String[] args) < try < Demo obj = new Demo("Apple", 55.983); Class c = obj.getClass(); Class[] arguments = new Class[2]; arguments[0] = String.class; arguments[1] = Double.class; Constructor constructor = c.getDeclaredConstructor(arguments); System.out.println("Constructor = " + constructor.toString()); >catch(NoSuchMethodException e) < System.out.println(e.toString()); >catch(SecurityException e) < System.out.println(e.toString()); >> >

output

Constructor = public Test.Demo(java.lang.String,java.lang.Double)

Now let us understand the above program.

An object of class Demo is created in the main() method. Then the array arguments[] stores the String.Class and Double.Class objects. Finally, the method getDeclaredConstructor() can be used to obtain the constructor object and this is displayed. A code snippet which demonstrates this is as follows −

Demo obj = new Demo("Apple", 55.983); Class c = obj.getClass(); Class[] arguments = new Class[2]; arguments[0] = String.class; arguments[1] = Double.class; Constructor constructor = c.getDeclaredConstructor(arguments); System.out.println("Constructor = " + constructor.toString());

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Accessing Private Constructor in Java

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1. Overview

In this tutorial, we’ll see why we’d use a private constructor for a class in Java and how to use it.

2. Why Use a Private Constructor?

In Java, we can declare a constructor as private using the private access specifier. If a constructor is declared private, we can’t create an object of the class, except within the class.

A private constructor is used when we want to limit the way objects of a class are instantiated. For example, we might do this if we want to create an object by only using a factory class that can create those objects. Or another situation is when we want to have only one object instance of that class.

Some of the most used cases of a private constructor are Singleton, Builder, and Factory, the creational design patterns.

Now, we can imagine any combination of these patterns as they can blend nicely and achieve a robust codebase.

3. Accessing the Private Constructor

Usually, in order to call the private constructor, the use cases listed above have other public methods that would call the private constructor within the class.

Alternatively, we can use the Java Reflection API to directly access the private constructor.

The Java Reflection API is an advanced feature that allows programs to examine and modify the runtime behavior of the application running within the JVM. Because of this, using this method isn’t recommended as it can lead to difficulty spotting and fixing bugs.

Using Reflection, we can see the methods and attributes of any class and modify or access them bypassing the access modifiers.

The most used case for using reflection is unit testing a class that has private methods. To unit test a private constructor or method using reflection, we’d need to do the following steps:

• get the class object for the class that we want to instantiate
• with the class object, call the getDeclaredContructor() method to get the Constructor object
• on the Constructor object, call the setAccessible() method and make the constructor accessible
• after the Constructor object is accessible, we can call the newInstance() method that will create a new object of that class

Let’s create a class with a private constructor. Then we’ll use the Java Reflection API to instantiate it and make sure that the private constructor was called:

private PrivateConstructorClass()

Let’s add a unit test where we instantiate this class using the private constructor:

@Test public void whenConstructorIsPrivate_thenInstanceSuccess() throws Exception < Constructorpcc = PrivateConstructorClass.class.getDeclaredConstructor(); pcc.setAccessible(true); PrivateConstructorClass privateConstructorInstance = pcc.newInstance(); Assertions.assertTrue(privateConstructorInstance instanceof PrivateConstructorClass); >

In the console output, we should see that the private constructor was called and the print inside the constructor displayed the message. In spite of the private access modifier, we can now call the private constructor and instantiate new objects.

4. Conclusion

In this article, we saw why we’d use a private constructor and a few different ways to use it. We also learned that we can create public methods to access a private constructor or use the advanced Java Reflection API for a more advanced approach.

As always, the full implementation of these examples can be found over on GitHub.

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Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

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Private Constructor in Java | Use of private constructor in Java

In this quick tutorial, we will discuss about private constructor in Java, its uses. How to declare a private constructor and call a private constructor in Java?

Before we answer that let’s make the basics clear. Java is an Object Oriented Programming language. It uses a predefined template called Class to create objects. Class is also referred to as an object factory.

A class contains data and methods together which can be accessed/used by creating an object of that class.

Well, then what is the story of Constructors?

A constructor is a special method in Java that is used to initialize Objects. As we mentioned that to use the members and methods of a class we need to create an object! This is how we create an object in Java.

Classname obj = new Classname();

Probably you have seen this before. But if you don’t know what a Constructor is? You might not know the purpose of the parenthesis'()’ at the end of the Classname. Every Class in Java has a default constructor! The statement ‘Classname( )‘ executes the default constructor of a class.

public class ConstructorDemo < int var; ConstructorDemo() < /*this ia a default constructor in Java. Whether we write it or not, it is already defined!*/ >void show() < System.out.println("hello world! my variable- "+var); >> 

//Creating object of class ConstructorDemo public class Test < public static void main(String[] args) < ConstructorDemo obj = new ConstructorDemo(); obj.show(); >> 

OUTPUT

hello world! my variable- 0 

Use of Java Constructor

A java constructor is used to initialize the member variables of a class. In the above example, we used a variable named ‘var’ . But when the show method is called from the main method of the Test class the value of my variable is printed as 0 . This is because we did not initialize var in the ConstructorDemo class and 0 is the default value of an int in Java. To solve this we can use a parameterized constructor. Here’s how.

public class ConstructorDemo < int var; ConstructorDemo(int value) < var = value; >void show() < System.out.println("hello world! my variable- "+var); >> 

As soon as we make changes in the ConstructorDemo class the Test class will show an error! It will say that ‘ConstructorDemo( )’ is not defined! The fact is that when we create our own parameterized constructor, the default constructor is removed! So to solve this we can do 2 things.

Either mention a default non-parameterized constructor for ConstructorDemo class or avoid calling the constructor with no parameters! Definitely, the first way is more authenticate.

Test Class:

OUTPUT

hello world! my variable- 5

Constructor vs Methods

The purpose of a constructor is to initialize the object of a class while the purpose of a method is to perform a task by executing java code. Constructors cannot be abstract, final, static, and synchronized while methods can be. Constructors do not have return types while methods do.

Private Constructors

By declaring a private constructor in Java, we are restricting its object creation of that class. A class does not have a real-life existence but an object does. It might occur that we need a limited object of a class.

Let’s say we are about to play a game! And objects of Players are being created. But our game allows only one player at a time. In this scenario, a Singleton class will be a solution. But our game can also be played by 2/3/4 players! What then? Well, we control the number of Player objects created in this way:

Player Class

public class Player < private static int playercount; String name; int id; private Player(String name,int id) void showPlayerInfo() < System.out.println("player_"+id+": "+"name = "+name); >static Player getPlayer(String name, int id) < playercount++; if(playercount >1) return new Player(name,id); > > 

Caller Class

player_1: name = Toto Max player limit reached!

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