Java Constructors

In this tutorial, we will learn about Java constructors, their types, and how to use them with the help of examples.

What is a Constructor?

A constructor in Java is similar to a method that is invoked when an object of the class is created.

Unlike Java methods, a constructor has the same name as that of the class and does not have any return type. For example,

class Test {
  Test() {
    // constructor body
  }
}

Here, Test() is a constructor. It has the same name as that of the class and doesn't have a return type.

Recommended Reading: Why do constructors not return values

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Example 1: Java Constructor

class Main {
  private String name;

  // constructor
  Main() {
    System.out.println("Constructor Called:");
    name = "Programiz";
  }

  public static void main(String[] args) {

    // constructor is invoked while
    // creating an object of the Main class
    Main obj = new Main();
    System.out.println("The name is " + obj.name);
  }
}

Run Code

Output:

Constructor Called:
The name is Programiz

In the above example, we have created a constructor named Main(). Inside the constructor, we are initializing the value of the name variable.

Notice the statement of creating an object of the Main class.

Main obj = new Main();

Here, when the object is created, the Main() constructor is called. And, the value of the name variable is initialized.

Hence, the program prints the value of the name variables as Programiz.

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Types of Constructor

In Java, constructors can be divided into 3 types:

1. No-Arg Constructor
2. Parameterized Constructor
3. Default Constructor

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1. Java No-Arg Constructors

Similar to methods, a Java constructor may or may not have any parameters (arguments).

If a constructor does not accept any parameters, it is known as a no-argument constructor. For example,

private Constructor() {
   // body of the constructor
}

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Example 2: Java private no-arg constructor

class Main {

  int i;

  // constructor with no parameter
  private Main() {
    i = 5;
    System.out.println("Constructor is called");
  }

  public static void main(String[] args) {

    // calling the constructor without any parameter
    Main obj = new Main();
    System.out.println("Value of i: " + obj.i);
  }
}

Run Code

Output:

Constructor is called
Value of i: 5

In the above example, we have created a constructor Main(). Here, the constructor does not accept any parameters. Hence, it is known as a no-arg constructor.

Notice that we have declared the constructor as private.

Once a constructor is declared private, it cannot be accessed from outside the class. So, creating objects from outside the class is prohibited using the private constructor.

Here, we are creating the object inside the same class. Hence, the program is able to access the constructor. To learn more, visit Java Implement Private Constructor.

However, if we want to create objects outside the class, then we need to declare the constructor as public.

Example 3: Java public no-arg constructors

class Company {
  String name;

  // public constructor
  public Company() {
    name = "Programiz";
  }
}

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

    // object is created in another class
    Company obj = new Company();
    System.out.println("Company name = " + obj.name);
  }
}

Run Code

Output:

Company name = Programiz

2. Java Parameterized Constructor

A Java constructor can also accept one or more parameters. Such constructors are known as parameterized constructors (constructor with parameters).

Example 4: Parameterized constructor

class Main {

  String languages;

  // constructor accepting single value
  Main(String lang) {
    languages = lang;
    System.out.println(languages + " Programming Language");
  }

  public static void main(String[] args) {

    // call constructor by passing a single value
    Main obj1 = new Main("Java");
    Main obj2 = new Main("Python");
    Main obj3 = new Main("C");
  }
}

Run Code

Output:

Java Programming Language
Python Programming Language
C Programming Language

In the above example, we have created a constructor named Main(). Here, the constructor takes a single parameter. Notice the expression,

Main obj1 = new Main("Java");

Here, we are passing the single value to the constructor. Based on the argument passed, the language variable is initialized inside the constructor.

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3. Java Default Constructor

If we do not create any constructor, the Java compiler automatically create a no-arg constructor during the execution of the program. This constructor is called default constructor.

Example 5: Default Constructor

class Main {

  int a;
  boolean b;

  public static void main(String[] args) {

    // A default constructor is called
    Main obj = new Main();

    System.out.println("Default Value:");
    System.out.println("a = " + obj.a);
    System.out.println("b = " + obj.b);
  }
}

Run Code

Output:

Default Value:
a = 0
b = false

Here, we haven't created any constructors. Hence, the Java compiler automatically creates the default constructor.

The default constructor initializes any uninitialized instance variables with default values.

Type	Default Value
boolean	false
byte	0
short	0
int	0
long	0L
char	\u0000
float	0.0f
double	0.0d
object	Reference null

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