Java Constructors

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

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);
  }
}

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.

Types of Constructor

In Java, constructors can be divided into 3 types:

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

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
}

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);
  }
}

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);
  }
}

Output:

Company name = Programiz

Recommended Reading: Java Access Modifier

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");
  }
}

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.

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);
  }
}

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.

In the above program, the variables a and b are initialized with default value 0 and false respectively.

The above program is equivalent to:

class Main {

  int a;
  boolean b;

   Main() {
    a = 0;
    b = false;
  }

  public static void main(String[] args) {
    // call the constructor
    Main obj = new Main();

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

The output of the program is the same as Example 5.

Important Notes on Java Constructors

• Constructors are invoked implicitly when you instantiate objects.
• The two rules for creating a constructor are:
  The name of the constructor should be the same as the class.
  A Java constructor must not have a return type.
• If a class doesn't have a constructor, the Java compiler automatically creates a default constructor during run-time. The default constructor initializes instance variables with default values. For example, the int variable will be initialized to 0
• Constructor types:
  No-Arg Constructor - a constructor that does not accept any arguments
  Parameterized constructor - a constructor that accepts arguments
  Default Constructor - a constructor that is automatically created by the Java compiler if it is not explicitly defined.
• A constructor cannot be abstract or static or final.
• A constructor can be overloaded but can not be overridden.

Constructors Overloading in Java

Similar to Java method overloading, we can also create two or more constructors with different parameters. This is called constructors overloading.

Example 6: Java Constructor Overloading

class Main {

  String language;

  // constructor with no parameter
  Main() {
    this.language = "Java";
  }

  // constructor with a single parameter
  Main(String language) {
    this.language = language;
  }

  public void getName() {
    System.out.println("Programming Langauage: " + this.language);
  }

  public static void main(String[] args) {

    // call constructor with no parameter
    Main obj1 = new Main();

    // call constructor with a single parameter
    Main obj2 = new Main("Python");

    obj1.getName();
    obj2.getName();
  }
}

Output:

Programming Language: Java
Programming Language: Python

In the above example, we have two constructors: Main() and Main(String language). Here, both the constructor initialize the value of the variable language with different values.

Based on the parameter passed during object creation, different constructors are called and different values are assigned.

It is also possible to call one constructor from another constructor. To learn more, visit Java Call One Constructor from Another.

Note: We have used this keyword to specify the variable of the class. To know more about this keyword, visit Java this keyword.