Default Constructor in Java

Last Updated on September 22, 2023 by Mayank Dham

The constructor concept in Java is critical in object-oriented design because it allows for the creation and initialization of objects. The default constructor in Java is unique among these constructors. The default constructor in Java is a fundamental building block that Java provides by default if a class does not define any constructors explicitly. The default constructor, as the foundation of object instantiation, initializes objects with default values, ensuring that the newly created instance is ready for use. While it may appear simple, the default constructor is a critical starting point for more complex object-oriented designs, allowing classes to be instantiated without the need for explicit constructor definitions. In this look at the default constructor in Java, we will look at its mechanics, its role in object creation, and how it interacts with other constructors to create robust and efficient Java applications.

What is a Constructor in Java?

A constructor in Java is a section of code that is identical to the method. When a class instance is created, it is called. Memory for the object is allocated in the memory at the time constructor is called.

Constructor is a special type of method that is used to initialize an instance of a class in java.

Every time a constructor is created using new keyword, then at least one constructor is called.

If there is no constructor available in the class then the default constructor is called. By default, the Java compiler provides a default constructor.

Point to Remember: Because it constructs the values when an object is created, it is termed a constructor. The creation of a constructor for a class is not necessary. It’s because if your class doesn’t have a constructor, the Java compiler produces one.

Conditions for Creating Constructors in Java

There are three conditions defined for the constructor in Java:-

1. The name of the constructor must be the same as its class name.
2. A Constructor must have no return type.
3. A Java constructor cannot be –
   • Static
   • Abstract
   • Final
   • Synchronized

Types of Constructors in Java

1. No-arg constructor.
2. Parameterized constructor.
3. Default constructor.

No-arg Constructor in Java

The No-arg constructor is a constructor that accepts no arguments. If we don’t define a constructor in a class, the compiler automatically builds one for the class (with no arguments). Additionally, the compiler does not produce a default constructor if we write a constructor with arguments or without arguments.

Parameterized Constructor in Java

Parameterized constructors are constructors that take parameters. Use a parameterized constructor if you wish to provide your own values as the default values for the class’s fields.

Default Constructor in Java

When the program is executed, the Java compiler automatically constructs a no-arg constructor if we don’t create one ourselves. The default constructor is the name given to this constructor.

Here is an Example:

public class Person {
  String firstName;
  String lastName;
  int age;
}
public static void main(String args[]) {
    Person person1= new Person();

    person1.firstName = "Manoj";
    person1.lastName = "Kumar";
    person1.age = 21;
    System.out.println(person1.age);   
    System.out.println(person1.firstName);
}

Output:

21
Manoj

Did you notice? we did not define any constructor before creating person1 to initialize the attributes created in the class.
This will not throw an error. Rather, the compiler will create an empty constructor but we will not see this constructor anywhere in the code – this happens behind the scenes.
When the compiler begins to run the code, it will seem as follows:

public class Person {
  String firstName;
  String lastName;
  int age;

  // default constructor
  Person() { }

}
public static void main(String args[]) {
    Person person1= new Person();

     person1.firstName = "Manoj";
     person1.lastName = "Kumar";
     person1.age = 21;

     System.out.println(person1.age);   
     System.out.println(person1.firstName);
}

Many people mistakenly confuse the default constructor with the no-arg constructor, however in Java, they are two different constructors. In Java, a constructor that was developed specifically by the programmer is not regarded as a default constructor.

Purpose of Default Constructor in Java

In Java, a default constructor is a constructor that is automatically generated by the compiler if no other constructors are defined by a programmer in a class. Its purpose is to initialize the object’s attributes to their default values.

Java has a default constructor, which takes no arguments and has an empty body. The default constructor is automatically created by the compiler if no constructors have been defined by the user in the class.

For example, if a class has an attribute x with a default value of 0, the default constructor would initialize the value of x to 0 for any new object created from that class.

It’s also possible to define a constructor with parameters in the class, this constructor will be called when an object is created with the provided parameters.

In summary, the purpose of a default constructor in Java is to provide a basic initialization of an object’s attributes when it is created, so that the object can be used without having to manually set the attribute values.

The Key Differences between Constructors and Methods in Java

In Java, a constructor is a special type of method that is used to initialize an object when it is created. Constructors have the same name as the class, and they do not have a return type. They are automatically called when an object is created using the "new" keyword.

A method, on the other hand, is a block of code that performs a specific task. Methods have a return type, and they can be called explicitly by the programmer. Methods can be used to perform operations on the object’s data, and they can also be used to return a result or a value.

In short:

• Constructors are used to initialize an instance when it is created.
• Methods are used to perform operations on an object’s data and can return a result or value.
• Constructors have the same name as the class and do not have a return type.
• Methods have a return type and can be called explicitly by the programmer.

Conclusion
In this article, we discussed what are constructors in Java, and what are conditions to follow in order to create a constructor in java, we also look at the types of constructors in java. We also talked about default constructors in java and how constructors are different from methods in java.

Frequently Asked Questions (FAQs)

Here are some of the frequently asked questions about the default constructor in Java.

Q1. What is a default constructor in Java?
A default constructor in Java is a constructor that is automatically provided by the Java compiler when a class does not explicitly define any constructors. It initializes object instances with default values, ensuring they are in a usable state upon creation.

Q2. What are default values assigned by the default constructor?
The default constructor initializes instance variables with default values, which are 0 for numeric types (like int, float), false for boolean, and null for object references.

Q3. How to explicitly define a default constructor in Java class?
Yes, you can explicitly define a default constructor in your class. However, if no constructor is defined at all, Java automatically provides a default constructor. If any constructor (including parameterized constructors) is defined, the default constructor is not generated.

Q4. When should you use the default constructor in Java?
The default constructor is useful when you want to create instances of a class with default values for its instance variables. It’s also important when you have subclasses, as they implicitly call the default constructor of the superclass if no constructor is explicitly invoked.

Q5. Can you change the default constructor’s behavior?
The behavior of the default constructor, which initializes instance variables with default values, is fixed and cannot be directly customized. However, you can define your own parameterized constructors to tailor the initialization process according to your needs.