Code360 powered by Coding Ninjas X Naukri.com. Code360 powered by Coding Ninjas X Naukri.com
Table of contents
1.
Introduction
2.
What is a Default Constructor in Java? 
3.
Example
3.1.
Java
4.
Purpose of a Default Constructor in Java
4.1.
Java
5.
How Java Constructors are Different From Java Methods?
6.
Advantages and Disadvantages of Default Constructor in Java
7.
Difference between Default and Parameterized Constructors?
8.
Frequently Asked Questions
8.1.
How many default constructors can a class have in Java?
8.2.
Is the default constructor always present in Java?
8.3.
What will happen when we do not create a default constructor in Java?
8.4.
Why is it called a default constructor?
9.
Conclusion 
Last Updated: May 3, 2024
Easy

Default Constructor in Java

Roadmap to SDE career at Amazon
Speaker
Anubhav Sinha
SDE-2 @
25 Jun, 2024 @ 01:30 PM

Introduction

Constructors are one of the most vital tools in a programmer's toolbox when writing code. In this blog, we'll understand the behavior and limitations of the default constructor in Java, which is essential to avoid surprising bugs and errors.

default constructor in java

What is a Default Constructor in Java? 

Java automatically provides a default constructor when a class is defined without explicit constructors. The default constructor initializes the object with default values, which means it sets the variables to their default values, like 0 for integers and null for objects.

Get the tech career you deserve, faster!
Connect with our expert counsellors to understand how to hack your way to success
User rating 4.7/5
1:1 doubt support
95% placement record
Akash Pal
Senior Software Engineer
326% Hike After Job Bootcamp
Himanshu Gusain
Programmer Analyst
32 LPA After Job Bootcamp
After Job
Bootcamp

Example

Here is a simple example of a default constructor in Java.

Example
 

  • Java

Java

import java.util.*;
class Car
{
private String make;
private String model;
private int year;
public void setMake(String make)
{
this.make = make;
}
public void setModel(String model)
{
this.model = model;
}
public void setYear(int year)
{
this.year = year;
}
public String getMake()
{
return make;
}
public String getModel()
{
return model;
}
public int getYear()
{
return year;
}
}
public class Main
{
public static void main(String[] args)
{
Car myCar = new Car();
System.out.println("Make: " + myCar.getMake());
System.out.println("Model: " + myCar.getModel());
System.out.println("Year: " + myCar.getYear());
}
}


Output

Make: null
Model: null
Year: 0

In this example, we have a class called Car with three private variables, make, model, and year. The default constructor is defined with the same name as the class and no parameters. The constructor initializes the variables make, model, and year with default values of "null" and 0, respectively.

Purpose of a Default Constructor in Java

The purpose of a default constructor in object-oriented programming is multifaceted, primarily concerning object instantiation and ensuring that a class always has the means to create an object with sensible default values.

When a new instance of a class is created, constructors are the unique methods that are called to initialize the object. A default constructor is a constructor that takes no parameters. Here are the key roles it plays:

  • Initialization: It initializes every object with default values. This is crucial because, without a default constructor, uninitialized objects might contain garbage values, leading to unpredictable behavior.
  • Creation of Objects: In many programming languages, if the programmer defines no constructor, the compiler provides a default constructor implicitly, allowing the creation of objects even when no specific initialization is required.
  • Flexibility in Object Creation: Having a default constructor allows for creating an object without the need to supply arguments. This can be especially useful in scenarios where the initial state of the object is not dependent on external parameters.
  • Overloading Constructors: In classes where multiple constructors are provided to allow for various initialization scenarios, the default constructor maintains a pathway to create an object with no customization.
  • Inheritance: In inheritance, a default constructor ensures that a base class can be properly initialized when a derived class is instantiated, especially when the derived class does not explicitly call a parent constructor.

 

Let's illustrate with an example in Java, a language known for its object-oriented features:

  • Java

Java

public class Vehicle {
private int maxSpeed;
// Default constructor
public Vehicle() {
// Initialize with default value
this.maxSpeed = 0;
}
public int getMaxSpeed() {
return this.maxSpeed;
}
// Method to display information about the vehicle
public void displayInfo() {
System.out.println("The vehicle has a max speed of " + maxSpeed + " km/h.");
}
// Other methods and constructors can be added here.
}
public class Main {
public static void main(String[] args) {
// Create a Vehicle object using the default constructor
Vehicle myVehicle = new Vehicle();
// The maxSpeed is initialized to 0 by the default constructor
myVehicle.displayInfo();
}
}

In this example, the Vehicle class has a default constructor, which initializes the maxSpeed of a vehicle to 0. The absence of this default constructor would mean that an object of Vehicle could not be instantiated without explicitly defining the max speed.

How Java Constructors are Different From Java Methods?

Java constructors and methods serve distinct roles in the context of a class, and they have several key differences:

FeatureConstructorsMethods
DeclarationDefined using the same name as the class.Defined using a unique name within the class.
Return TypeNo return type specified, not even void.Must have a return type, or void if no return value.
InvokedAutomatically invoked upon object creation.Invoked explicitly using the method name.
PurposeUsed to initialize object state.Perform operations or computations.
OverloadingCan be overloaded by defining multiple constructors with different parameters.Can be overloaded within the same class.
InheritanceConstructors are not inherited.Methods are inherited by subclasses.
Default ExistenceA default constructor is provided by the compiler if no constructors are defined.No default method is provided.
Implicit Super CallInvokes superclass constructor implicitly if not explicitly called using "super()".Superclass methods must be called explicitly if overridden.

Advantages and Disadvantages of Default Constructor in Java

In the following discussion, we will explore the benefits and drawbacks of default constructors in Java.

Advantages

Disadvantages

Simplifies the code: Using a default constructor can simplify the code by reducing the need for additional constructors with default values.Limited initialization: Default constructors can only initialize the instance variables of a class with default values. A default constructor may be insufficient if a class requires more complex initialization logic, such as input validation.
Ensures object initialization: The default constructor ensures that every class object is initialized with default values, even if the programmer explicitly forgets to provide a constructor. It may not be applicable in all cases: Some classes may not be suitable for default constructors, especially if they have specific requirements for initialization. In such cases, explicit constructors with parameters may be necessary.
Easy to use: Since the default constructor doesn't require any parameters, it's easy to use and understand. Can lead to ambiguity: A class with both a default constructor and explicit constructors with parameters can lead to ambiguity and confusion when creating objects. 

Difference between Default and Parameterized Constructors?

AspectDefault ConstructorParameterized Constructor
Parameter ListHas no parametersIncludes parameters to pass specific values during creation.
Object Initialization Initializes objects with default valuesInitializes objects with given values for attributes
Compiler Generation Provided by the compiler if no constructor existsThe programmer must explicitly define it
Inheritance Behavior Implicitly called if a subclass has no constructorIt must be explicitly invoked in a subclass using ‘super(params)’
Constructor Overloading If present alone, no overloading is involvedIts presence alongside a default constructor signifies overloading
Explicit Definition Required Not required; created automatically if no other constructors are definedAlways required; the compiler never generates it

Frequently Asked Questions

How many default constructors can a class have in Java?

A class in Java can have only one default constructor. The default constructor is the no-argument constructor automatically provided by the compiler if no other constructors are explicitly defined by the programmer.

Is the default constructor always present in Java?

Yes, there is a default constructor present in Java if the programmer does not define its own constructor explicitly. Constructors are automatically invoked whenever an object is created, and the default constructor has the same name as the class.

What will happen when we do not create a default constructor in Java?

At the times when the programmer does not define its own constructor, then the compiler adds one constructor itself that initializes the data variables to their default values

Why is it called a default constructor?

It's called a default constructor because it's automatically provided by the compiler if no constructors are explicitly defined in a class. This default constructor initializes instance variables to their default values.

Conclusion 

Through this comprehensive discussion, we have thoroughly understood how default constructors in Java work and when they can be effectively used in Java programming. So, let's embrace the power of default constructors and use them wisely to build better Java programs!

Also, do refer to default constructor-related articles:


Moreover, apart from this blog, you can read more such articles on our platform, Coding Ninjas Studio. If you want to improve or practice coding questions visit our website Code360. For interview preparation, you can read Interview Experiences of popular companies. 

Happy Coding!

Live masterclass