Constructors in Java are a type of Java method used for initializing objects of a class. They are an important building block to create an object and assign its value. Objects may result in unpredictable behaviour or issues when not using the proper constructor. But the question is what is the correct way to use them?
This blog is your way to understanding Java constructors through its meaning, types and other aspects. Let’s see what it takes to write an efficient Java program using them.
Constructors in Java are special methods used to initialize newly created objects. They are invoked automatically when an object is instantiated using the new keyword. The primary purpose of a constructor is to set up the initial state of an object by initializing its attributes to default or specified values. Here are some characteristics of them -
The next thing you should understand is the working of Java constructors. Take a look at the example given below. It will help you understand how constructors are naturally called and when to build objects in Java.
// Java Program to demonstrate
// Constructor usage
// Class Definition
class IgmGuru {
// Constructor
IgmGuru() {
super();
System.out.println("Constructor Called");
}
// Main method
public static void main(String[] args) {
// Creating an object of IgmGuru
IgmGuru obj = new IgmGuru();
}
}
|
Constructor Called |
There are various types of constructors we can use in Java programming. So, the next question is how to choose the right one. Let’s explore them with examples to understand their perfect use:
A constructor without parameters is called a default constructor. The compiler naturally provides a default constructor if no constructors are explicitly defined within a class. This default constructor is implicit and initializes the object with default values. For example, false for booleans, null for reference types and 0 for numeric types. It can either be explicit or implicit -
// Java Program to demonstrate
// Default Constructor
// Driver class
class IgmGuru {
// Default Constructor
IgmGuru() {
System.out.println("Default constructor");
}
// Main method
public static void main(String[] args) {
// Creating an object of IgmGuru
IgmGuru hello = new IgmGuru();
}
}
|
Default constructor |
A constructor with parameters is called a parameterized constructor. Use these when you want to set up the class’s fields with your own starting values. Here is an example -
// Java Program for Parameterized Constructor
public class IgmGuru {
// Data members of the class
String name;
int id;
// Parameterized Constructor
IgmGuru(String name, int id) {
this.name = name;
this.id = id;
}
public static void main(String[] args) {
// Invoke the parameterized constructor
IgmGuru user1 = new IgmGuru("Sweta", 68);
System.out.println("UserName: " + user1.name
+ " and UserId: " + user1.id);
}
}
|
UserName: Sweta and UserId: 68 |
Copy constructors are different as they take another object as input instead of the usual way of making new objects. They copy the data from that object into the new one you are making. Here is an example -
// Java Program for Copy Constructor
public class IgmGuru {
// Data members of the class
String name;
int id;
// Parameterized Constructor
IgmGuru(String name, int id) {
this.name = name;
this.id = id;
}
// Copy Constructor
IgmGuru(IgmGuru obj2) {
this.name = obj2.name;
this.id = obj2.id;
}
public static void main(String[] args) {
// This would invoke the parameterized constructor
System.out.println("First Object");
IgmGuru user1 = new IgmGuru("Sweta", 68);
System.out.println("UserName: " + user1.name
+ " and UserId: " + user1.id);
System.out.println();
// This would invoke the copy constructor
IgmGuru user2 = new IgmGuru(user1);
System.out.println("Copy Constructor used for Second Object");
System.out.println("UserName: " + user2.name
+ " and UserId: " + user2.id);
}
}
|
First Object UserName: Sweta and UserId: 68 Copy Constructor used for Second Object UserName: Sweta and UserId: 68 |
A private constructor is a constructor that cannot be accessed from outside the class. It is mainly used when you want to restrict object creation and control how instances of a class are created. Private constructors are commonly used in design patterns like Singleton and in utility classes that should not be instantiated.
// Java Program for Private Constructor
class IgmGuru {
// Private Constructor
private IgmGuru() {
System.out.println("Private Constructor Called");
}
public static void main(String[] args) {
// Object can be created inside the same class
IgmGuru obj = new IgmGuru();
}
}
|
Private Constructor Called |
Constructor chaining is a process where one constructor calls another constructor within the same class or from the parent class. It helps reduce code duplication and ensures that object initialization is performed in a structured manner. In Java, constructor chaining can be achieved using the this() keyword for the current class and super() for the parent class.
// Java Program for Constructor Chaining
class IgmGuru {
IgmGuru() {
this("Java");
System.out.println("Default Constructor");
}
IgmGuru(String course) {
System.out.println("Course: " + course);
}
public static void main(String[] args) {
IgmGuru obj = new IgmGuru();
}
}
|
Course: Java Default Constructor |
Some learners often get confused between constructors and methods. Both have some similarities and some significant differences. Let’s understand how they differ with given table:
| Features | Constructor | Method |
| Name | A constructor must share the exact name as the class name | A method can carry any name as long as it is valid |
| Return Type | It does not include any return type | It must declare a return type or use void when no value is returned |
| Invocation | Constructors are invoked naturally whenever the new keyword is used to create an object | They must be called explicitly |
| Purpose | Their main task is to initialize objects | They are used to perform operations |
When you deep dive into Java programming, a new concept is introduced, known as Overloading. Constructor overloading is a main concept in object-oriented programming. It allows you to create multiple constructors within the same class, each with its own set of parameters.
Here is an example of how constructor overloading works: you can have several constructors that all initialize an object, but each one takes different arguments.
// Java Program to illustrate constructor overloading
// using same task (addition operation) for different
// types of arguments
public class IgmGuru {
// Constructor with one argument
IgmGuru(String name) {
System.out.println("Constructor with one argument - String: " + name);
}
// Constructor with two arguments
IgmGuru(String name, int age) {
System.out.println("Constructor with two arguments - String and Integer: "
+ name + " " + age);
}
// Constructor with one argument but with different type
IgmGuru(long id) {
System.out.println("Constructor with one argument - Long: " + id);
}
public static void main(String[] args) {
// Creating the objects of the class 'IgmGuru' by passing different arguments
// Invoke the constructor with one argument of type 'String'
IgmGuru user1 = new IgmGuru("Sweta");
// Invoke the constructor with two arguments (String, int)
IgmGuru user2 = new IgmGuru("Amiya", 28);
// Invoke the constructor with one argument of type 'Long'
IgmGuru user3 = new IgmGuru(325614567L); // use L for long literal
}
}
|
Constructor with one argument - String: Sweta Constructor with two arguments - String and Integer: Amiya 28 Constructor with one argument - Long: 325614567 |
It is safe to conclude that constructors are important for object initialization in Java. It makes sure that every object starts its life in a valid and consistent state. Understanding conductors is important for writing clean and maintainable code. I have created this guide to help you lay a solid foundation for building strong Java applications.
Read Related Articles:
Constructors can be overloaded which means you can have multiple constructors with the same name but different parameter lists.
Java would provide a default constructor that initializes object attributes to default values if you do not define it.
Constructors cannot have a return type as it would make a regular method instead of a constructor.
Constructors are used to give initial values to an object’s variables. They make sure the object starts in a proper and usable state.
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