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Understanding Constructors in Java

 

What is a Constructor?

In Java, a constructor is a special block of code similar to a method. It is called when an instance of the class is created. The primary purpose of a constructor is to initialize the newly created object.

When a constructor is invoked, memory for the object is allocated in the memory. Every time an object is created using the new() keyword, at least one constructor is called. Java provides a default constructor if no constructor is explicitly defined in the class.

Characteristics of a Constructor

  • Initialization: A constructor initializes the object.
  • Invocation: Called automatically when a new object is created.
  • Default Constructor: If no constructor is defined, Java provides a default constructor.

Rules for Creating a Java Constructor

  1. Constructor Name: Must be the same as the class name.
  2. Return Type: Must not have an explicit return type.
  3. Modifiers: Cannot be abstract, static, final, or synchronized.

Note: Access modifiers can be used to control object creation, allowing for private, protected, public, or default constructors in Java.

Example: Basic Constructor

public class Constructor {
    String name;
    int age;

    Constructor(String name, int age) {
        this.name = name;
        this.age = age;
    }

    void displayInfo() {
        System.out.println(name + "\n" + age);
    }

    public static void main(String[] args) {
        Constructor p1 = new Constructor("Lalit", 20);
        p1.displayInfo();
    }
}

In the example above, the Constructor class has a constructor that initializes the name and age fields. When a new object p1 is created using new Constructor("Lalit", 20);, the constructor initializes the fields, and the displayInfo method prints them.

Overloading Constructors

Constructor overloading allows a class to have more than one constructor with different parameter lists. It provides multiple ways to initialize objects.

Example: Overloaded Constructors

public class Constructor {
    String name;
    int age;

    // Default constructor
    Constructor() {
        this.name = "Unknown";
        this.age = 0;
    }

    // Parameterized constructor
    Constructor(String name, int age) {
        this.name = name;
        this.age = age;
    }

    void displayInfo() {
        System.out.println(name + "\n" + age);
    }

    public static void main(String[] args) {
        Constructor p1 = new Constructor();
        Constructor p2 = new Constructor("Lalit", 20);
        p1.displayInfo();
        p2.displayInfo();
    }
}

In this example, there are two constructors:

  • Default Constructor: Initializes name to "Unknown" and age to 0.
  • Parameterized Constructor: Initializes name and age with the provided values.

Both objects p1 and p2 are created using different constructors, demonstrating constructor overloading.

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