Default Constructor in Java

 

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In Java, a default constructor is a constructor that is automatically generated by the compiler if no constructors are explicitly defined in a class. It is a no-argument constructor that initializes the object with default values.

What Does the Default Constructor Do?

1. Initialization: The default constructor initializes the object with default values. For primitive data types, these default values are:

   • int, short, byte, long: 0
   • float, double: 0.0
   • char: '\u0000' (null character)
   • boolean: false
   • For reference types (like objects), the default value is null.

2. Implicit Creation: If you do not define any constructor in your class, the Java compiler automatically provides a default constructor.

Example

Let’s look at an example to understand this better:

Java

class Student {
    String name;
    int age;

    // No explicit constructor defined
}

public class Main {
    public static void main(String[] args) {
        Student student = new Student(); // Default constructor is called
        System.out.println("Name: " + student.name); // Output: Name: null
        System.out.println("Age: " + student.age);   // Output: Age: 0
    }
}

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Explanation:

• Class Definition: The Student class does not have any explicitly defined constructors.
• Object Creation: When we create an object of the Student class using new Student(), the default constructor is called.
• Default Values: The name attribute is a String, so its default value is null. The age attribute is an int, so its default value is 0.