Java Constructors: Types, Overloading & Destructors

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Last Updated on: 25th Oct 2025 13:42:18 PM

Welcome to the complete beginner-friendly tutorial on Constructors in Java — one of the most important concepts in Object-Oriented Programming (OOP)!

 

What is a Constructor?

A constructor is a special method in a class that is automatically called when an object is created using the new keyword.

 

Purpose:

• Initialize object properties (fields)

• Set default or custom values

• Allocate resources

 

Key Features of Constructors

Feature	Description
Name	Same as the class name
Return Type	None (not even void)
Called	Automatically with new
Can be overloaded	Yes
Can call other constructors	Using this()
Cannot be static, final, or abstract	Belongs to object, not class

 

Types of Constructors in Java

 

There are three main types of constructors in Java:

1. Default Constructor

   • Created automatically by the compiler if no constructor is defined.

   • Initializes object with default values (0, null, false, etc.)..

2. No-Argument Constructor

   • Created by the programmer explicitly.

   • Used to perform custom initialization or display a message.

3. Parameterized Constructor

   • Takes parameters to initialize object with specific values.

4. Copy Constructor (user-defined)

   • Used to copy data from one object to another.

   • Not built-in like in C++, but can be implemented manually.

 

Real-Life Example: Student Enrollment System

Think of a university registration desk:

• Some students register with basic info → Default Constructor

• Some with full details → Parameterized Constructor

• Some are transfers (copied from another record) → Copy Constructor

 

1. Default Constructor

class Student {
    String name;
    int age;
    String course;

    // Default Constructor
    Student() {
        name = "Unknown";
        age = 18;
        course = "General";
        System.out.println("Default student created.");
    }

    void display() {
        System.out.println("Name: " + name + ", Age: " + age + ", Course: " + course);
    }
}

 

Usage:

Student s1 = new Student();  // Default constructor called
s1.display();

 

Output:

Default student created.
Name: Unknown, Age: 18, Course: General

 

2. Parameterized Constructor

class Student {
    String name;
    int age;
    String course;

    // Parameterized Constructor
    Student(String name, int age, String course) {
        this.name = name;
        this.age = age;
        this.course = course;
        System.out.println("Custom student enrolled.");
    }

    void display() {
        System.out.println("Name: " + name + ", Age: " + age + ", Course: " + course);
    }
}

 

Usage:

Student s2 = new Student("Alice", 20, "Computer Science");
s2.display();

 

Output:

Custom student enrolled.
Name: Alice, Age: 20, Course: Computer Science

 

3. Copy Constructor

class Student {
    String name;
    int age;
    String course;

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

    // Copy Constructor
    Student(Student other) {
        this.name = other.name;
        this.age = other.age;
        this.course = other.course;
        System.out.println("Student record copied.");
    }

    void display() {
        System.out.println("Name: " + name + ", Age: " + age + ", Course: " + course);
    }
}

 

Usage:

Student s1 = new Student("Bob", 22, "Mathematics");
Student s3 = new Student(s1);  // Copy constructor
s3.display();

 

Output:

Student record copied.
Name: Bob, Age: 22, Course: Mathematics

 

Constructor Overloading

Constructor Overloading = Having multiple constructors in the same class with different parameter lists.

 

Real-Life Example: Pizza Order System

Order Type	Constructor Used
Just size	Pizza("Large")
Size + toppings	Pizza("Medium", "Pepperoni")
Full custom	Pizza("Small", "Veggie", "Thin")

 

Example: Constructor Overloading

class Pizza {
    String size;
    String toppings;
    String crust;
    double price;

    // Constructor 1: Size only
    Pizza(String size) {
        this.size = size;
        this.toppings = "Cheese";
        this.crust = "Regular";
        this.price = getBasePrice();
        System.out.println("Basic pizza: " + size);
    }

    // Constructor 2: Size + Toppings
    Pizza(String size, String toppings) {
        this.size = size;
        this.toppings = toppings;
        this.crust = "Regular";
        this.price = getBasePrice() + 2.0;
        System.out.println("Pizza with " + toppings);
    }

    // Constructor 3: Full custom
    Pizza(String size, String toppings, String crust) {
        this.size = size;
        this.toppings = toppings;
        this.crust = crust;
        this.price = getBasePrice() + 2.0 + 1.5;
        System.out.println("Fully custom pizza!");
    }

    double getBasePrice() {
        return switch (size) {
            case "Small" -> 8.99;
            case "Medium" -> 10.99;
            case "Large" -> 12.99;
            default -> 10.99;
        };
    }

    void display() {
        System.out.printf("Pizza: %s, %s, %s crust, $%.2f%n", 
                         size, toppings, crust, price);
    }
}

 

Main Class:

public class PizzaShop {
    public static void main(String[] args) {
        Pizza p1 = new Pizza("Large");                    // Constructor 1
        Pizza p2 = new Pizza("Medium", "Pepperoni");      // Constructor 2
        Pizza p3 = new Pizza("Small", "Veggie", "Thin");  // Constructor 3

        p1.display();
        p2.display();
        p3.display();
    }
}

 

Output:

Basic pizza: Large
Pizza with Pepperoni
Fully custom pizza!
Pizza: Large, Cheese, Regular crust, $12.99
Pizza: Medium, Pepperoni, Regular crust, $12.99
Pizza: Small, Veggie, Thin crust, $12.49

 

this() – Constructor Chaining

Call one constructor from another to avoid code duplication.

class Employee {
    String name;
    int id;
    double salary;

    // Full constructor
    Employee(String name, int id, double salary) {
        this.name = name;
        this.id = id;
        this.salary = salary;
    }

    // Overloaded: name and id only
    Employee(String name, int id) {
        this(name, id, 50000.0);  // Reuse full constructor
        System.out.println("Default salary applied.");
    }
}

 

Destructors in Java? → NO!

Language	Destructor?	Memory Management
C++	Yes (~ClassName())	Manual
Java	No	Garbage Collector (GC)

 

Java Uses Garbage Collection

• Automatically frees memory when object is no longer referenced

• You cannot predict when GC runs

• No need for destructors

 

finalize() Method (Deprecated!)

@Override
protected void finalize() throws Throwable {
    System.out.println("Object is being cleaned up!");
}

 

 

Full Example: All Types + Overloading

class BankAccount {
    String holder;
    long accountNumber;
    double balance;
    String type;

    // 1. Default Constructor
    BankAccount() {
        holder = "Unknown";
        accountNumber = 0;
        balance = 0.0;
        type = "Savings";
        System.out.println("Default account created.");
    }

    // 2. Parameterized: Holder + Balance
    BankAccount(String holder, double balance) {
        this.holder = holder;
        this.accountNumber = System.currentTimeMillis();
        this.balance = balance;
        this.type = "Savings";
        System.out.println("Account for " + holder + " created.");
    }

    // 3. Full Parameterized
    BankAccount(String holder, long accountNumber, double balance, String type) {
        this.holder = holder;
        this.accountNumber = accountNumber;
        this.balance = balance;
        this.type = type;
        System.out.println("Premium account created.");
    }

    // 4. Copy Constructor
    BankAccount(BankAccount other) {
        this.holder = other.holder;
        this.accountNumber = other.accountNumber + 1000;
        this.balance = other.balance;
        this.type = other.type;
        System.out.println("Account duplicated for branch transfer.");
    }

    void display() {
        System.out.printf("Holder: %s, Acc: %d, Balance: $%.2f, Type: %s%n",
                         holder, accountNumber, balance, type);
    }
}

 

Main Class:

public class Bank {
    public static void main(String[] args) {
        BankAccount a1 = new BankAccount();                           // Default
        BankAccount a2 = new BankAccount("Alice", 5000.0);            // Parameterized
        BankAccount a3 = new BankAccount("Bob", 123456, 10000, "Current"); // Full
        BankAccount a4 = new BankAccount(a2);                         // Copy

        a1.display();
        a2.display();
        a3.display();
        a4.display();
    }
}

 

Sample Output:

Default account created.
Account for Alice created.
Premium account created.
Account duplicated for branch transfer.
Holder: Unknown, Acc: 0, Balance: $0.00, Type: Savings
Holder: Alice, Acc: 1712345678901, Balance: $5000.00, Type: Savings
Holder: Bob, Acc: 123456, Balance: $10000.00, Type: Current
Holder: Alice, Acc: 1712345679901, Balance: $5000.00, Type: Savings

 

Summary Table

Constructor Type	Parameters	Use Case
Default	None	Basic object with defaults
Parameterized	1+ args	Custom initialization
Copy	Object	Duplicate object
Overloading	Multiple	Flexible object creation

Feature	Java
Destructor	Not supported
Memory Cleanup	Garbage Collector
finalize()	Deprecated

 

Common Interview Questions

Question	Answer
Can constructor return a value?	No
Can we overload constructors?	Yes
What if no constructor is written?	Java adds default constructor
Can constructor be private?	Yes (for Singleton pattern)
Can we call constructor explicitly?	Only via this()

 

Practice Exercise

Create a MobilePhone class with:

1. Default constructor

2. Parameterized (brand, model, price)

3. Copy constructor

4. Overloaded constructor (brand, model)

5. display() method

Test all 4 constructors.

 

Final Words

You’ve mastered:

• All types of constructors

• Constructor overloading

• Copy constructor

• Why no destructors in Java

• Real-world examples (Student, Pizza, Bank)

 

Constructors = Object Initialization Engine
Garbage Collector = Automatic Cleanup Crew

 

Keep coding! 

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