Constructors in Java: Definition, Types, and Best Practices

Introduction

When you create an object in Java, the object must be properly initialized before it can be used safely. An uninitialized object can lead to an inconsistent state, runtime errors, or invalid data. This is where constructors come in.

Constructors are responsible for preparing an object for use. They assign initial values, enforce validation rules, and guarantee that the object starts in a valid and predictable state.
Without constructors, object-oriented programming would be unsafe and unreliable.

A well-designed constructor ensures that every object begins its life correctly initialized.

1. What Is a Constructor?

A constructor in Java is a special member of a class used to initialize new objects.

More formally:

A constructor is a special method that has the same name as the class, has no return type, and is automatically executed when an object is instantiated using the new keyword.

A constructor:

• Has the same name as the class
• Has no return type (not even void)
• Executes automatically when an object is created

class User {
    String name;

    User(String name) {
        this.name = name;
    }
}

Usage:

User user = new User("John");

What Happens If You Don’t Call a Constructor?

In Java, you cannot create an object without calling a constructor.

User user; // Only declares a reference

At this point, user is just a reference variable — no object exists yet.

Trying to use it will result in a NullPointerException:

user.name = "John"; // NullPointerException

The new keyword:

1. Allocates memory
2. Calls the constructor
3. Returns the initialized object reference

Without calling a constructor, no usable object exists.

2. Default Constructor

If you don’t define any constructors, Java automatically provides a default constructor.

class Product {
    // No constructor defined
}

Java internally provides:

Product() {}

Important rules:

• The default constructor is provided only if no constructor is declared.
• If you define even one constructor, Java does not generate the default one.

Example:

class Product {
    Product(String name) {}
}

Product p = new Product(); // Compilation error

Frameworks like Spring, Hibernate, and Jackson often require a no-argument constructor to instantiate objects via reflection.

3. Parameterized Constructor

A parameterized constructor allows passing values at object creation time.

class Product {
    String name;
    double price;

    Product(String name, double price) {
        this.name = name;
        this.price = price;
    }
}

Parameterized constructors:

• Enforce required fields
• Prevent partially initialized objects
• Improve immutability design
• Increase object safety

Example with validation:

Product(String name, double price) {
    if (price < 0) {
        throw new IllegalArgumentException("Price cannot be negative");
    }
    this.name = name;
    this.price = price;
}

4. Constructor Overloading

Constructor overloading means defining multiple constructors with different parameter lists.

class Account {
    String owner;
    double balance;

    Account(String owner) {
        this(owner, 0);
    }

    Account(String owner, double balance) {
        this.owner = owner;
        this.balance = balance;
    }
}

Overloading allows:

• Flexible object creation
• Default values
• Cleaner API design

5. Using this()

You may use the keyword this to call another constructor in the same class.

Rules:

• Must be the first statement
• Avoids code duplication(DRY principle)
• Improves maintainability

Example:

Account(String owner) {
    this(owner, 0);
}

6. Using super()

The expression super() calls the parent class constructor. To learn more about class hierarchy, visit our article on inheritance in Java.

class Person {
    Person(String name) {
        System.out.println("Person: " + name);
    }
}

class Employee extends Person {
    Employee(String name) {
        super(name);
        System.out.println("Employee created");
    }
}

Important rules:

• super() must be the first statement.
• If not written, Java inserts a no-arg super() automatically (if available).
• If the parent has no no-arg constructor, you must call the correct one explicitly.

7. Private Constructors

A constructor can be declared private.

Utility class:

class Utils {
    private Utils() {}
}

Singleton pattern:

class DatabaseConnection {
    private static final DatabaseConnection INSTANCE = new DatabaseConnection();

    private DatabaseConnection() {}

    public static DatabaseConnection getInstance() {
        return INSTANCE;
    }
}

Private constructors control how objects are created.

8. Best Practices

✔ Initialize required fields
✔ Validate constructor parameters
✔ Keep constructors simple
✔ Avoid heavy operations (I/O, database calls)
✔ Use constructor overloading wisely
✔ Prefer immutability when possible

A constructor should establish invariants and ensure the object starts in a valid state.

Conclusion

Constructors define how objects are created and initialized.

They ensure:

• Objects are never in an invalid state
• Required data is provided at creation time
• Inheritance hierarchies are properly initialized

Understanding default constructors, parameterized constructors, overloading, this(), super(), and access control will help you design safer and more maintainable object-oriented systems.

A constructor defines the starting life of every object. Design it carefully.

You can find the complete code of this article here on GitHub.