How do I call one constructor from another in Java?

In Java, constructors are special methods used to initialize new objects. Sometimes, you might want multiple constructors that provide different ways to create an instance of a class. Instead of repeating initialization logic, you can call one constructor directly from another. This technique, known as constructor chaining, promotes cleaner, more maintainable code.

In this detailed guide, we’ll explore how to use this() and super() for constructor chaining, the rules you must follow, and practical examples for applying this technique. We’ll also recommend top-notch courses and resources to help you refine your Java design, patterns, and system design skills.

What is Constructor Chaining?
Using this() to Call Another Constructor in the Same Class
Using super() to Call a Constructor in the Superclass
Rules and Restrictions
Practical Examples
Best Practices for Constructor Chaining
Recommended Courses to Enhance Your Java Skills
Additional Resources for Interview Preparation
Conclusion

1. What is Constructor Chaining?

Constructor chaining means calling one constructor from another within the same class or from a subclass to its superclass. It ensures you don’t repeat common initialization code in every constructor. By centralizing initialization logic, you make your code more concise, maintainable, and easier to debug.

2. Using `this()` to Call Another Constructor in the Same Class

When you have multiple constructors, you can use this() to call a constructor defined in the same class:

Example:

public class Example {
    private int x;
    private int y;

    public Example() {
        this(0, 0); // Calls the constructor with two arguments
    }

    public Example(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Key Points:

this(...) must be the first statement in the constructor.
By using this(), you avoid duplicating logic, ensuring that all initialization flows through a single path.

3. Using `super()` to Call a Constructor in the Superclass

If you’re working with inheritance, super() allows you to call a constructor from the superclass:

Example:

public class Parent {
    public Parent(int value) {
        System.out.println("Parent initialized with value: " + value);
    }
}

public class Child extends Parent {
    public Child() {
        super(10); // Calls Parent(int value)
    }
}

Key Points:

super(...) must be the first statement if you use it in a constructor.
If you don’t explicitly call super(...), Java implicitly calls super() with no arguments (assuming a no-arg constructor exists in the superclass).
Ensures that the superclass’s state is properly initialized before the subclass adds its own logic.

4. Rules and Restrictions

Must Be the First Statement:
Both this() and super() calls must appear as the first statement in the constructor. You cannot call this() or super() after any other statement.
Only One Is Allowed:
You can’t call both this() and super() in the same constructor, because only one can be the first statement.
Preventing Recursive Loops:
Avoid creating recursive constructor calls. For example, if constructor A calls B, and B calls A again, you’ll end up in an infinite loop, causing a stack overflow.

5. Practical Examples

Chaining Constructors in the Same Class:

public class Rectangle {
    private int width;
    private int height;

    public Rectangle() {
        this(10); // calls Rectangle(int width)
    }

    public Rectangle(int width) {
        this(width, 20); // calls Rectangle(int width, int height)
    }

    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }
}

Chaining Constructors in Inheritance:

public class Vehicle {
    private String type;

    public Vehicle(String type) {
        this.type = type;
    }
}

public class Car extends Vehicle {
    public Car() {
        super("Car"); // Initialize Vehicle part of Car
    }
}

6. Best Practices for Constructor Chaining

Centralize Common Logic:
Put all complex initialization in the most specific constructor. Then, let other constructors call that one with appropriate defaults.
Maintain Readability:
Don’t overuse constructor chaining if it makes the code confusing. Keep it simple and intuitive.
Consider Factory Methods:
Sometimes, using static factory methods instead of multiple constructors can clarify initialization logic and make code more expressive.

7. Recommended Courses to Enhance Your Java Skills

Mastering constructor chaining is one piece of the puzzle. To become a top-tier Java developer, you must also understand design principles, patterns, and large-scale system design concepts.

Recommended Courses from DesignGurus.io:

Grokking SOLID Design Principles
Learn how to structure classes and methods following SOLID principles for cleaner, maintainable code.
Grokking Design Patterns for Engineers and Managers
Understanding common design patterns helps you choose the right approach for initialization logic and object creation.

For broad interview readiness and system design mastery:

8. Additional Resources for Interview Preparation

Blogs by DesignGurus.io:

YouTube Channel: Check out the DesignGurus YouTube Channel for system design tutorials and interview insights.

Mock Interviews and Services:

Get personalized feedback from ex-FAANG engineers and sharpen your coding, design, and communication skills.

9. Conclusion

Constructor chaining using this() and super() is a valuable technique for eliminating code duplication and improving maintainability in Java classes. By consolidating initialization logic and clearly defining your class hierarchies, you create code that’s both cleaner and more consistent.

Apply these concepts alongside strong design principles, patterns, and system design fundamentals to write robust, scalable Java applications. With practice and a deeper understanding of these core concepts, you’ll be ready to handle any object creation challenge.

Streamline your object initialization: harness constructor chaining, follow best practices, and build cleaner, more maintainable Java code.