🚀 Java Constructors – Masterclass with Examples

constructors:

• ✅ Basic explanation
• 🤔 Why we use constructors
• 📌 Types of constructors
• 💭 Real-life examples
• 🧠 Most asked constructor-based interview questions (with answers)
• 🧪 Puzzles and tricky constructor questions for practice

🚀 Java Constructors – Masterclass with Examples

✅ What is a Constructor?

A constructor is a special method in a class:

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

🤔 Why Do We Use Constructors?

• To initialize objects
• To assign default or custom values
• To run specific logic when an object is created

📌 Types of Constructors in Java

💡 Example:

public class Student {
    String name;
    int age;

    // Constructor
    Student(String n, int a) {
        name = n;
        age = a;
    }

    void display() {
        System.out.println(name + " - " + age);
    }

    public static void main(String[] args) {
        Student s1 = new Student("Arun", 21);
        s1.display();
    }
}

🧠 Top Java Constructor Interview Questions with Answers

Q1: Can a constructor be overloaded?

✅ Yes! Just like methods, you can create multiple constructors with different parameter lists.

public class Person {
    Person() { System.out.println("No-arg"); }
    Person(String name) { System.out.println("Name: " + name); }
}

Q2: What happens if you don't write a constructor?

✅ Java provides a default constructor (no-args) automatically.

But if you write any constructor, Java won’t generate the default one.

Q3: Can constructors be private?

✅ Yes. Used in Singleton Design Pattern to restrict object creation.

private ConstructorExample() {
    // Cannot create from outside
}

Q4: Is it possible to call a constructor from another constructor?

✅ Yes, using this() keyword.

Student() {
    this("Default Name");
}

Student(String name) {
    System.out.println(name);
}

Q5: Can constructors be inherited?

❌ No. Constructors are not inherited, but the subclass can call the superclass constructor using super().

🔍 Tricky Constructor Puzzle Questions

🔸 Puzzle 1:

class Test {
    Test() {
        System.out.println("Constructor");
    }

    void Test() {
        System.out.println("This is a method");
    }

    public static void main(String[] args) {
        Test t = new Test();
        t.Test();
    }
}

✅ Output:

Constructor
This is a method

📌 Why? Because method name can be same as class name, but it still acts like a method due to return type.

🔸 Puzzle 2:

class Demo {
    Demo() {
        this(10);
        System.out.println("Default");
    }

    Demo(int x) {
        System.out.println("Parameterized: " + x);
    }

    public static void main(String[] args) {
        new Demo();
    }
}

✅ Output:

Parameterized: 10
Default

🔸 Puzzle 3:

class A {
    A() {
        System.out.println("A constructor");
    }
}

class B extends A {
    B() {
        System.out.println("B constructor");
    }
}

public class Main {
    public static void main(String[] args) {
        new B();
    }
}

✅ Output:

A constructor  
B constructor

📌 Reason: Super constructor is always called first.

✨ Summary