Constructors in Java

# CHAPTER 14

Constructors in Java #

1. Introduction #

2. Learning Objectives #

• Understand what constructors are and why they're needed.

• Create default and parameterized constructors.

• Implement constructor overloading.

• Use this() for constructor chaining.

• Understand the copy constructor pattern.

3. What is a Constructor? #

public class Car {
    String model;
    int year;
    
    // Constructor
    Car(String model, int year) {
        this.model = model;
        this.year = year;
    }
}

Car myCar = new Car("Tesla", 2024); // Constructor called automatically

4. Default Constructor #

public class Book {
    String title;
    // Java provides: Book() {} implicitly
}

Book b = new Book(); // Uses default constructor

Important: If you write ANY constructor, Java no longer provides the default one!

5. Parameterized Constructor #

public class Student {
    String name;
    int age;
    
    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    void display() {
        System.out.println(name + " - " + age);
    }
    
    public static void main(String[] args) {
        Student s = new Student("Alice", 20);
        s.display(); // Alice - 20
    }
}

6. Constructor Overloading #

public class Rectangle {
    double length, width;
    
    Rectangle() {
        this.length = 1.0;
        this.width = 1.0;
    }
    
    Rectangle(double side) {
        this.length = side;
        this.width = side;  // Square
    }
    
    Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }
    
    double area() { return length * width; }
}

7. Constructor Chaining with this() #

public class Employee {
    String name;
    String dept;
    double salary;
    
    Employee() {
        this("Unknown", "Unassigned", 0.0); // Calls 3-param constructor
    }
    
    Employee(String name) {
        this(name, "General", 50000.0);
    }
    
    Employee(String name, String dept, double salary) {
        this.name = name;
        this.dept = dept;
        this.salary = salary;
    }
}

8. Copy Constructor #

public class Point {
    int x, y;
    
    Point(int x, int y) { this.x = x; this.y = y; }
    
    // Copy constructor
    Point(Point other) {
        this.x = other.x;
        this.y = other.y;
    }
}

Point p1 = new Point(10, 20);
Point p2 = new Point(p1); // Copy of p1

9. Common Mistakes #

• Adding a return type to a constructor: void Car() is a method, not a constructor!

• Forgetting to call this.field: Without this, the parameter shadows the field.

• Not providing a no-arg constructor when needed: Some frameworks (like Hibernate) require one.

10. MCQ Quiz with Answers #

11. Interview Questions #

• Q: What is the difference between a constructor and a method?

• Q: What is constructor chaining?

• Q: Can a constructor call another constructor? How?

• Q: Why would you make a constructor private?

12. Summary #

13. Next Chapter Recommendation #