Constructors

Student s = new Student("Alice", 11);   // constructor runs here

public class Student {
    private String name;
    private int grade;
    private double gpa;
    
    // Constructor
    public Student(String name, int grade, double gpa) {
        this.name = name;     // assign parameter to instance variable
        this.grade = grade;
        this.gpa = gpa;
    }
}

public class Student {
    private String name;
    private int grade;
    private double gpa;
    
    // No-argument constructor
    public Student() {
        name = "Unknown";
        grade = 9;
        gpa = 0.0;
    }
}

Student s = new Student();   // uses default values

public class Rectangle {
    private double width;
    private double height;
    
    // No-arg constructor — default values
    public Rectangle() {
        width = 1.0;
        height = 1.0;
    }
    
    // One-arg constructor — square
    public Rectangle(double side) {
        width = side;
        height = side;
    }
    
    // Two-arg constructor — custom dimensions
    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
}

// All three work:
Rectangle r1 = new Rectangle();          // 1.0 × 1.0
Rectangle r2 = new Rectangle(5.0);       // 5.0 × 5.0
Rectangle r3 = new Rectangle(3.0, 7.0);  // 3.0 × 7.0

public class Point {
    private int x;
    private int y;
    // No constructor written — Java provides one automatically
}

Point p = new Point();   // x = 0, y = 0

public class Point {
    private int x;
    private int y;
    
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

// Point p = new Point();   // COMPILE ERROR — no no-arg constructor!
Point p = new Point(3, 5);  // works

public class Student {
    private String name;
    private int grade;
    private double gpa;
    
    public Student(String name, int grade, double gpa) {
        this.name = name;
        
        // Validate grade is 9-12
        if (grade >= 9 && grade <= 12) {
            this.grade = grade;
        } else {
            this.grade = 9;  // default if invalid
        }
        
        // Validate GPA is 0.0-4.0
        if (gpa >= 0.0 && gpa <= 4.0) {
            this.gpa = gpa;
        } else {
            this.gpa = 0.0;
        }
    }
}

Student s = new Student("Alice", 11, 3.8);

public class Dog {
    private String name;
    private int age;
    
    public Dog(String name) {
        this.name = name;
        this.age = 0;   // all dogs start at age 0
    }
    
    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String toString() {
        return name + " (age " + age + ")";
    }
}

Dog d1 = new Dog("Max");       // name="Max", age=0
Dog d2 = new Dog("Bella", 3);  // name="Bella", age=3
System.out.println(d1);         // "Max (age 0)"
System.out.println(d2);         // "Bella (age 3)"