BJava and Pseudocode Quick Reference

public class Student {                       // class name: PascalCase
    private static int count = 0;            // shared by all objects
    private final String id;                 // set once
    private String name;                     // attributes: private
    private int marks;

    public Student(String name, int marks) { // constructor: no return type
        this.id = "S" + (++count);
        this.name = name;
        setMarks(marks);
    }

    public String getName() { return name; } // getter
    public void setMarks(int marks) {        // setter with validation
        if (marks >= 0 && marks <= 100) this.marks = marks;
    }

    @Override
    public String toString() { return id + " " + name + " " + marks; }
}

Student s = new Student("Nimal", 80);         // create an object
System.out.println(s.getName());              // call a method

abstract class Shape {                     // cannot be instantiated
    abstract double area();                 // no body: subclasses must implement
}

interface Drawable {                        // a contract
    void draw();
}

class Circle extends Shape implements Drawable {   // one superclass, many interfaces
    private final double r;
    Circle(double r) { this.r = r; }

    @Override double area() { return Math.PI * r * r; }
    @Override public void draw() { System.out.println("Drawing a circle"); }
}

Shape s = new Circle(2);                    // upcasting
if (s instanceof Circle c) c.draw();        // downcast after a check (Java 16+ pattern)