core5ControlStatements

1package pkg1core;2 3/*4 * core5ControlStatements.java5 * ----------------------6 * if / else if / else, classic switch, modern switch expressions, and7 * pattern matching for switch (Java 21).8 *9 * EXPLANATION:10 *  - switch EXPRESSIONS (->) return a value and don't fall through.11 *  - Pattern matching lets switch branch on the runtime TYPE of an object.12 */13public class core5ControlStatements {14    sealed interface Shape permits Circle, Rectangle {}15    record Circle(double r) implements Shape {}16    record Rectangle(double w, double h) implements Shape {}17 18    public static void main(String[] args) {19        int score = 82;20 21        // if / else if / else22        String grade;23        if (score >= 90) grade = "A";24        else if (score >= 80) grade = "B";25        else if (score >= 70) grade = "C";26        else grade = "F";27        System.out.println("Grade: " + grade);28 29        // Classic switch (statement)30        int day = 3;31        switch (day) {32            case 1: System.out.println("Monday"); break;33            case 3: System.out.println("Wednesday"); break;34            default: System.out.println("Other day");35        }36 37        // Switch EXPRESSION with arrow and yield38        String type = switch (day) {39            case 1, 2, 3, 4, 5 -> "Weekday";40            case 6, 7 -> "Weekend";41            default -> {42                yield "Invalid";   // yield returns from a block43            }44        };45        System.out.println("Day type: " + type);46 47        // Pattern matching for switch (Java 21) + guarded patterns48        for (Shape sh : new Shape[]{ new Circle(2), new Rectangle(3, 4) }) {49            double area = switch (sh) {50                case Circle c -> Math.PI * c.r() * c.r();51                case Rectangle r when r.w() == r.h() -> r.w() * r.w(); // guard52                case Rectangle r -> r.w() * r.h();53            };54            System.out.printf("Area of %s = %.2f%n", sh, area);55        }56 57        // instanceof pattern matching (binds the variable)58        Object o = "hello";59        if (o instanceof String str && str.length() > 3) {60            System.out.println("String of length " + str.length());61        }62    }63}