Practice

if ((y % 4 == 0 && y % 100 != 0) || y % 400 == 0)
    System.out.println("Leap");
else
    System.out.println("Not leap");

1. 7 / 2 = 3 (integer division).
2. 7 % 2 = 1.
3. 7 / 2.0 = 3.5 (a double operand).
4. 3 + 1 + 3.5 = 7.5

int max = (a > b) ? a : b;

1. x += E means x = x + E; the left x is read first: 10.
2. x++ gives 10, x becomes 11.
3. ++x makes x 12 and gives 12.
4. x = 10 + (10 + 12) = 32.

int s = 0;
for (int i = 2; i <= 100; i += 2) s += i;
System.out.println(s);   // 2550

int i = 5;
while (i > 0) {
    System.out.print(i);
    i--;
}

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= i; j++) System.out.print(j + " ");
    System.out.println();
}

int count = 0;
while (n > 0) { n /= 10; count++; }

outer:
for (int n = 2; n <= 30; n++) {
    for (int d = 2; d * d <= n; d++)
        if (n % d == 0) continue outer;   // not prime
    System.out.print(n + " ");
}

2 3 5 7 11 13 17 19 23 29

int tries = 0, pin;
do {
    System.out.print("PIN: ");
    pin = in.nextInt();
    tries++;
} while (pin != 1234 && tries < 3);
System.out.println(pin == 1234 ? "Welcome" : "Card blocked");

double s = 0;
for (double v : a) s += v;
double avg = s / a.length;

for (int i = 0; i < m.length; i++) {
    int s = 0;
    for (int j = 0; j < m[i].length; j++) s += m[i][j];
    System.out.println("Row " + i + ": " + s);
}

for (int i = 0, j = a.length - 1; i < j; i++, j--) {
    int t = a[i]; a[i] = a[j]; a[j] = t;
}

1. Pass 1: {1, 4, 2, 5}
2. Pass 2: {1, 2, 4, 5}
3. Pass 3: {1, 2, 4, 5} (no swaps — already sorted)

1. lo=0, hi=7, mid=3 → 12 < 23 → lo = 4
2. lo=4, hi=7, mid=5 → 23 found at index 5

public boolean withdraw(double amt) {
    if (amt <= 0 || amt > balance) return false;
    balance -= amt;
    return true;
}

public class Rectangle {
    private double width, height;
    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
    public double area()      { return width * height; }
    public double perimeter() { return 2 * (width + height); }
}

1. Two objects: #1 (first new) and #2.
2. c → #1, a → #2, b → null.
3. Both are still referenced → 0 eligible.

Time() { this(0, 0); }
Time(int h) { this(h, 0); }
Time(int h, int m) { this.h = h; this.m = m; }

static int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }

Point midpoint(Point p) {
    return new Point((x + p.x) / 2, (y + p.y) / 2);
}

static int count(String s, char c) {
    if (s.isEmpty()) return 0;
    return (s.charAt(0) == c ? 1 : 0) + count(s.substring(1), c);
}
// count("banana", 'a') → 3

class Circle {
    private static int count;
    private double r;
    Circle(double r) { this.r = r; count++; }
    static int getCount() { return count; }
}

• Pay itself.
• Every class in package hr.
• Subclasses of Pay in other packages — through inheritance (their own objects).
• No other classes.

class Order {
    private static int next = 1001;
    private final int no;
    Order() { no = next++; }
    int getNo() { return no; }
}

class Manager extends Employee {
    private double bonus;
    Manager(String n, double base, double bonus) { super(n, base); this.bonus = bonus; }
    @Override double salary() { return super.salary() + bonus; }
}

class Shape {
    protected String color;
    Shape(String color) { this.color = color; }
}
class Circle extends Shape {
    private double radius;
    Circle(String color, double radius) {
        super(color);
        this.radius = radius;
    }
}

for (Animal a : zoo)
    if (a instanceof Dog d)   // Java 16 pattern
        d.speak();

1. Does not compile: an Animal is not necessarily a Dog.
2. Compiles and works.
3. Compiles, but throws ClassCastException at run time because the object is a Dog.

abstract class Employee {
    protected String name;
    Employee(String name) { this.name = name; }
    abstract double earnings();
}
class Salaried extends Employee {
    private double monthly;
    Salaried(String n, double m) { super(n); monthly = m; }
    double earnings() { return monthly; }
}
class Hourly extends Employee {
    private double rate; private int hours;
    Hourly(String n, double r, int h) { super(n); rate = r; hours = h; }
    double earnings() { return rate * hours; }
}

interface Resizable { void resize(double factor); }
class Circle implements Resizable {
    private double r;
    public void resize(double f) { r *= f; }
}

class Robot extends Machine implements Movable, Chargeable { ... }

static double sqrt(double x) {
    if (x < 0) throw new IllegalArgumentException("negative: " + x);
    return Math.sqrt(x);
}

class InvalidMarksException extends RuntimeException {
    InvalidMarksException(String msg) { super(msg); }
}

static int readScore(Scanner in) {
    while (true) {
        try {
            return Integer.parseInt(in.nextLine().trim());
        } catch (NumberFormatException e) {
            System.out.println("Please type a whole number.");
        }
    }
}

Thread[] ts = new Thread[3];
for (int i = 0; i < 3; i++) {
    ts[i] = new Thread(() -> System.out.println(Thread.currentThread().getName()), "T" + i);
    ts[i].start();
}
for (Thread t : ts) t.join();
System.out.println("done");

static boolean isPal(String s) {
    s = s.toLowerCase();
    for (int i = 0, j = s.length() - 1; i < j; i++, j--)
        if (s.charAt(i) != s.charAt(j)) return false;
    return true;
}

static String subtract(String a, String b) {
    StringBuilder sb = new StringBuilder();
    int borrow = 0;
    for (int i = a.length() - 1, j = b.length() - 1; i >= 0; i--, j--) {
        int d = a.charAt(i) - '0' - borrow - (j >= 0 ? b.charAt(j) - '0' : 0);
        borrow = d < 0 ? 1 : 0;
        sb.append(d < 0 ? d + 10 : d);
    }
    String r = sb.reverse().toString().replaceFirst("^0+(?!$)", "");
    return r;
}

String t = "  OOP   is   fun ";
String[] w = t.trim().split("\\s+");     // one or more spaces
System.out.println(w.length);            // 3

1. 9 + 1 + 0 = 10 → write 0, carry 1
2. 9 + 0 + 1 = 10 → write 0, carry 1
3. 9 + 0 + 1 = 10 → write 0, carry 1
4. carry 1 left → write 1

gc.setFill(Color.RED);
gc.fillOval(60, 60, 80, 80);   // top-left corner, width, height

tf.setOnAction(e -> lb.setText("Length: " + tf.getText().length()));

List<Student> list = new ArrayList<>();
try (ResultSet rs = ps.executeQuery()) {
    while (rs.next())
        list.add(new Student(rs.getString("name"), rs.getDouble("cgpa")));
}