Java Basics & JVM Fundamentals Quiz

javac -d build src/com/company/app/*.java

public class Welcome {
    public static void main(String[] args) {
        String lang = "Java";
        int year = 1995;
        System.out.printf("%s was released in %d!%n", lang, year);
        System.out.println("Still going strong in 2025!");
    }
}

class Vehicle {
    String move() { return "moving"; }
}
class Car extends Vehicle {
    String move() { return "driving on road"; }
}
public class Test {
    public static void main(String[] args) {
        Vehicle v = new Car();
        System.out.println(v.move());
    }
}

java com.example.demo.App

public class BuilderTest {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        sb.append("Java ");
        sb.append(17);
        sb.insert(5, "SE ");
        sb.delete(8, 10);
        System.out.println(sb);
    }
}

public class ArrayDemo {
    public static void main(String[] args) {
        int[] primes = {2, 3, 5, 7, 11, 13, 17, 19};
        for (int i = 0; i < primes.length; i += 2) {
            System.out.print(primes[i] + " ");
        }
        System.out.println("\nTotal: " + primes.length + " primes");
    }
}

// File: Greeting.java
public class Greeting {
    private String message;
    public Greeting(String msg) { this.message = msg; }
    public void print() { System.out.println(message); }
}

// File: Main.java
public class Main {
    public static void main(String[] args) {
        new Greeting("Hello from multi-file Java!").print();
    }
}

public class LanguageLoop {
    public static void main(String[] args) {
        String[] jvmLangs = {"Java", "Kotlin", "Scala", "Groovy", "Clojure"};
        for (String lang : jvmLangs) {
            if (lang.length() >= 5) {
                System.out.println(lang + " (" + lang.length() + ")");
            }
        }
    }
}

public class Pattern {
    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }
    }
}

public class ArgsDemo {
    public static void main(String[] args) {
        System.out.println("Arguments count: " + args.length);
        for (int i = 0; i < args.length; i++) {
            System.out.println("arg[" + i + "] = " + args[i]);
        }
    }
}

public class VarDemo {
    public static void main(String[] args) {
        var number = 42;
        var text = "The answer";
        var result = text + " is " + number;
        System.out.println(result);
    }
}

public class SwitchDemo {
    public static void main(String[] args) {
        int day = 3;
        String type = switch (day) {
            case 1, 2, 3, 4, 5 -> "Weekday";
            case 6, 7 -> "Weekend";
            default -> "Invalid";
        };
        System.out.println(type);
    }
}

public class Countdown {
    static void count(int n) {
        if (n <= 0) {
            System.out.println("Blast off!");
            return;
        }
        System.out.print(n + "...");
        count(n - 1);
    }
    public static void main(String[] args) {
        count(5);
    }
}

public class ImmutableDemo {
    public static void main(String[] args) {
        String a = "hello";
        String b = a;
        a = a + " world";
        System.out.println("a = " + a);
        System.out.println("b = " + b);
    }
}

public class LabeledBreak {
    public static void main(String[] args) {
        outer: for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (i == 2 && j == 2) {
                    break outer;
                }
                System.out.print("(" + i + "," + j + ") ");
            }
        }
        System.out.println("Done");
    }
}

public record Point(int x, int y) {
    public static void main(String[] args) {
        var p1 = new Point(10, 20);
        var p2 = new Point(10, 20);
        System.out.println(p1);
        System.out.println(p1.equals(p2));
    }
}

public class EverythingDemo {
    static String version = "Java 21";
    
    public static void main(String[] args) {
        var builder = new StringBuilder();
        builder.append("Welcome to ").append(version).append("!");
        
        for (int i = 0; i < 3; i++) {
            builder.append(" ").append(i * 7);
        }
        
        System.out.println(builder.reverse());
        System.out.println("Length: " + builder.length());
    }
}

public class InnerDemo {
    public static void main(String[] args) {
        class Helper {
            String greet() { return "Hello from inner class!"; }
        }
        Helper h = new Helper();
        System.out.println(h.greet());
    }
}

public class TextBlockDemo {
    public static void main(String[] args) {
        String json = """
            {
                "name": "Java",
                "year": 1995
            }
            """;
        System.out.println(json.trim());
    }
}

public class FinalDemo {
    public static void main(String[] args) {
        final int MAX = 100;
        final var list = java.util.List.of(1, 2, 3);
        // list.add(4); // Would not compile
        System.out.println("Max: " + MAX + ", List size: " + list.size());
    }
}

sealed interface Vehicle permits Car, Bike {}
non-sealed class Car implements Vehicle {}
final class Bike implements Vehicle {}
public class SealedDemo {
    public static void main(String[] args) {
        Vehicle v = new Car();
        System.out.println(v.getClass().getSimpleName());
    }
}

public class UltimateHello {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("Java runs on " + Runtime.getRuntime().availableProcessors() + " CPU cores");
        System.out.println("Max memory: " + (Runtime.getRuntime().maxMemory() / 1024 / 1024) + " MB");
    }
}

enum Level { LOW, MEDIUM, HIGH }
public class EnumDemo {
    public static void main(String[] args) {
        Level l = Level.HIGH;
        switch (l) {
            case LOW -> System.out.println("Easy");
            case MEDIUM -> System.out.println("Normal");
            case HIGH -> System.out.println("Hard");
        }
    }
}

public class PatternMatch {
    public static void main(String[] args) {
        Object obj = "Hello";
        if (obj instanceof String s && s.length() > 3) {
            System.out.println(s.toUpperCase() + "!");
        }
    }
}

public class VirtualThreads {
    public static void main(String[] args) throws Exception {
        try (var executor = java.util.concurrent.Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 0; i < 10; i++) {
                executor.submit(() -> System.out.print(Thread.currentThread() + " "));
            }
        }
        System.out.println("Done");
    }
}

public class JavaJourney {
    public static void main(String[] args) {
        System.out.println("Java: Born 1995, Still #1 in 2025");
        var jvm = Runtime.getRuntime();
        System.out.printf("Running on %,d cores with %,d MB max heap%n",
            jvm.availableProcessors(),
            jvm.maxMemory() / 1024 / 1024);
        System.out.println("Write Once, Run Anywhere — proven for 30 years!");
    }
}

public class TheEssence {
    public static void main(String[] args) {
        System.out.println("Hello from the JVM!");
        // This single line triggered:
        // • Class loading
        // • Bytecode verification
        // • Memory allocation
        // • JIT compilation
        // • Garbage collection readiness
        // • And 30 years of engineering excellence
    }
}