☕ Java MCQ Questions – Page 27

public class MixedArrayInit {
    public static void main(String[] args) {
        int[] data = new int[]{10, 20};
        // Attempt to create an array using both dimension expression and initializer list
        data = new int[2]{30, 40};
        System.out.println(data[0]);
    }
}

public class OverloadDemo3 {
    void calculate(int a, int b) {
        System.out.println("Sum of ints: " + (a + b));
    }
    void calculate(double a, double b) {
        System.out.println("Sum of doubles: " + (a + b));
    }
    public static void main(String[] args) {
        OverloadDemo3 obj = new OverloadDemo3();
        obj.calculate(5, 5);
    }
}

public enum Permission {
    READ("read"),
    WRITE("write"),
    EXECUTE("execute");

    private final String code;
    private String description; // Private mutable field

    private Permission(String code) {
        this.code = code;
        this.description = "Permission to " + code;
    }

    public String getCode() { return code; }
}

public class PermissionChecker {
    public static void main(String[] args) {
        // Attempt to access a private field of an enum constant
        System.out.println(Permission.READ.description); 
    }
}

class Test {
    public static void main(String[] args) {
        do {
            System.out.println("Attempting operation...");
            throw new java.io.IOException("File error");
        } while (false);
        System.out.println("Operation finished.");
    }
}

public class Test {
    public static void main(String[] args) {
        try {
            System.out.print("Outer Try ");
            try {
                System.out.print("Inner Try ");
                throw new Exception("Inner Exception");
            } finally {
                System.out.print("Inner Finally ");
            }
        } catch (Exception e) {
            System.out.print("Outer Catch: " + e.getMessage());
        } finally {
            System.out.print("Outer Finally ");
        }
        System.out.print("End");
    }
}

import java.util.concurrent.locks.ReentrantLock;

public class LockUnlockError {
    private static ReentrantLock resourceLock = new ReentrantLock();

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            // t1 does not acquire the lock
            System.out.println("T1 trying to unlock without owning...");
            resourceLock.unlock(); 
            System.out.println("T1 unlocked (should not happen)");
        });

        t1.start();
    }
}

import java.util.PriorityQueue;

public class QueueError {
    public static void main(String[] args) {
        PriorityQueue<String> pq = new PriorityQueue<>();
        pq.add("Hello");
        pq.add(null); // Adding a null element
    }
}

public class PrimitiveSwitchNullCaseError {
    public static void main(String[] args) {
        int number = 10;
        String category = switch (number) { // Error expected here
            case 5 -> "Small";
            case 10 -> "Medium";
            case null -> "Null Input"; // Invalid case
            default -> "Large";
        };
        System.out.println(category);
    }
}

public class VolatileRunnable {
    private static volatile boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        Runnable stopTask = () -> {
            System.out.println(Thread.currentThread().getName() + " started.");
            while (running) {
                // busy-wait
            }
            System.out.println(Thread.currentThread().getName() + " stopped.");
        };

        Thread worker = new Thread(stopTask, "WorkerThread");
        worker.start();

        Thread.sleep(100); // Give worker a chance to start
        running = false; // Change volatile flag

        Thread.sleep(100); // Give worker a chance to see the change
        System.out.println("Main thread finished setting running to false.");
    }
}

import java.util.stream.IntStream;
import java.util.List;
import java.util.stream.Collectors;

public class StreamTest {
    public static void main(String[] args) {
        List<Integer> numbers = IntStream.range(1, 4)
                                         .map(n -> n * 10)
                                         .boxed()
                                         .collect(Collectors.toList());
        System.out.println(numbers);
    }
}

import java.io.*;

public class DeserializationError4 {
    public static void main(String[] args) {
        // This byte array represents a serialized object of a class named 'MyRemoteClass'
        // that has serialVersionUID = 1L. The actual definition of 'MyRemoteClass' is not provided.
        // We assume it was serialized previously.
        byte[] serializedData = { (byte)0xac, (byte)0xed, (byte)0x00, (byte)0x05, (byte)0x73, (byte)0x72, (byte)0x00, (byte)0x0e, (byte)0x4d, (byte)0x79, (byte)0x52, (byte)0x65, (byte)0x6d, (byte)0x6f, (byte)0x74, (byte)0x65, (byte)0x43, (byte)0x6c, (byte)0x61, (byte)0x73, (byte)0x73, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x01, (byte)0x02, (byte)0x00, (byte)0x01, (byte)0x4c, (byte)0x00, (byte)0x04, (byte)0x6e, (byte)0x61, (byte)0x6d, (byte)0x65, (byte)0x74, (byte)0x00, (byte)0x12, (byte)0x4c, (byte)0x6a, (byte)0x61, (byte)0x76, (byte)0x61, (byte)0x2f, (byte)0x6c, (byte)0x61, (byte)0x6e, (byte)0x67, (byte)0x2f, (byte)0x53, (byte)0x74, (byte)0x72, (byte)0x69, (byte)0x6e, (byte)0x67, (byte)0x3b, (byte)0x78, (byte)0x70, (byte)0x74, (byte)0x00, (byte)0x06, (byte)0x48, (byte)0x65, (byte)0x6c, (byte)0x6c, (byte)0x6f, (byte)0x21 };

        // Current class definition for deserialization
        class MyRemoteClass implements Serializable {
            private static final long serialVersionUID = 2L; // Changed SUID
            String name;
            public MyRemoteClass() { this.name = "Default"; }
            public MyRemoteClass(String name) { this.name = name; }
            @Override public String toString() { return "MyRemoteClass: " + name; }
        }

        try (ByteArrayInputStream bis = new ByteArrayInputStream(serializedData);
             ObjectInputStream ois = new ObjectInputStream(bis)) {
            MyRemoteClass obj = (MyRemoteClass) ois.readObject();
            System.out.println(obj);
        } catch (Exception e) {
            System.out.println("Error: " + e.getClass().getName() + ": " + e.getMessage().split(";")[0]);
        }
    }
}

import java.io.FileWriter;
import java.io.IOException;
import java.io.FileReader;
import java.io.BufferedReader;
import java.nio.file.Files;
import java.nio.file.Path;

public class FileWriterOverwriteEmpty {
    public static void main(String[] args) {
        String filename = "overwrite_empty.txt";
        try {
            try (FileWriter fw = new FileWriter(filename)) { fw.write("Original content."); }

            try (FileWriter fw = new FileWriter(filename)) {
                // Nothing written here, but FileWriter opens in non-append mode
            }

            try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
                String line = br.readLine();
                System.out.println("Content: '" + (line != null ? line : "[Empty]") + "'");
            }
        } catch (IOException e) { System.out.println("Error: " + e.getMessage());
        } finally { try { Files.deleteIfExists(Path.of(filename)); } catch (IOException ignored) {} }
    }
}

public class StringError {
    public static void main(String[] args) {
        String str = null;
        System.out.println(str.length());
    }
}