☕ Java MCQ Questions – Page 78

public class ArrayError {
    public static void main(String[] args) {
        int[] numbers = new int[3] {1, 2, 3};
        System.out.println(numbers[0]);
    }
}

public class MissingStart implements Runnable {
    @Override
    public void run() {
        System.out.println("This should not print.");
    }

    public static void main(String[] args) {
        Runnable r = new MissingStart();
        // No Thread created or started
        System.out.println("Program finished.");
    }
}

public class Counter {
    private int count;

    public Counter() {
        this.count = 0;
    }

    public void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }

    public static void main(String[] args) {
        Counter c = new Counter();
        c.increment();
        c.increment();
        System.out.println(c.getCount());
    }
}

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class LockConditionTest {
    private final ReentrantLock lock = new ReentrantLock();
    private final Condition condition = lock.newCondition();
    private boolean flag = false;

    public void worker1() throws InterruptedException {
        lock.lock();
        try {
            while (!flag) { condition.await(); }
            System.out.println("Worker1 done.");
        } finally { lock.unlock(); }
    }

    public void worker2() {
        lock.lock();
        try { flag = true; condition.signalAll(); }
        finally { lock.unlock(); }
    }

    public static void main(String[] args) throws InterruptedException {
        LockConditionTest test = new LockConditionTest();
        new Thread(() -> { try { test.worker1(); } catch (InterruptedException e) {} }).start();
        Thread.sleep(100); // Give worker1 time to await
        new Thread(test::worker2).start();
        Thread.sleep(500); // Allow threads to finish
    }
}

import java.util.HashSet;
import java.util.Objects;

class KeyWithoutInstanceof {
    int value;
    public KeyWithoutInstanceof(int value) { this.value = value; }
    @Override
    public boolean equals(Object o) { // Missing instanceof check
        KeyWithoutInstanceof other = (KeyWithoutInstanceof) o; // Dangerous cast
        return this.value == other.value;
    }
    @Override
    public int hashCode() { return Objects.hashCode(value); }
}

public class Main {
    public static void main(String[] args) {
        HashSet<KeyWithoutInstanceof> set = new HashSet<>();
        set.add(new KeyWithoutInstanceof(10));
        System.out.println(set.contains("some_string")); // Triggering equals with incompatible type
    }
}

public class OperatorChallenge {
    public static void main(String[] args) {
        int maxInt = Integer.MAX_VALUE;
        long l = (long) maxInt + 1;
        int i = (int) l;
        System.out.println(i);
    }
}

class Box {
    Box() {
        System.out.println("Box created.");
    }
}
public class Main {
    public static void main(String[] args) {
        Box b1 = new Box();
        Box b2 = new Box();
    }
}

import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        TreeMap<Integer, String> treeMap = new TreeMap<>();
        treeMap.put(10, "Ten");
        treeMap.put(20, "Twenty");
        System.out.println(treeMap.containsKey(10) + " " + treeMap.containsKey(30));
    }
}

public class StringError {
    public static void main(String[] args) {
        String text = "abcdef";
        boolean matches = text.matches("[a-z+");
        System.out.println(matches);
    }
}

public class LoopTest {
    public static void main(String[] args) {
        for (int i = 1; i <= 2; i++) {
            System.out.println("Hello");
        }
    }
}

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("ABC");
        sb.insert(1, "X").insert(3, "Y").insert(0, "P");
        System.out.println(sb);
    }
}

import java.io.FileReader;
import java.io.IOException;

public class FileReaderError6 {
    public static void main(String[] args) {
        String filePath = null; // Null file path
        try (FileReader fr = new FileReader(filePath)) { // Constructor with null String
            int data = fr.read();
            System.out.println("Read: " + (char)data);
        } catch (IOException e) {
            System.err.println(e.getClass().getSimpleName() + ": " + e.getMessage());
        } catch (NullPointerException e) {
            System.err.println(e.getClass().getSimpleName() + ": Path must not be null");
        }
    }
}

public class ConditionCheck {
    public static void main(String[] args) {
        int num = 10;
        if (num > 5) {
            String message = "Number is large";
            if (num > 15) {
                message = "Number is very large";
            }
        } else {
            System.out.println(message);
        }
    }
}

class Task implements Runnable {
    public void run() throws Exception {
        System.out.println("Task is running...");
        if (Math.random() > 0.5) {
            throw new Exception("Something went wrong");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Task myTask = new Task();
        new Thread(myTask).start();
    }
}

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Programming");
        sb.delete(3, 6);
        sb.deleteCharAt(4);
        System.out.println(sb);
    }
}

public class ArrayDefaultValues {
    public static void main(String[] args) {
        int[] intArray = new int[3];
        String[] stringArray = new String[2];
        Object[] objectArray = new Object[1];
        Boolean[] booleanArray = new Boolean[1];

        System.out.print(intArray[0]);
        System.out.print(stringArray[0] == null);
        System.out.print(objectArray[0] == null);
        System.out.println(booleanArray[0]);
    }
}

class MyClass {
    final MyClass() {
        System.out.println("Final constructor");
    }
}
public class Main {
    public static void main(String[] args) {
        MyClass obj = new MyClass();
    }
}

public class Main {
    public static void main(String[] args) {
        String word = "Java";
        System.out.println(word.charAt(4));
    }
}