Java MCQ Questions – Page 70 of 200

Q: What does this code print?

A) Violations: 1. The `@Valid` annotation on `Box.item` triggers validation on the `Item` object. Since `Item.name` is `@NotNull` and set to `null`, it results in one validation violation.

Q: How is a `Runnable` object typically executed in a separate thread?

B) By passing the `Runnable` object to a `Thread` constructor and then calling `start()` on the `Thread` object.. To execute a `Runnable` in a new thread, you instantiate a `Thread` object with the `Runnable` instance and then call `start()` on the `Thread` object. Calling `run()` directly executes it in the current thread.

Q: What is the output of this code?

A) 15. The `finally` block always executes, even when an exception is caught or `continue` is called. For `x=1` and `x=3`, `count` increments by 1 (in try) and 1 (in finally). For `x=2`, an exception is thrown, `count` increments by 10 (in catch) and 1 (in finally), totaling 15.

Q: Which operator checks if two operands are equal in Java?

B) ==. The equality operator (==) compares two operands and returns true if they are equal, otherwise false.

Q: Which operator provides a shorthand for `x = x + y`?

C) x += y. The `+=` operator is a compound assignment operator that adds the right operand to the left operand and assigns the result to the left operand.

Q: What does this code print?

B) false. The logical AND operator `&&` returns `true` only if both operands are `true`. Since `q` is `false`, the entire expression `p && q` evaluates to `false`.

Q: Which method is used to retrieve, but not remove, the head of a Queue, returning null if the queue is empty?

B) peek(). The `peek()` method inspects the head of the queue without removing it. It returns `null` if the queue is empty, indicating no element is available.

Q: Under ideal conditions (good hash function, few collisions), what is the average time complexity for `get()` and `put()` operations in a `HashMap`?

A) O(1) - Constant time. In the average case, with a well-distributed hash function, `HashMap` operations like `get` and `put` take constant time, O(1), as they involve direct calculation of the bucket index.

Q: What is the main purpose of the `volatile` keyword in Java?

B) To ensure that changes to a variable by one thread are immediately visible to other threads, preventing caching issues.. `volatile` ensures visibility across threads by forcing reads and writes directly to main memory, preventing a thread from using its cached value. It also prevents reordering of instructions related to the volatile variable.

Q: What is the fundamental data structure that `ArrayList` uses internally to store its elements?

B) A dynamically resizable array. `ArrayList` is implemented using a dynamic array, which provides fast random access but requires resizing when capacity is exceeded.

Q1381 medium code output

What does this code print?

java

import jakarta.validation.*;
import jakarta.validation.constraints.NotNull;
import java.util.Set;

public class Test {
    static class Item {
        @NotNull public String name;
        public Item(String name) { this.name = name; }
    }
    static class Box {
        @Valid public Item item;
        public Box(Item item) { this.item = item; }
    }
    public static void main(String[] args) {
        Validator validator = Validation.buildDefaultValidatorFactory().getValidator();
        Box box = new Box(new Item(null));
        Set<ConstraintViolation<Box>> violations = validator.validate(box);
        System.out.println("Violations: " + violations.size());
    }
}

Q1382 easy

How is a `Runnable` object typically executed in a separate thread?

Q1383 hard code output

What is the output of this code?

java

public class WhileFinally {
    public static void main(String[] args) {
        int count = 0;
        int x = 0;
        while (x < 3) {
            try {
                x++;
                if (x == 2) {
                    throw new RuntimeException("Error at 2");
                }
                count++;
            } catch (RuntimeException e) {
                count += 10;
                continue; 
            } finally {
                count += 1; 
            }
        }
        System.out.println(count);
    }
}

Q1384 easy

Which operator checks if two operands are equal in Java?

Q1385 easy

Which operator provides a shorthand for `x = x + y`?

Q1386 easy code output

What does this code print?

java

public class Main {
    public static void main(String[] args) {
        boolean p = true;
        boolean q = false;
        System.out.println(p && q);
    }
}

Q1387 easy

Which method is used to retrieve, but not remove, the head of a Queue, returning null if the queue is empty?

Q1388 medium

Under ideal conditions (good hash function, few collisions), what is the average time complexity for `get()` and `put()` operations in a `HashMap`?

Q1389 medium

What is the main purpose of the `volatile` keyword in Java?

Q1390 medium

What is the fundamental data structure that `ArrayList` uses internally to store its elements?

Q1391 hard code output

What is the expected outcome of running this code?

java

public class DeadlockExample {
    private final Object lockA = new Object();
    private final Object lockB = new Object();

    public void task1() {
        synchronized (lockA) {
            System.out.println(Thread.currentThread().getName() + " acquired LockA");
            try { Thread.sleep(50); } catch (InterruptedException e) {}
            synchronized (lockB) {
                System.out.println(Thread.currentThread().getName() + " acquired LockB");
            }
        }
    }

    public void task2() {
        synchronized (lockB) {
            System.out.println(Thread.currentThread().getName() + " acquired LockB");
            try { Thread.sleep(50); } catch (InterruptedException e) {}
            synchronized (lockA) {
                System.out.println(Thread.currentThread().getName() + " acquired LockA");
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        DeadlockExample example = new DeadlockExample();
        new Thread(example::task1, "Thread-1").start();
        new Thread(example::task2, "Thread-2").start();
        Thread.sleep(500);
        System.out.println("Main thread exiting.");
    }
}

Q1392 easy code output

What is the output of this Java program?

java

import java.io.*;

class Nullable implements Serializable {
    private String data;
    private Integer number;

    public Nullable(String data, Integer number) {
        this.data = data;
        this.number = number;
    }

    public String toString() {
        return "Data: " + data + ", Number: " + number;
    }
}

public class Main {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Nullable obj = new Nullable(null, null); 
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(obj);
        oos.close();

        ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
        ObjectInputStream ois = new ObjectInputStream(bis);
        Nullable deserializedObj = (Nullable) ois.readObject();
        ois.close();

        System.out.println(deserializedObj);
    }
}

Q1393 medium code error

What is the result of attempting to run the following Java code?

java

public class Main {
    public static void main(String[] args) {
        Object lock = new Object();
        try {
            lock.notify(); // This line
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("Notified.");
    }
}

Q1394 hard code output

What is the output of this code?

java

public class Q2_ThreadJoinState {
    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(() -> {
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });
        t.start();
        Thread.sleep(10); // Ensure t is running
        System.out.println(t.getState()); // State 1
        t.join(); // Main thread waits for t to finish
        System.out.println(t.getState()); // State 2
    }
}

Q1395 hard code output

What is the output of this Java code, specifically the sequence of messages related to `Thread.join()`?

java

public class ThreadJoinRunnable {
    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            try {
                System.out.println(Thread.currentThread().getName() + " started.");
                Thread.sleep(200);
                System.out.println(Thread.currentThread().getName() + " finished.");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        };

        Thread t1 = new Thread(task, "T1");
        Thread t2 = new Thread(task, "T2");

        t1.start();
        t2.start();

        System.out.println("Main waiting for T1.");
        t1.join();
        System.out.println("T1 joined, Main waiting for T2.");
        t2.join();
        System.out.println("T2 joined, Main finished.");
    }
}

Q1396 easy code output

What does this code print?

java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<Integer> numbers = new LinkedList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.remove();
        System.out.println(numbers.element());
    }
}

Q1397 hard code error

What compile-time error occurs when attempting to compile this Java code?

java

interface A {
    default void perform() {
        System.out.println("A's perform");
    }
}

interface B {
    default void perform() {
        System.out.println("B's perform");
    }
}

class MyClass implements A, B {
    // No implementation for perform()
}

Q1398 medium

Which of the following scenarios would typically result in an `IllegalStateException` when using `java.util.Iterator`?

Q1399 easy code output

What does this Java code print to the console?

java

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

Q1400 easy code output

What will this Java code print?

java

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        Iterator<Integer> it = numbers.iterator();
        try {
            it.remove();
        } catch (IllegalStateException e) {
            System.out.println("Caught: " + e.getClass().getSimpleName());
        }
    }
}

☕ Java MCQ Questions – Page 70