Java MCQ Questions – Page 72 of 200

Q: What is the output of this code, demonstrating a 'this' reference escape during constructor execution?

B) Logged: Notifier created (ID: null). The `this` reference is 'escaping' in the `Notifier` constructor when `logger.logEvent()` is called before the `id` field (a `final` field) has been fully initialized. At the point of the `logEvent` call, `this.id` holds its default value for object references, which is `null`, leading to 'null' in the log message.

Q: What is the output of this code?

A) true true 0. `HashSet` relies on the `hashCode()` and `equals()` contract. Since `MyKey` correctly implements both, `new MyKey(10)` is considered equivalent to the `MyKey(10)` object added to the set, even though it's a different instance. Thus, `contains()` returns true and `remove()` successfully removes the element.

Q: What is the ultimate superclass for all exception and error types in Java, including custom exceptions?

C) java.lang.Throwable. All exceptions and errors in Java, including custom ones, are direct or indirect subclasses of `java.lang.Throwable`.

Q: What error will occur when compiling or running this Java code?

B) Compilation Error: possible lossy conversion from double to int. Assigning a double literal (3.14) directly to an `int` type array element without explicit casting will result in a compile-time error due to possible loss of precision.

Q: Which method is used to add an element to an `ArrayList`?

C) `add()`. The `add()` method is the standard way to insert an element into an `ArrayList`, either at the end of the list or at a specific index.

Q: What is the primary reason for creating a custom exception in Java?

B) To provide more specific and meaningful error information to the user or calling code.. Custom exceptions allow developers to define specific error types relevant to their application's domain, making error handling more precise and informative.

Q: Can an abstract class be instantiated?

C) No, it must be subclassed and implemented first. An abstract class cannot be instantiated directly. It serves as a blueprint for other classes, and subclasses must implement its abstract methods (if any) before they can be instantiated.

Q: What is the error in this Java code?

B) A NullPointerException will occur at runtime.. While 'null' can be stored as an Integer value in a HashMap, attempting to unbox a 'null' Integer to a primitive 'int' type will result in a NullPointerException at runtime.

Q: What does this code print?

A) Static Block 1 Static Block 2 Main method started Constructor called Constructor called. Static Initialization Blocks (SIBs) execute exactly once, when the class is first loaded into the JVM, before any static members are accessed or objects are created. They run in the order they appear. The `main` method starts after class loading, then constructors run for each object creation.

Q: What does this Java code snippet print?

A) ABXDCE. 1. `StringBuffer sb = new StringBuffer("ABCDE");` -> "ABCDE" 2. `sb.setCharAt(2, 'X');` -> Replaces 'C' with 'X': "ABXDE" 3. `sb.charAt(3)` -> Returns the character at index 3, which is 'D'. 4. `sb.insert(1, sb.charAt(3));` -> Inserts 'D' at index 1: "ADBXDE". So the output is "ADBXDE".

Q1421 hard code output

What is the output of this code, demonstrating a 'this' reference escape during constructor execution?

java

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

class EventLogger {
    private List<String> logs = new ArrayList<>();
    public void logEvent(String event) { logs.add(event); System.out.println("Logged: " + event); }
}

class Notifier {
    private final String id;
    public Notifier(EventLogger logger, String id) {
        logger.logEvent("Notifier created (ID: " + this.id + ")");
        this.id = id;
    }
}

public class Main {
    public static void main(String[] args) {
        new Notifier(new EventLogger(), "N_001");
    }
}

Q1422 hard code output

What is the output of this code?

java

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

class MyKey {
    int value;
    public MyKey(int value) { this.value = value; }
    @Override public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MyKey myKey = (MyKey) o;
        return value == myKey.value;
    }
    @Override public int hashCode() { return Objects.hash(value); }
}

public class Main {
    public static void main(String[] args) {
        HashSet<MyKey> set = new HashSet<>();
        set.add(new MyKey(10));
        System.out.println(set.contains(new MyKey(10)));
        System.out.println(set.remove(new MyKey(10)));
        System.out.println(set.size());
    }
}

Q1423 easy

What is the ultimate superclass for all exception and error types in Java, including custom exceptions?

Q1424 medium code error

What error will occur when compiling or running this Java code?

java

public class ArrayError {
    public static void main(String[] args) {
        int[] numbers = new int[5];
        numbers[0] = 3.14;
    }
}

Q1425 easy

Which method is used to add an element to an `ArrayList`?

Q1426 easy

What is the primary reason for creating a custom exception in Java?

Q1427 easy

Can an abstract class be instantiated?

Q1428 easy code error

What is the error in this Java code?

java

import java.util.HashMap;
import java.util.Map;

public class Test {
    public static void main(String[] args) {
        Map<String, Integer> myMap = new HashMap<>();
        myMap.put("score", null);
        int finalScore = myMap.get("score");
        System.out.println(finalScore);
    }
}

Q1429 medium code output

What does this code print?

java

class Company {
    static {
        System.out.println("Static Block 1");
    }
    public Company() {
        System.out.println("Constructor called");
    }
    static {
        System.out.println("Static Block 2");
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("Main method started");
        Company c1 = new Company();
        Company c2 = new Company();
    }
}

Q1430 medium code output

What does this Java code snippet print?

java

public class StringBufferTest {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("ABCDE");
        sb.setCharAt(2, 'X');
        sb.insert(1, sb.charAt(3));
        System.out.print(sb);
    }
}

Q1431 easy code error

What is the error in the following Java code snippet?

java

public class ArrayError {
    public static void main(String[] args) {
        int[] numbers;
        numbers[0] = 10;
        System.out.println(numbers[0]);
    }
}

Q1432 medium code error

What error does this Java code produce when executed?

java

public class Main {
    public static void main(String[] args) {
        String country = "Canada";
        System.out.println(country.substring(7));
    }
}

Q1433 medium

What is the primary purpose of the `java.util.Iterator` interface in Java?

Q1434 easy code output

What does this code print?

java

public class User {
    private String username;

    public User(String username) {
        this.username = username;
    }

    public String getUsername() {
        return username;
    }

    public static void main(String[] args) {
        User user = new User("alice");
        System.out.println(user.getUsername());
    }
}

Q1435 medium

When is a `ClassCastException` most likely to occur in the context of polymorphism?

Q1436 easy code output

What is the output of this code?

java

public class DataTypeTest {
    public static void main(String[] args) {
        byte b1 = 10;
        byte b2 = 20;
        int sum = b1 + b2;
        System.out.println(sum);
    }
}

Q1437 easy code error

What is the output or error of the following Java code?

java

public class ExceptionTest {
    public static void main(String[] args) {
        try {
            int result = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Caught ArithmeticException");
        } catch (Exception e) {
            System.out.println("Caught general Exception");
        }
    }
}

Q1438 hard code output

What is the output of this Java code?

java

import java.util.PriorityQueue;
import java.util.Comparator;

class MutableInt {
    int value;
    MutableInt(int v) { this.value = v; }
    void setValue(int v) { this.value = v; }
    @Override public String toString() { return String.valueOf(value); }
}

public class QueueTest {
    public static void main(String[] args) {
        PriorityQueue<MutableInt> pq = new PriorityQueue<>(Comparator.comparingInt(mi -> mi.value));

        MutableInt m1 = new MutableInt(5);
        MutableInt m2 = new MutableInt(10);
        MutableInt m3 = new MutableInt(2);

        pq.offer(m1);
        pq.offer(m2);
        pq.offer(m3);

        m3.setValue(15);

        String result = "";
        result += pq.poll().value + " ";
        result += pq.poll().value + " ";
        result += pq.poll().value;

        System.out.println(result);
    }
}

Q1439 easy code output

What is the output of this Java code?

java

class Product {
    String name;
    Product(String name) {
        this.name = name;
        System.out.println("Product: " + this.name);
    }
}
public class Main {
    public static void main(String[] args) {
        Product p = new Product("Laptop");
    }
}

Q1440 medium code output

What is the output of this code?

java

class Counter {
    int count = 0;
    static int totalCount = 0;

    public Counter() {
        count++;
        totalCount++;
    }
}

public class Main {
    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        c1.count++;
        Counter.totalCount++;
        System.out.println("c1.count: " + c1.count);
        System.out.println("c2.count: " + c2.count);
        System.out.println("totalCount: " + Counter.totalCount);
    }
}

☕ Java MCQ Questions – Page 72