Java MCQ Questions – Page 17 of 200

Q: What does this code print?

A) Laptop - 0.0 Mouse - 25.0. The constructor `Product(String name)` uses `this(name, 0.0)` to call the two-argument constructor, initializing price to 0.0. The `toString()` method then formats the output correctly.

Q: What is the compilation error in the following Java code snippet?

A) Compile error: Incompatible types, cannot convert from ArrayList to ArrayList .. Java generics are invariant. `ArrayList ` is not a subtype of `ArrayList `, even though `Integer` is a subtype of `Number`. This assignment causes a compile-time error.

Q: What error will occur when this code is executed?

B) NullPointerException. The `StringBuffer` variable `sb` is explicitly assigned `null`. Attempting to call an instance method like `append()` on a `null` object will result in a `NullPointerException` at runtime.

Q: What is the output of this code?

C) Class AClass B. In the overridden `display` method of class `B`, `super.display()` explicitly calls the `display` method of the superclass `A`, before executing its own print statement. This leads to both outputs concatenated.

Q: Identify the error produced by the following Java code:

A) Compile-time error: 'add(int,int)' has private access in 'Calculator'. The 'add' method is declared as private within the 'Calculator' class, making it inaccessible from outside the class, including the 'App' class's main method.

Q: What is the output of the following Java code?

A) Generating error... Caught Error: Memory exhausted! End of program.. The `generateError` method throws an `OutOfMemoryError`, which is a subclass of `Error`, not `Exception`. The first `catch` block for `Exception` will not catch it. The second `catch` block for `Error` will catch it, printing its message, and the program then continues normally.

Q: What does this code print?

A) Base ctor. Derived display: x=0 Derived ctor.. When `new Derived()` is called, the `Base` constructor executes first. Inside `Base`'s constructor, `display()` is called. Due to polymorphism, `Derived`'s `display()` is invoked. At this point, `Derived`'s instance fields (like `x`) have not yet been initialized to their explicit values, only to default values (0 for `int`). After `Base`'s constructor completes, `Derived`'s constructor and field initializations run.

Q: Regarding `null` elements in a `HashSet`, which statement is true?

A) A `HashSet` can contain at most one `null` element, and it is internally handled as a special case in `HashMap` (its backing map).. `HashSet` permits exactly one `null` element. Its backing `HashMap` stores `null` keys as a special entry at index 0, separate from normal hashing logic.

Q: What is the final value of `counter` printed?

A) Final counter: 5. The semaphore limits concurrent access to the critical section (where `Thread.sleep` and `counter++` happen) to two threads. All five threads will eventually acquire a permit, perform the task, and increment the counter. The `synchronized(this)` ensures the `counter++` is atomic, and the semaphore ensures only a limited number of threads access it at once, but all threads eventually get a turn.

Q: What kind of error will occur when compiling this Java code?

D) Syntax Error: Unexpected token ';'.. The semicolon immediately after the 'if' condition `if (num == 7));` makes the 'if' statement an empty statement. The subsequent curly braces `{ System.out.println("Seven"); }` then form a separate, unconditional block, leading to an unexpected token or syntax error depending on the compiler's interpretation. The most direct cause is the extraneous semicolon making the 'if' condition itself an empty statement.

Q321 medium code output

What does this code print?

java

class Product {
    String name;
    double price;

    public Product(String name) {
        this(name, 0.0);
    }

    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String toString() {
        return name + " - " + price;
    }
}

public class Main {
    public static void main(String[] args) {
        Product p1 = new Product("Laptop");
        Product p2 = new Product("Mouse", 25.0);
        System.out.println(p1);
        System.out.println(p2);
    }
}

Q322 easy code error

What is the compilation error in the following Java code snippet?

java

import java.util.ArrayList;

public class Question8 {
    public static void main(String[] args) {
        ArrayList<Integer> intList = new ArrayList<>();
        intList.add(10);
        ArrayList<Number> numList = intList;
        System.out.println(numList);
    }
}

Q323 easy code error

What error will occur when this code is executed?

java

public class Main {
    public static void main(String[] args) {
        StringBuffer sb = null;
        sb.append("Hello");
        System.out.println(sb);
    }
}

Q324 easy code output

What is the output of this code?

java

class A {
    public void display() {
        System.out.print("Class A");
    }
}

class B extends A {
    @Override
    public void display() {
        super.display();
        System.out.print("Class B");
    }
}

public class Main {
    public static void main(String[] args) {
        B obj = new B();
        obj.display();
    }
}

Q325 easy code error

Identify the error produced by the following Java code:

java

class Calculator {
    private int add(int a, int b) {
        return a + b;
    }
}

public class App {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        int sum = calc.add(5, 3);
        System.out.println(sum);
    }
}

Q326 medium code output

What is the output of the following Java code?

java

class Example {
    static void generateError() {
        System.out.println("Generating error...");
        throw new OutOfMemoryError("Memory exhausted!"); // Error type
    }

    public static void main(String[] args) {
        try {
            generateError();
        } catch (Exception e) {
            System.out.println("Caught Exception: " + e.getMessage());
        } catch (Error e) {
            System.out.println("Caught Error: " + e.getMessage());
        }
        System.out.println("End of program.");
    }
}

Q327 hard code output

What does this code print?

java

class Base {
    public Base() {
        System.out.print("Base ctor. ");
        display();
    }
    public void display() {
        System.out.println("Base display.");
    }
}
class Derived extends Base {
    private int x = 10;
    public Derived() {
        System.out.print("Derived ctor. ");
    }
    @Override
    public void display() {
        System.out.println("Derived display: x=" + x);
    }
}
public class Test {
    public static void main(String[] args) {
        new Derived();
    }
}

Q328 hard

Regarding `null` elements in a `HashSet`, which statement is true?

Q329 hard code output

What is the final value of `counter` printed?

java

import java.util.concurrent.Semaphore;

public class SemaphoreTest {
    private final Semaphore semaphore = new Semaphore(2); // 2 permits
    private int counter = 0;

    public void performTask() {
        try {
            semaphore.acquire();
            Thread.sleep(50); // Simulate work
            synchronized (this) { // Ensure counter increment is atomic
                counter++;
            }
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } finally {
            semaphore.release();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        SemaphoreTest test = new SemaphoreTest();
        for (int i = 0; i < 5; i++) {
            new Thread(test::performTask).start();
        }
        Thread.sleep(1000); // Allow all threads to complete
        System.out.println("Final counter: " + test.counter);
    }
}

Q330 easy code error

What kind of error will occur when compiling this Java code?

java

public class Main {
    public static void main(String[] args) {
        int num = 7;
        if (num == 7)); {
            System.out.println("Seven");
        }
    }
}

Q331 medium code error

What compile-time error will occur in this Java code snippet?

java

public class LoopError {
    public static void main(String[] args) {
        for (int i = 0; i < 3; i++) {
            int i = 10;
            System.out.println(i);
        }
    }
}

Q332 easy code output

What does this 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<String> words = new ArrayList<>();
        Iterator<String> it = words.iterator();
        if (!it.hasNext()) {
            System.out.println("List is empty");
        } else {
            System.out.println(it.next());
        }
    }
}

Q333 medium

Can an `ArrayList` store `null` elements?

Q334 easy

What is the underlying data structure used by `java.util.LinkedList`?

Q335 easy code output

What will be the content of the file 'output.txt' after this code executes?

java

import java.io.FileWriter;
import java.io.IOException;

public class FileWriterTest {
    public static void main(String[] args) {
        char[] data = {'J', 'a', 'v', 'a', ' ', 'I', 'O'};
        try (FileWriter writer = new FileWriter("output.txt")) {
            writer.write(data, 0, 4); // Write 'Java'
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Q336 hard code error

Does this Java code compile? If not, what is the compile-time error?

java

import java.io.IOException;
class MyClass {
    public MyClass() {
        this(true);
    }
    public MyClass(boolean flag) throws IOException {
        if (flag) {
            throw new IOException("Failed to initialize");
        }
    }
}
public class Main {
    public static void main(String[] args) {
        // MyClass obj = new MyClass();
    }
}

Q337 hard code error

What is the output of this code?

java

import java.util.LinkedList;
import java.util.Iterator;
import java.util.Arrays;

public class MyClass {
    public static void main(String[] args) {
        LinkedList<Integer> numbers = new LinkedList<>(Arrays.asList(1, 2, 3, 4, 5));
        Iterator<Integer> iterator = numbers.iterator();
        while (iterator.hasNext()) {
            Integer num = iterator.next();
            if (num % 2 == 0) {
                numbers.remove(num); // Modifying list directly, not via iterator
            }
        }
        System.out.println(numbers);
    }
}

Q338 easy code output

What is the output of the following Java program?

java

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

Q339 medium

To significantly improve the performance of writing many small pieces of character data using `FileWriter`, what other Java I/O class should it typically be wrapped with?

Q340 hard

In a Java 8+ interface, what is the nature of `static` methods with respect to abstraction and inheritance?

☕ Java MCQ Questions – Page 17