Java MCQ Questions – Page 102 of 200

Q: What is the compilation error, if any, for the provided Java code snippet?

B) error: reference to handle is ambiguous. Both `String` and `Integer` are reference types, and `null` can be assigned to either. Since neither type is a subtype of the other, the compiler cannot determine which overloaded `handle` method is more specific, resulting in an ambiguous method call error.

Q: Which method is commonly used to write a `String` to a file using `FileWriter`?

D) write(String s). The `write(String s)` method is a convenient way to output an entire string of characters to the file.

Q: What does this code print?

A) --- Composed (addThree then multiplyByTwo) --- Add 3: 5 Multiply by 2: 8 Result: 16 --- AndThen (multiplyByTwo then addThree) --- Multiply by 2: 5 Add 3: 10 Result: 13. `Function.compose(before)` applies the `before` function first, then `this` function. `Function.andThen(after)` applies `this` function first, then the `after` function. The print statements help trace the execution flow.

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

A) Runtime error: UnsupportedOperationException.. The 'items' list is wrapped using 'Collections.unmodifiableList', making it immutable from outside the DataStore class. Any attempt to modify this list (e.g., by calling 'add()') after it has been made unmodifiable will result in an UnsupportedOperationException at runtime.

Q: What is the output of this code?

A) Hello World. The `put` and `take` methods use `wait()` and `notifyAll()` correctly within `synchronized` blocks to implement a simple producer-consumer pattern. The consumer waits until a message is available, and the producer puts a message and notifies the consumer, ensuring "Hello World" is printed.

Q: What does this code print?

A) Try 0 Finally 0 Try 1 After continue 1 Finally 1 End loop 1. The `finally` block executes even if control flow is altered by `continue` or `break`. For `i=0`, `finally` executes, then `continue` skips the rest of the loop body for that iteration. For `i=1`, all statements execute.

Q: What is the output of this code?

A) true true 1 true true 0. `HashSet` allows exactly one `null` element. `set.contains(null)` returns `true` because null was added. `set.remove("Hello")` returns `true` and removes "Hello". `set.remove(new String("World"))` returns `true` because `String`'s `equals()` and `hashCode()` correctly identify the existing "World" element. Finally, `set.contains(null)` is still true as null was never removed, and the size becomes 0 after removing all other elements.

Q: What is the output of this code?

C) 0. After adding 1 and 2, and removing 1, the queue contains [2]. The `clear()` method then removes all elements from the queue, making its size 0.

Q: What does this code print?

A) List is empty.. For an empty list, `it.hasNext()` immediately returns `false`. Therefore, the `else` block is executed, printing 'List is empty.'.

Q: What is the output of this code?

A) 1: TIMED_WAITING Child thread finished. 2: TERMINATED. A thread calling `Thread.sleep(long)` enters the TIMED_WAITING state. After the sleep duration, it transitions back to RUNNABLE, finishes its `run()` method, and then becomes TERMINATED.

Q2021 hard code error

What is the compilation error, if any, for the provided Java code snippet?

java

class Overloader {
    public void handle(String s) {}
    public void handle(Integer i) {}

    public static void main(String[] args) {
        Overloader o = new Overloader();
        o.handle(null); // Line X
    }
}

Q2022 easy

Which method is commonly used to write a `String` to a file using `FileWriter`?

Q2023 hard code output

What does this code print?

java

import java.util.function.Function;

public class FunctionComposition {
    public static void main(String[] args) {
        Function<Integer, Integer> multiplyByTwo = x -> {
            System.out.println("Multiply by 2: " + x);
            return x * 2;
        };
        Function<Integer, Integer> addThree = x -> {
            System.out.println("Add 3: " + x);
            return x + 3;
        };

        Function<Integer, Integer> composed = multiplyByTwo.compose(addThree);
        Function<Integer, Integer> andThen = multiplyByTwo.andThen(addThree);

        System.out.println("--- Composed (addThree then multiplyByTwo) ---");
        System.out.println("Result: " + composed.apply(5));

        System.out.println("--- AndThen (multiplyByTwo then addThree) ---");
        System.out.println("Result: " + andThen.apply(5));
    }
}

Q2024 medium code error

What is the error in the following Java code?

java

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

class DataStore {
    private final List<String> items;

    public DataStore(List<String> initialItems) {
        this.items = Collections.unmodifiableList(new ArrayList<>(initialItems));
    }

    public List<String> getItems() {
        return items;
    }
}

public class MainProcessor {
    public static void main(String[] args) {
        List<String> original = new ArrayList<>();
        original.add("Item A");
        DataStore store = new DataStore(original);
        store.getItems().add("Item B"); // Attempt to modify unmodifiable list
    }
}

Q2025 medium code output

What is the output of this code?

java

class MessageQueue {
    private String message;
    private boolean empty = true;

    public synchronized void put(String msg) {
        while (!empty) {
            try { wait(); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
        }
        this.message = msg;
        this.empty = false;
        notifyAll();
    }

    public synchronized String take() {
        while (empty) {
            try { wait(); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
        }
        empty = true;
        notifyAll();
        return message;
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        MessageQueue queue = new MessageQueue();
        Thread producer = new Thread(() -> queue.put("Hello"));
        Thread consumer = new Thread(() -> System.out.println(queue.take() + " World"));

        consumer.start();
        producer.start();

        producer.join();
        consumer.join();
    }
}

Q2026 hard code output

What does this code print?

java

public class Test {
    public static void main(String[] args) {
        for (int i = 0; i < 2; i++) {
            try {
                System.out.print("Try " + i + " ");
                if (i == 0) {
                    continue;
                }
                System.out.print("After continue " + i + " ");
            } finally {
                System.out.print("Finally " + i + " ");
            }
            System.out.print("End loop " + i + " ");
        }
    }
}

Q2027 hard code output

What is the output of this code?

java

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add(null);
        set.add("Hello");
        set.add("World");

        System.out.println(set.contains(null));
        System.out.println(set.remove("Hello"));
        System.out.println(set.size());
        System.out.println(set.remove(new String("World")));
        System.out.println(set.contains(null));
        System.out.println(set.size());
    }
}

Q2028 easy code output

What is the output of this code?

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(1);
        numbers.add(2);
        numbers.remove();
        numbers.clear();
        System.out.println(numbers.size());
    }
}

Q2029 medium 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> emptyList = new ArrayList<>();
        Iterator<String> it = emptyList.iterator();
        if (it.hasNext()) {
            System.out.println("Found: " + it.next());
        } else {
            System.out.println("List is empty.");
        }
    }
}

Q2030 medium code output

What is the output of this code?

java

public class ThreadStateDemo2 {
    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(() -> {
            try {
                Thread.sleep(200); // Simulate work
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
            System.out.println("Child thread finished.");
        });
        t.start();
        Thread.sleep(50); // Give thread time to enter sleep
        System.out.println("1: " + t.getState());
        Thread.sleep(200); // Wait for the thread to finish
        System.out.println("2: " + t.getState());
    }
}

Q2031 easy

When you create a `new File("myFile.txt")` object in Java, what happens on the file system?

Q2032 medium code error

What happens when you run this Java code?

java

public class LoopTest {
    public static void main(String[] args) {
        int[] arr = {10, 20};
        int index = 0;
        do {
            System.out.println(arr[index]);
            index++;
        } while (index <= arr.length);
    }
}

Q2033 medium

When iterating over a `java.util.List` using its `Iterator` and calling `remove()` for an element, what happens to the subsequent elements' indices within the original `List`?

Q2034 hard

Which statement about `String` objects in Java is most accurate regarding their data type characteristics?

Q2035 medium code output

What is the output of this code?

java

String sentence = "Java is a great language, Java!";
int first = sentence.indexOf("Java");
int last = sentence.lastIndexOf("Java");
int missing = sentence.indexOf("Python");
System.out.println(first + "," + last + "," + missing);

Q2036 medium code error

What is the error in this Java code?

java

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

public class MyClass {
    public static void main(String[] args) {
        // Attempt to instantiate an interface directly
        List<String> shoppingList = new List<>();
        shoppingList.add("Milk");
        System.out.println(shoppingList);
    }
}

Q2037 hard code error

What kind of error will occur when executing the following Java code?

java

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

class MyItem {
    String value;
    MyItem(String v) { this.value = v; }
}

public class QError9 {
    public static void main(String[] args) {
        Comparator<MyItem> comparator = (a, b) -> a.value.compareTo(b.value);
        PriorityQueue<MyItem> pq = new PriorityQueue<>(comparator);
        pq.add(new MyItem("alpha"));
        pq.add(new MyItem(null));
        System.out.println(pq.poll().value);
    }
}

Q2038 easy code output

What is the output of this code?

java

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<Character> ts = new TreeSet<>();
        ts.add('c');
        ts.add('a');
        ts.add('b');
        System.out.println(ts);
    }
}

Q2039 hard code error

What is the result of executing this Java code snippet?

java

public class ArrayComponentCast {
    public static void main(String[] args) {
        Object[] stringObjects = new String[2];
        stringObjects[0] = "Hello";
        stringObjects[1] = "World";

        Integer[] integers = (Integer[]) stringObjects; 
        System.out.println(integers[0]);
    }
}

Q2040 hard

Consider a custom functional interface: java @FunctionalInterface interface Transformer<T, R> { R transform(T t); } If we instantiate this interface with `Transformer<String, ? extends Number>`, which of the following lambda expressions would be valid?

☕ Java MCQ Questions – Page 102