Java MCQ Questions – Page 125 of 200

Q: What error will this code produce at compile time?

A) incompatible types: int cannot be converted to boolean. The middle part of a `for` loop is the condition, which must evaluate to a boolean. `int j = 0` is a variable declaration, not a boolean expression, leading to a compile-time error.

Q: When a variable is declared as `volatile` in Java, which crucial guarantee is provided regarding its value in a multi-threaded environment?

C) It guarantees that reads of the variable will always see the most recent write to that variable by any thread, and writes will be immediately visible to other threads.. `volatile` ensures memory visibility, meaning changes to the variable by one thread are immediately visible to others, establishing a happens-before relationship. It does not guarantee atomicity for compound operations.

Q: Can a Java class implement the `Runnable` interface and also extend another class simultaneously?

B) Yes, a class can extend one class and implement multiple interfaces.. Java supports single inheritance for classes but multiple inheritance for interfaces. Therefore, a class can extend one superclass and implement any number of interfaces, including `Runnable`.

Q: What does this code print?

A) true true false true false. The combined predicate evaluates as `(num % 2 == 0 && num > 0) || (num >= 10)`. Each `test` call applies this logic sequentially. For example, `test(-3)` is `(false && false) || false` which is `false`.

Q: What is the output of this code?

A) false false false true. toLowerCase() creates a new String object if the case is changed. s2 will be a new 'hello' object on the heap. s3 is a literal 'hello' from the String Pool. '==' compares references, 'equals()' compares content.

Q: What keyword is used to call another constructor within the same class from a constructor in Java?

B) this(). The `this()` keyword is used for constructor chaining, allowing one constructor to call another constructor within the same class.

Q: Which of the following built-in functional interfaces is commonly used with lambda expressions to represent a boolean-valued function of one argument?

D) `java.util.function.Predicate`. The `Predicate` functional interface defines a single abstract method `test(T t)` that returns a boolean, making it ideal for filtering or checking conditions.

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

A) java.lang.NullPointerException. PriorityQueue does not permit null elements. Adding a null element to a PriorityQueue will result in a java.lang.NullPointerException.

Q: What is the output of this Java code?

A) 024. The loop checks numbers from 0 to 4. It prints only the numbers that are divisible by 2 (even numbers): 0, 2, and 4.

Q: If the `indexOf()` method is used to search for a character or substring that does not exist within a given String, what value does it return?

B) -1. The `indexOf()` method returns the index of the first occurrence of the specified substring or character. If the substring or character is not found, it returns -1.

Q2481 hard code error

What error will this code produce at compile time?

java

public class InvalidForCondition {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 0; int j = 0; i < 5; i++, j++) {
            sum += i + j;
        }
        System.out.println(sum);
    }
}

Q2482 hard

When a variable is declared as `volatile` in Java, which crucial guarantee is provided regarding its value in a multi-threaded environment?

Q2483 easy

Can a Java class implement the `Runnable` interface and also extend another class simultaneously?

Q2484 hard code output

What does this code print?

java

import java.util.function.Predicate;

public class PredicateChaining {
    public static void main(String[] args) {
        Predicate<Integer> isEven = num -> num % 2 == 0;
        Predicate<Integer> isPositive = num -> num > 0;
        Predicate<Integer> isLessThanTen = num -> num < 10;

        Predicate<Integer> combinedPredicate = 
            isEven.and(isPositive).or(isLessThanTen.negate());

        System.out.println(combinedPredicate.test(4));
        System.out.println(combinedPredicate.test(15));
        System.out.println(combinedPredicate.test(-3));
        System.out.println(combinedPredicate.test(10));
        System.out.println(combinedPredicate.test(9));
    }
}

Q2485 medium code output

What is the output of this code?

java

String s1 = "HELLO";
String s2 = s1.toLowerCase();
String s3 = "hello";
System.out.println(s1 == s2);
System.out.println(s2 == s3);
System.out.println(s1.equals(s2));
System.out.println(s2.equals(s3));

Q2486 easy

What keyword is used to call another constructor within the same class from a constructor in Java?

Q2487 medium

Which of the following built-in functional interfaces is commonly used with lambda expressions to represent a boolean-valued function of one argument?

Q2488 hard code error

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

java

import java.util.PriorityQueue;
import java.util.Queue;

public class QError1 {
    public static void main(String[] args) {
        Queue<String> pq = new PriorityQueue<>();
        pq.add("apple");
        pq.add(null);
        System.out.println(pq.poll());
    }
}

Q2489 easy code output

What is the output of this Java code?

java

public class LoopTest {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            if (i % 2 == 0) {
                System.out.print(i);
            }
        }
    }
}

Q2490 medium

If the `indexOf()` method is used to search for a character or substring that does not exist within a given String, what value does it return?

Q2491 medium code output

What is the output of this code?

java

String str1 = "Hello";
String str2 = "Hello";
String str3 = new String("Hello");
System.out.println(str1 == str2);
System.out.println(str1.equals(str2));
System.out.println(str1 == str3);
System.out.println(str1.equals(str3));

Q2492 easy code error

What error will this Java code produce when executed?

java

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

Q2493 hard code output

What is the output of this code?

java

import java.lang.reflect.Constructor;
class Restricted {
    private String name;
    private Restricted(String name) {
        this.name = name;
        System.out.println("Restricted instance: " + name);
    }
}
public class Main {
    public static void main(String[] args) throws Exception {
        Constructor<Restricted> ctor = Restricted.class.getDeclaredConstructor(String.class);
        ctor.setAccessible(true);
        Restricted r1 = ctor.newInstance("First");
        Restricted r2 = ctor.newInstance("Second");
        System.out.println("Main method ends.");
    }
}

Q2494 easy code error

What is the error in the following Java code?

java

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(10);
        numbers.add("20");
    }
}

Q2495 medium code error

What error will occur when running this Java code snippet?

java

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

public class Test {
    public static void main(String[] args) {
        Map<String, String> settings = new HashMap<>();
        settings.put("theme", "dark");
        settings.put("language", null); // Storing a null value
        
        String lang = settings.get("language");
        System.out.println(lang.toUpperCase()); // Attempt to call method on null
    }
}

Q2496 hard code output

Consider a custom `BadReader` class that extends `FileReader` and overrides `read()` to always throw an `IOException` and also overrides `close()` to always throw an `IOException`. Assume `data.txt` exists. What is the output of this code?

java

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

public class FileReaderTryWithResourcesException {
    static class BadReader extends FileReader {
        public BadReader(File file) throws IOException { super(file); }
        @Override
        public int read() throws IOException { throw new IOException("Read error!"); }
        @Override
        public void close() throws IOException { throw new IOException("Close error!"); }
    }

    public static void main(String[] args) {
        // Assume 'data.txt' exists
        try (BadReader reader = new BadReader(new File("data.txt"))) {
            reader.read();
        } catch (IOException e) {
            System.out.println("Caught: " + e.getMessage());
            Throwable[] suppressed = e.getSuppressed();
            if (suppressed.length > 0) {
                System.out.println("Suppressed: " + suppressed[0].getMessage());
            }
        }
    }
}

Q2497 easy

What is the default name given to the first user-created `Thread` if not specified explicitly?

Q2498 medium code output

What is the output of this code?

java

public class StringBufferTest {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("Hello");
        sb.append(" World").append(123);
        System.out.print(sb.capacity());
    }
}

Q2499 easy

Encapsulation is often described as bundling data (attributes) and methods (behaviors) that operate on the data into a single unit. What is this unit called in Java?

Q2500 easy code output

What is the output of this Java code?

java

public class SleepExample {
    public static void main(String[] args) {
        Thread thread = new Thread(() -> {
            try {
                Thread.sleep(100);
                System.out.println("Thread woke up.");
            } catch (InterruptedException e) {
                System.out.println("Thread interrupted.");
            }
        });
        thread.start();
        System.out.println("Main finished.");
    }
}

☕ Java MCQ Questions – Page 125