Java MCQ Questions – Page 19 of 200

Q: Why are `String` objects immutable in Java?

B) To enhance security, enable thread-safety, and allow `String` to be used as hash map keys.. Immutability of `String` objects is crucial for security (e.g., when used as network parameters or file paths), thread-safety (no need for synchronization), and performance when used as keys in hash-based collections as their hash code is cached and stable.

Q: What does this code print?

A) Generating error... Caught generic exception: NullPointerException Process complete.. The `generateError` method throws a `NullPointerException`. The `main` method's `catch (Exception e)` block is a polymorphic catch that can handle any `Exception` and its subclasses, including `NullPointerException`. It then prints the simple name of the caught exception class before the program continues.

Q: Consider the following code snippet: java interface MyFunc1 { void apply(String s); } interface MyFunc2 { void apply(Object o); } public class Ambiguity { public static void process(MyFunc1 func) { System.out.println("Processing MyFunc1"); } public static void process(MyFunc2 func) { System.out.println("Processing MyFunc2"); } public static void main(String[] args) { // Which lambda expression would cause a compile-time error due to ambiguity? } } Which lambda expression, if used in the `main` method, would cause a compile-time error due to ambiguity?

C) `process(s -> {});`. The lambda expression `s -> {}` has an untyped parameter `s`. The compiler cannot infer whether `s` should be `String` (for `MyFunc1`) or `Object` (for `MyFunc2`) because both target types are equally applicable, leading to an ambiguous method call.

Q: In which scenario would `java.util.LinkedList` typically offer better performance than `java.util.ArrayList`?

C) Frequent insertion or deletion of elements in the middle or at the ends of the list.. LinkedList excels in scenarios with frequent insertions and deletions, especially at the ends or middle of the list, due to its O(1) or O(n) (for middle, better than ArrayList's O(n) if shifting many elements) performance for these operations without requiring array reallocations.

Q: What does this code print to the console?

B) false. Boolean arrays, when initialized, have their elements set to `false` by default if not explicitly assigned a value.

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

B) Compilation Error: cannot find symbol method length(). Arrays in Java use the `length` field (without parentheses) to get their size, not the `length()` method which is for String objects. Using `length()` on an array results in a compile-time error.

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

A) The code will compile but throw a ClassCastException at runtime.. Using a raw `TreeSet` (`TreeSet rawSet = new TreeSet();`) allows adding different types of objects, but `TreeSet` relies on natural ordering (or a `Comparator`) to sort elements. When trying to add a `String` after an `Integer`, the `TreeSet` attempts to compare them, resulting in a `ClassCastException`.

Q: What is the output of this Java code?

A) Compile-time error. The variable `x` is declared as a local variable and accessed from a lambda expression. Local variables accessed from lambdas must be 'effectively final'. Reassigning `x` with `x = 15;` after `closure` is defined makes `x` no longer effectively final, resulting in a compile-time error.

Q: What is the output of this code?

A) Outer Try Inner Try Inner Finally After Inner Blocks Outer Finally. Execution proceeds sequentially. The outer try prints, then the inner try, then the inner finally. After inner blocks complete, the outer try continues, then the outer finally executes.

Q: What is a primary advantage of using immutable objects in a multi-threaded Java application?

B) They are inherently thread-safe and do not require explicit synchronization mechanisms.. Since their state cannot change, immutable objects can be safely shared across multiple threads without the need for locks or synchronization, simplifying concurrent programming.

Q361 medium

Why are `String` objects immutable in Java?

Q362 easy code output

What does this code print?

java

public class MyClass {
    public static void generateError() {
        System.out.println("Generating error...");
        throw new NullPointerException("Object is null");
    }

    public static void main(String[] args) {
        try {
            generateError();
        } catch (Exception e) {
            System.out.println("Caught generic exception: " + e.getClass().getSimpleName());
        }
        System.out.println("Process complete.");
    }
}

Q363 hard

Consider the following code snippet: java interface MyFunc1 { void apply(String s); } interface MyFunc2 { void apply(Object o); } public class Ambiguity { public static void process(MyFunc1 func) { System.out.println("Processing MyFunc1"); } public static void process(MyFunc2 func) { System.out.println("Processing MyFunc2"); } public static void main(String[] args) { // Which lambda expression would cause a compile-time error due to ambiguity? } } Which lambda expression, if used in the `main` method, would cause a compile-time error due to ambiguity?

Q364 medium

In which scenario would `java.util.LinkedList` typically offer better performance than `java.util.ArrayList`?

Q365 easy code output

What does this code print to the console?

java

public class Test {
    public static void main(String[] args) {
        boolean[] flags = new boolean[2];
        flags[0] = true;
        System.out.println(flags[1]);
    }
}

Q366 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 = {1, 2, 3, 4, 5};
        for (int i = 0; i < numbers.length(); i++) {
            System.out.println(numbers[i]);
        }
    }
}

Q367 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 rawSet = new TreeSet();
        rawSet.add(10);
        rawSet.add("Twenty");
    }
}

Q368 medium code output

What is the output of this Java code?

java

import java.util.function.Supplier;

public class Test {
    public static void main(String[] args) {
        int x = 10;
        Supplier<Integer> closure = () -> {
            return x * 2;
        };
        x = 15; 
        System.out.println(closure.get());
        System.out.println(x);
    }
}

Q369 easy code output

What is the output of this code?

java

public class Main {
    public static void main(String[] args) {
        try {
            System.out.print("Outer Try ");
            try {
                System.out.print("Inner Try ");
            } finally {
                System.out.print("Inner Finally ");
            }
            System.out.print("After Inner Blocks ");
        } finally {
            System.out.print("Outer Finally ");
        }
    }
}

Q370 medium

What is a primary advantage of using immutable objects in a multi-threaded Java application?

Q371 hard code output

What does this code print?

java

public class Test {
    public static int modifyValue() {
        int x = 0;
        try {
            x = 10;
            System.out.print("Try ");
            return x;
        } finally {
            x = 20;
            System.out.print("Finally ");
        }
    }
    public static void main(String[] args) {
        System.out.println(modifyValue());
    }
}

Q372 hard code output

What is the output of this code? (Assuming Java 14+)

java

public class SwitchTest {
    public static void main(String[] args) {
        int day = 7;
        String dayType = switch (day) {
            case 1, 7 -> "Weekend";
            case 2, 3, 4, 5, 6 -> "Weekday";
            default -> "Invalid Day";
        };
        System.out.println(dayType);
    }
}

Q373 hard code output

What does this code print to the console?

java

class Product { public String getItem() { return "Generic Product"; } }
class Book extends Product { public String getTitle() { return "My Book"; } }
class MyStore { public Product sell() { return new Product(); } }
class Bookstore extends MyStore { @Override public Book sell() { return new Book(); } }
public class Main {
    public static void main(String[] args) {
        MyStore store = new Bookstore();
        Product p = store.sell();
        System.out.println(p.getClass().getSimpleName());
    }
}

Q374 medium code output

What will be printed to the console by this Java program?

java

public class StringBufferTest {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("Start");
        sb.append(10).insert(3, 'X').reverse();
        System.out.print(sb);
    }
}

Q375 medium code output

What does this Java code print?

java

import java.util.HashSet;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Set<String> fruits = new HashSet<>();
        fruits.add("apple");
        fruits.add("banana");
        fruits.add("cherry");
        fruits.remove("banana");
        System.out.println(fruits.contains("banana"));
    }
}

Q376 medium code output

What does this code print?

java

import java.util.TreeMap;

public class TreeMapRemoval {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(10, "A");
        map.put(20, "B");
        map.put(30, "C");
        map.remove(20);
        System.out.println(map.containsKey(20) + "," + map.size());
    }
}

Q377 medium code output

What is the output of this program?

java

class ConfigurationError extends Error {
    public ConfigurationError(String message) {
        super(message);
    }
}

public class Main {
    public static void loadConfig() {
        throw new ConfigurationError("Missing config file");
    }

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

Q378 easy code error

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

java

public class LoopError {
    public static void main(String[] args) {
        int num = 0;
        do {
            System.out.println(num);
            num++;
        }
        while (num < 2);
    }
}

Q379 easy

Can an `ArrayList` store `null` values?

Q380 hard

Which of the following `BlockingQueue` implementations offers a constructor parameter to configure 'fairness', influencing the order in which waiting producer and consumer threads acquire access to the queue?

☕ Java MCQ Questions – Page 19