Java MCQ Questions – Page 109 of 200

Q: Which of the following statements correctly describes how `PriorityQueue` handles elements when no explicit `Comparator` is provided?

B) Elements must implement the `Comparable` interface.. If no `Comparator` is provided, `PriorityQueue` uses the natural ordering of its elements, meaning the elements must implement the `Comparable` interface to define this natural ordering.

Q: What is the output of this Java code snippet using a `switch` expression with a `null` case (Java 17+)?

B) Null Input. In Java 17+, `switch` expressions can explicitly handle `null` using `case null`. Since 'input' is `null`, it matches `case null`, and "Null Input" is yielded, preventing a `NullPointerException`.

Q: What is the compile-time error in this Java code?

A) Compile-time error: unreported exception InterruptedException; must be caught or declared to be thrown (at Line 3). The `run()` method defined in the `Runnable` interface does not declare any checked exceptions. Therefore, an overridden `run()` method cannot throw a checked exception like `InterruptedException` without catching it internally.

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

A) Compile-time error: Incompatible types, String cannot be converted to double.. The array `prices` is declared to hold `double` values. Attempting to assign a `String` literal ("thirty") to a `double` array element causes a compile-time type mismatch error.

Q: What is the output of this Java code?

A) Child priority: 10 Main priority: 5. The child thread `t` is explicitly set to `Thread.MAX_PRIORITY` (10). The main thread, by default, runs at `Thread.NORM_PRIORITY` (5). `t.join()` ensures the child thread completes and prints its priority before the main thread prints its own.

Q: What does this code print?

A) Log: Application started. Abstract classes can contain `final` concrete methods. These methods are inherited by subclasses and can be called directly, but cannot be overridden. The `logMessage` method is called from the `ConsoleLogger` instance.

Q: When an `int` array is declared and initialized with a specific size (e.g., `new int[3]`) but without explicit values, what are the default values of its elements?

C) 0. For numeric primitive types like `int`, elements of a newly created array are automatically initialized to 0.

Q: What will happen when this Java code is compiled?

C) A compilation error: incompatible types: boolean cannot be converted to int.. In Java, a boolean value cannot be directly assigned to an integer variable, and vice-versa, without explicit conversion logic. This is a type mismatch error caught at compile time.

Q: In an inheritance hierarchy, what is the correct order of execution for instance initialization blocks and constructors when a subclass object is instantiated?

B) Superclass instance blocks, superclass constructor, subclass instance blocks, subclass constructor.. When an object is created, the initialization process flows from the top of the hierarchy down. For each class, instance initializer blocks run first, followed by the constructor. This means superclass instance blocks run, then the superclass constructor, then subclass instance blocks, and finally the subclass constructor.

Q: What is the compilation error in the provided Java code?

A) Illegal combination of modifiers: abstract and static. An abstract method cannot be declared static. Abstract methods are instance methods meant to be overridden by concrete subclasses, while static methods belong to the class itself and cannot be overridden. Also, this `Button` class is not declared `abstract` while having an `abstract` method, which is another error, but the `static abstract` combination is the primary, most direct compilation error for that specific line.

Q2161 medium

Which of the following statements correctly describes how `PriorityQueue` handles elements when no explicit `Comparator` is provided?

Q2162 medium code output

What is the output of this Java code snippet using a `switch` expression with a `null` case (Java 17+)?

java

public class SwitchTest {
    public static void main(String[] args) {
        String input = null;
        String output;
        try {
            output = switch (input) {
                case "VALID" -> "Processed";
                case null -> "Null Input";
                default -> "Other";
            };
        } catch (NullPointerException e) {
            output = "NPE Caught";
        }
        System.out.println(output);
    }
}

Q2163 hard code error

What is the compile-time error in this Java code?

java

public class TaskWithCheckedException implements Runnable {
    @Override
    public void run() throws InterruptedException { // Line 3
        Thread.sleep(100);
        System.out.println("Task finished.");
    }

    public static void main(String[] args) {
        new Thread(new TaskWithCheckedException()).start();
    }
}

Q2164 easy code error

What is the error in the following Java code snippet?

java

public class ArrayError {
    public static void main(String[] args) {
        double[] prices = new double[5];
        prices[0] = 10.5;
        prices[1] = 20;
        prices[2] = "thirty";
    }
}

Q2165 medium code output

What is the output of this Java code?

java

class PriorityTask implements Runnable {
    public void run() {
        System.out.println("Child priority: " + Thread.currentThread().getPriority());
    }
}
public class Main {
    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(new PriorityTask(), "HighPri");
        t.setPriority(Thread.MAX_PRIORITY);
        t.start();
        t.join();
        System.out.println("Main priority: " + Thread.currentThread().getPriority());
    }
}

Q2166 easy code output

What does this code print?

java

abstract class Logger {
    final void logMessage(String message) {
        System.out.println("Log: " + message);
    }
    abstract void logDetails();
}

class ConsoleLogger extends Logger {
    @Override
    void logDetails() {
        System.out.println("Details logged to console.");
    }
}

public class Test {
    public static void main(String[] args) {
        ConsoleLogger cl = new ConsoleLogger();
        cl.logMessage("Application started");
    }
}

Q2167 easy

When an `int` array is declared and initialized with a specific size (e.g., `new int[3]`) but without explicit values, what are the default values of its elements?

Q2168 easy code error

What will happen when this Java code is compiled?

java

public class MyClass {
    public static void main(String[] args) {
        boolean isValid = true;
        int status = isValid;
        System.out.println(status);
    }
}

Q2169 hard

In an inheritance hierarchy, what is the correct order of execution for instance initialization blocks and constructors when a subclass object is instantiated?

Q2170 medium code error

What is the compilation error in the provided Java code?

java

abstract class Component {
    abstract void render();
}

public class Button extends Component {
    void render() {
        System.out.println("Rendering Button");
    }
    
    public static abstract void click(); // Problematic method
}

Q2171 medium code error

What error will this Java code generate during compilation?

java

public class LoopTest {
    public static void main(String[] args) {
        boolean flag = false;
        while (flag = "true") {
            System.out.println("Inside loop");
        }
    }
}

Q2172 easy code output

What is the output of this code?

java

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

public class Main {
    public static void main(String[] args) {
        Map<Character, Integer> counts = new HashMap<>();
        counts.put('A', 1);
        counts.put('B', 2);
        counts.put('C', 3);
        counts.remove('B');
        System.out.println(counts.size());
    }
}

Q2173 medium

Which characteristic typically leads to an infinite `for` loop in Java?

Q2174 easy code error

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

java

abstract class Employee {
    abstract void calculateSalary();
    public void details() {
        System.out.println("Employee details");
    }
}

class Manager extends Employee {
    // Manager class does not implement calculateSalary()
    public void report() {
        System.out.println("Manager report");
    }
}

public class Main {
    public static void main(String[] args) {
        // ... (code would try to create Manager object if no compile error)
    }
}

Q2175 hard code output

What does this code print?

java

import java.util.TreeSet;

public class App {
    static class Item implements Comparable<Item> {
        int id; String description;
        public Item(int id, String description) { this.id = id; this.description = description; }
        @Override public int compareTo(Item other) { return Integer.compare(this.id, other.id); } // Orders by ID
        @Override public boolean equals(Object obj) {
            if (this == obj) return true;
            if (!(obj instanceof Item)) return false;
            Item item = (Item) obj;
            return id == item.id && description.equals(item.description);
        }
        @Override public String toString() { return id + "-" + description; }
    }
    public static void main(String[] args) {
        TreeSet<Item> items = new TreeSet<>();
        items.add(new Item(1, "Pen"));
        items.add(new Item(2, "Book"));
        items.remove(new Item(1, "Pencil")); // Same ID, different description
        System.out.println(items.size());
    }
}

Q2176 easy code output

What is the output of this code?

java

import java.util.ArrayList;

public class Test {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        System.out.println(names.contains("Bob"));
    }
}

Q2177 easy code error

What error will occur when compiling the following Java code?

java

public class Main {
    public static void main(String[] args) {
        int value = 5;
        (value + 10) = 20;
        System.out.println(value);
    }
}

Q2178 medium

When using the `throw` keyword, which of the following types of objects must be thrown?

Q2179 hard code output

What is the output of this code?

java

public class OperatorChallenge {
    public static void main(String[] args) {
        int x = -10;
        System.out.println(x >> 2);
        System.out.println(x >>> 2);
    }
}

Q2180 medium

Which of the following is a key characteristic that distinguishes a `while` loop from a `do-while` loop?

☕ Java MCQ Questions – Page 109