Java MCQ Questions – Page 144 of 200

Q: If a functional interface has a single abstract method with the signature `void process(String message)`, which of the following method references would be compatible?

B) `System.out::println`. The `System.out::println` method reference is compatible because `System.out.println(String)` has a `void` return type and accepts a single `String` argument, matching the `void process(String message)` signature of the functional interface.

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

A) Compile-time error: 'continue' outside of loop. The `continue` statement is exclusively used to skip the rest of the current iteration of a loop. It has no meaning and is not allowed inside a `switch` statement unless the `switch` itself is contained within a loop and `continue` targets that outer loop. Using `continue` directly in a `switch` like this results in a compile-time error.

Q: Which of the following scenarios is a `do-while` loop particularly well-suited for in Java?

B) Executing a block of code at least once, typically for user input validation or menu displays, before checking a termination condition.. The `do-while` loop's guarantee of at least one execution makes it ideal for situations where initial setup or input is needed before a condition can be meaningfully checked.

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

A) error: 'else' without 'if'. The semicolon after `if (value > 5)` terminates the `if` statement, making its body an empty statement. The subsequent block `{...}` is then treated as a standalone block, leading to the `else` statement having no `if` to pair with.

Q: What is the runtime error when the `main` method is executed?

A) java.lang.NullPointerException. In Java, you cannot throw a `null` reference. If an expression whose type is `Throwable` evaluates to `null` and is used in a `throw` statement, a `java.lang.NullPointerException` will be thrown at runtime.

Q: What is the output of this Java code?

A) Error: java.io.EOFException. The code attempts to deserialize an object from a byte array that has been intentionally truncated. When `readObject` tries to read beyond the available bytes in the stream, it results in an `EOFException` (End Of File Exception).

Q: What error occurs when compiling this code, assuming 'data.txt' exists?

A) Compile-time error: unreported exception java.io.IOException; must be caught or declared to be thrown. The `read()` method of `FileReader` (inherited from `Reader`) is declared to throw `IOException`. While `FileNotFoundException` is caught, a general `IOException` from `read()` is not explicitly handled by a `try-catch` block or declared in the `main` method's signature, leading to a compile-time error.

Q: When performing a large number of `String` concatenations in a loop (e.g., `str = str + char;`), which approach is generally recommended for better performance?

C) Using `StringBuilder` or `StringBuffer`.. Repeated `+` string concatenation creates many intermediate `String` objects, leading to performance overhead and increased memory usage. `StringBuilder` (or `StringBuffer` for thread-safety) provides a mutable sequence of characters, allowing efficient appending without creating new objects for each modification.

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

A) Compile-time error: 'the switch case block does not contain a result expression'. In a `switch` expression, if an `->` branch uses a block (`{}`), it must explicitly `yield` a value. Omitting the `yield` statement within the block results in a compile-time error because the expression branch needs a value.

Q: What compilation error occurs in this Java code due to encapsulation rules for final fields?

A) Compile-time error: 'variable x might not have been initialized'. A final instance variable must be initialized exactly once, either at its declaration or in every constructor. Since 'x' is not initialized in the constructor, the compiler reports an error about the variable potentially not being initialized.

Q2861 medium

If a functional interface has a single abstract method with the signature `void process(String message)`, which of the following method references would be compatible?

Q2862 medium code error

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

java

public class Test {
    public static void main(String[] args) {
        int day = 3;
        switch (day) {
            case 1:
                System.out.println("Monday");
                break;
            case 3:
                System.out.println("Wednesday");
                continue;
            case 5:
                System.out.println("Friday");
                break;
        }
    }
}

Q2863 medium

Which of the following scenarios is a `do-while` loop particularly well-suited for in Java?

Q2864 medium code error

What error will occur when compiling this Java code?

java

public class ConditionCheck {
    public static void main(String[] args) {
        int value = 10;
        if (value > 5);
        {
            System.out.println("Value is greater than 5.");
        }
        else {
            System.out.println("Value is 5 or less.");
        }
    }
}

Q2865 hard code error

What is the runtime error when the `main` method is executed?

java

class MyCustomException extends Exception {
    public MyCustomException(String message) { super(message); }
}

public class Main {
    public static MyCustomException createNullException() {
        return null;
    }

    public static void main(String[] args) {
        MyCustomException e = createNullException();
        throw e; // This line attempts to throw a null reference, causing a runtime error
    }
}

Q2866 medium code error

What is the output of this Java code?

java

import java.io.*;

class MySimpleObject implements Serializable {
    String data;
    public MySimpleObject(String data) { this.data = data; }
}

public class DeserializationError9 {
    public static void main(String[] args) {
        byte[] fullData = null;
        try (ByteArrayOutputStream bos = new ByteArrayOutputStream();
             ObjectOutputStream oos = new ObjectOutputStream(bos)) {
            oos.writeObject(new MySimpleObject("Complete"));
            fullData = bos.toByteArray();
        } catch (IOException e) { e.printStackTrace(); }

        // Create a truncated byte array
        byte[] truncatedData = new byte[fullData.length / 2];
        System.arraycopy(fullData, 0, truncatedData, 0, truncatedData.length);

        try (ByteArrayInputStream bis = new ByteArrayInputStream(truncatedData);
             ObjectInputStream ois = new ObjectInputStream(bis)) {
            MySimpleObject obj = (MySimpleObject) ois.readObject();
            System.out.println("Deserialized: " + obj.data);
        } catch (Exception e) {
            System.out.println("Error: " + e.getClass().getName() + ": " + e.getMessage());
        }
    }
}

Q2867 medium code error

What error occurs when compiling this code, assuming 'data.txt' exists?

java

import java.io.FileReader;
import java.io.FileNotFoundException;
import java.io.IOException;

public class MyClass {
    public static void main(String[] args) {
        FileReader reader = null;
        try {
            reader = new FileReader("data.txt");
            int charCode = reader.read(); // Potential IOException here
            System.out.println("Character read: " + (char)charCode);
        } catch (FileNotFoundException e) {
            System.err.println("File not found: " + e.getMessage());
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    System.err.println("Error closing reader: " + e.getMessage());
                }
            }
        }
    }
}

Q2868 medium

When performing a large number of `String` concatenations in a loop (e.g., `str = str + char;`), which approach is generally recommended for better performance?

Q2869 hard code error

What is the compile-time error in the following Java code snippet?

java

public class SwitchBlockNoYieldError {
    public static void main(String[] args) {
        int x = 1;
        String result = switch (x) {
            case 1 -> {
                System.out.println("Case 1 executed"); // Error expected here
            }
            case 2 -> {
                System.out.println("Case 2 executed");
                yield "Two";
            }
            default -> "Other";
        };
        System.out.println(result);
    }
}

Q2870 easy code error

What compilation error occurs in this Java code due to encapsulation rules for final fields?

java

class Coordinate {
    private final int x;
    public Coordinate() {
        // x = 0; // Missing initialization
    }
}

public class Map {
    public static void main(String[] args) {
        Coordinate point = new Coordinate();
    }
}

Q2871 hard code error

What is the output of this code?

java

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

public class RestrictedDirectoryFileCreation {
    public static void main(String[] args) {
        File restrictedDir = new File("restricted_test_dir");
        File newFile = new File(restrictedDir, "new_restricted_file.txt");
        try {
            restrictedDir.mkdir();
            // Make directory unwritable for everyone
            restrictedDir.setWritable(false, false);
            System.out.println("Directory writable: " + restrictedDir.canWrite());

            // Attempt to create a file in a non-writable directory
            boolean created = newFile.createNewFile();
            System.out.println("File created: " + created);

        } catch (IOException e) {
            System.out.println("Error: " + e.getClass().getSimpleName() + ": " + e.getMessage());
        } finally {
            // Cleanup: Need to make it writable again to delete contents and directory
            restrictedDir.setWritable(true, false);
            newFile.delete();
            restrictedDir.delete();
        }
    }
}

Q2872 medium

What are the default values for elements in a newly declared `int[][] myMatrix = new int[2][3];` if no explicit values are assigned?

Q2873 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> cities = new ArrayList<>();
        cities.add("London");
        cities.add("Paris");
        StringBuilder result = new StringBuilder();
        for (String city : cities) {
            result.append(city).append(" ");
        }
        System.out.println(result.toString().trim());
    }
}

Q2874 easy code error

What error will this code produce?

java

public class LoopError {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30};
        for (int i = 0; i <= numbers.length; i++) {
            System.out.println(numbers[i]);
        }
    }
}

Q2875 medium code error

What is the compilation error in the following code?

java

class A {
    protected void printMessage() {
        System.out.println("Hello from A");
    }
}

class B extends A {
    @Override
    private void printMessage() {
        System.out.println("Hello from B");
    }
}

Q2876 medium

When an unlabeled `continue` statement is encountered within the innermost loop of a nested loop structure, what does it do?

Q2877 hard code error

What will be the output when executing this Java program that attempts to manage immutability for a list?

java

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

final class ImmutableConfig {
    private final List<String> items;

    public ImmutableConfig(List<String> items) {
        this.items = Collections.unmodifiableList(new ArrayList<>(items)); // Defensive copy
    }

    public List<String> getItems() {
        return items; // Returns the unmodifiable list directly
    }
}

public class ImmutableWrapperError {
    public static void main(String[] args) {
        List<String> mutableSource = new ArrayList<>();
        mutableSource.add("Setting1");
        ImmutableConfig config = new ImmutableConfig(mutableSource);
        config.getItems().add("Setting2"); // Attempt to modify through getter
    }
}

Q2878 easy

Which of the following is true about a mutable object in Java?

Q2879 easy

What is the average time complexity for `get()` and `put()` operations in a Java HashMap?

Q2880 hard code output

What is the output of this code?

java

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

public class Test {
    public static void main(String[] args) {
        LinkedList<String> list = new LinkedList<>();
        list.add("A");
        list.add("B");
        list.add("C");
        Iterator<String> it = list.iterator();
        while (it.hasNext()) {
            String element = it.next();
            if (element.equals("B")) {
                list.remove(element); 
            }
        }
        System.out.println(list);
    }
}

☕ Java MCQ Questions – Page 144