Java MCQ Questions – Page 153 of 200

Q: What error occurs when executing the following Java code?

A) java.lang.ClassCastException. Due to type erasure, the compiler cannot prevent adding a `String` to a raw `ArrayList` which was declared with a generic type `Integer`. At runtime, when attempting to cast the retrieved `String` to an `Integer`, a `ClassCastException` occurs.

Q: What error will this Java code produce when executed?

A) ArrayIndexOutOfBoundsException. The `matrix` array is declared with 2 rows (indices 0 and 1). Attempting to access `matrix[2]` goes beyond the valid range, resulting in an `ArrayIndexOutOfBoundsException` at runtime.

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

A) TestShape.java:10: error: Shape is abstract; cannot be instantiated. Abstract classes cannot be instantiated directly using the `new` keyword because they may contain unimplemented abstract methods. They must be subclassed, and the subclass must provide implementations for all abstract methods (unless the subclass is also abstract).

Q: What typically happens if a `HashSet` is structurally modified (e.g., adding or removing elements) while it is being iterated over using its default iterator?

C) A `ConcurrentModificationException` is thrown.. The iterators returned by `HashSet` are fail-fast. Any structural modification to the set during iteration (except through the iterator's own `remove()` method) will cause a `ConcurrentModificationException`.

Q: What is the output of this Java code snippet?

A) 5. The value at `numbers[0]` is initially 1, but it is then reassigned to 5. The `println` statement outputs the updated value.

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

A) br cannot be resolved to a variable. Variables declared inside a `try` block (or any block) are scoped to that block. `br` is not accessible outside the `try` block, leading to a 'cannot be resolved to a variable' error.

Q: In Java's `AbstractList` implementation, the `modCount` field tracks structural modifications. Which of the following modifications to an `ArrayList` instance, performed *after* its `Iterator` has been obtained but *before* the `Iterator` completes, would *not* reliably lead to a `ConcurrentModificationException` upon a subsequent `next()` or `hasNext()` call, assuming it's a single-threaded environment and the modification happens externally to the iterator's own `remove()` method?

C) Changing an element's value using `List.set(index, element)` without changing the list's size.. `ConcurrentModificationException` is typically triggered by structural changes (add, remove, clear) that alter `modCount`. `List.set(index, element)` modifies an existing element but does not change the list's size or internal structure, thus it usually does not increment `modCount` and avoids the exception.

Q: Is `java.util.HashSet` synchronized by default?

B) No, it is not synchronized.. `HashSet` is not synchronized by default. If multiple threads access a hash set concurrently and at least one of the threads modifies the set, it must be synchronized externally.

Q: What is the error in this Java code?

A) This code will throw an `IOException` at runtime because the parent directory `nonexistent_dir` does not exist.. `FileWriter` can create a file if it doesn't exist, but it cannot create parent directories. If `nonexistent_dir` does not exist, creating `nonexistent_dir/output.txt` will result in an `IOException` (specifically, `FileNotFoundException` which is a subclass of `IOException`).

Q: What is the error in this Java code?

B) Compile-time error: cannot inherit from final BaseClass. A class declared as `final` cannot be extended by any other class. Attempting to do so results in a compile-time error.

Q3041 hard code error

What error occurs when executing the following Java code?

java

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

public class Main {
    public static void main(String[] args) {
        ArrayList rawList = new ArrayList<Integer>();
        rawList.add("This is a string"); // Adding a String to a raw ArrayList
        Integer value = (Integer) rawList.get(0);
        System.out.println(value);
    }
}

Q3042 medium code error

What error will this Java code produce when executed?

java

public class ArrayError {
    public static void main(String[] args) {
        int[][] matrix = new int[2][3];
        matrix[0][0] = 1;
        matrix[1][0] = 2;
        System.out.println(matrix[2][0]);
    }
}

Q3043 medium code error

What is the compilation error in the provided Java code?

java

abstract class Shape {
    public abstract void draw();
    public void display() {
        System.out.println("Displaying shape");
    }
}

public class TestShape {
    public static void main(String[] args) {
        Shape s = new Shape(); // ERROR: cannot instantiate abstract class
        s.display();
    }
}

Q3044 medium

What typically happens if a `HashSet` is structurally modified (e.g., adding or removing elements) while it is being iterated over using its default iterator?

Q3045 easy code output

What is the output of this Java code snippet?

java

public class ArrayTest {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3};
        numbers[0] = 5;
        System.out.println(numbers[0]);
    }
}

Q3046 easy code error

What compilation error will occur when compiling this Java code?

java

import java.io.BufferedReader;
import java.io.StringReader;
import java.io.IOException;

public class MyClass {
    public static void main(String[] args) {
        try {
            BufferedReader br = new BufferedReader(new StringReader("data"));
            String line = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println(br);
    }
}

Q3047 hard

In Java's `AbstractList` implementation, the `modCount` field tracks structural modifications. Which of the following modifications to an `ArrayList` instance, performed *after* its `Iterator` has been obtained but *before* the `Iterator` completes, would *not* reliably lead to a `ConcurrentModificationException` upon a subsequent `next()` or `hasNext()` call, assuming it's a single-threaded environment and the modification happens externally to the iterator's own `remove()` method?

Q3048 easy

Is `java.util.HashSet` synchronized by default?

Q3049 easy code error

What is the error in this Java code?

java

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

public class FileWriterError5 {
    public static void main(String[] args) {
        FileWriter writer = null;
        try {
            // Trying to create a file in a non-existent directory
            writer = new FileWriter("nonexistent_dir/output.txt");
            writer.write("Directory issues.");
        } catch (IOException e) {
            System.err.println("Error: " + e.getMessage());
        } finally {
            if (writer != null) {
                try { writer.close(); } catch (IOException e) { /* ignore */ }
            }
        }
    }
}

Q3050 easy code error

What is the error in this Java code?

java

final class BaseClass {
    public void method() {
        System.out.println("Base method");
    }
}

class DerivedClass extends BaseClass { // Line 7
    public void anotherMethod() {
        System.out.println("Derived method");
    }
}

public class Main {
    public static void main(String[] args) {
        // DerivedClass d = new DerivedClass(); // If uncommented, would show the error
    }
}

Q3051 easy code output

What is the output of this Java code snippet?

java

public class LoopTest {
    public static void main(String[] args) {
        int start = 10;
        do {
            System.out.print("Execute ");
            start++;
        } while (start < 10);
    }
}

Q3052 hard code error

What compilation error will occur when compiling this Java code?

java

import java.util.Arrays;
import java.util.List;

public class LambdaBreak {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3);
        numbers.forEach(n -> {
            if (n == 2) {
                break;
            }
            System.out.println(n);
        });
    }
}

Q3053 easy

When `thread.join()` is invoked on the current thread, causing it to wait for `thread` to die, what state does the current thread enter?

Q3054 medium

For a method to successfully override another method in Java, which components of the method signature must be identical?

Q3055 easy code error

What will happen when this Java code is compiled?

java

public class MyClass {
    public static void main(String[] args) {
        final int FIXED_VALUE = 25;
        FIXED_VALUE = 30;
        System.out.println(FIXED_VALUE);
    }
}

Q3056 easy code output

What is the output of this Java code?

java

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

public class Main {
    public static void validateAge(int age) {
        if (age < 0) {
            throw new ValidationException("Age cannot be negative"); // Compiler error here
        }
        System.out.println("Age is valid: " + age);
    }

    public static void main(String[] args) {
        validateAge(-5);
    }
}

Q3057 medium

What is a fundamental distinction between a Java constructor and a regular method?

Q3058 easy

What are the default values for elements in a newly created `boolean` array in Java?

Q3059 medium code output

What does this code print?

java

import java.io.BufferedReader;
import java.io.StringReader;
import java.io.IOException;

public class BufferedReaderTest {
    public static void main(String[] args) {
        String fileContent = "Line A\nLine B\nLine C";
        try (BufferedReader br = new BufferedReader(new StringReader(fileContent))) {
            String line = br.readLine();
            System.out.println(line.charAt(line.length() - 1));
            System.out.println(br.readLine());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Q3060 hard code error

What is the compile-time error in the provided Java code snippet using record patterns in a switch expression?

java

record Point(int x, int y) {}

public class RecordPatternMismatchError {
    public static void main(String[] args) {
        Object obj = new Point(1, 2);
        String description = switch (obj) {
            case Point(int a, double b) -> "Point with a: " + a + ", b: " + b; // Error expected here
            default -> "Unknown";
        };
        System.out.println(description);
    }
}

☕ Java MCQ Questions – Page 153