Java MCQ Questions – Page 183 of 200

Q: What is the effect of declaring a class as `final` in Java?

C) It cannot be inherited (subclassed) by any other class.. When a class is declared `final`, it means it cannot be extended by any other class. This prevents further specialization or modification of its behavior through inheritance.

Q: What error will this Java code produce when compiled and run?

A) java.lang.ArrayStoreException. While Java arrays are covariant (meaning `String[]` can be assigned to `Object[]`), at runtime, the actual component type of the array is checked. Attempting to store an `Integer` object into an `Object[]` that was originally created as a `String[]` will throw an `ArrayStoreException`.

Q: What is the output of the following Java code?

B) Red. The 'fruit' variable is 'Apple', which exactly matches 'case "Apple"'. The code prints 'Red' and the 'break' statement exits the switch.

Q: What is the result of executing this Java code snippet?

A) A ClassCastException occurs at runtime on the line with the cast to Integer[][].. An array of `String` arrays (`String[][]`) can be assigned to `Object[][]` due to array covariance. However, the runtime type of `objMatrix` remains `String[][]`. An array of `String` arrays cannot be cast to an array of `Integer` arrays (`Integer[][]`), even though they are both multi-dimensional object arrays. This incompatible array type cast results in a `ClassCastException` at runtime.

Q: What error will be thrown when `main` attempts to execute the `synchronized` block?

B) java.lang.NullPointerException. Attempting to synchronize on a `null` reference will result in a `java.lang.NullPointerException` at runtime. The Java compiler allows this syntax, but the JVM will throw the exception when it tries to acquire a lock on a non-existent object.

Q: Which annotation is commonly used in Java to indicate that a method is intended to override a method in a superclass or implement an interface method?

A) `@Override`. The `@Override` annotation is a compile-time check that ensures the annotated method correctly overrides a superclass method or implements an interface method.

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

A) Error: Constructor call must be the first statement in a constructor. Calls to `this()` or `super()` to invoke another constructor must always be the very first statement within a constructor. In `MyClass()`, `System.out.println()` appears before `this(0)`, leading to a compile-time error.

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

A) java.lang.StringIndexOutOfBoundsException: String index out of range: -1. String indices must be non-negative. Accessing a character at a negative index will immediately throw a StringIndexOutOfBoundsException.

Q: What is an abstract method in Java?

A) A method declared with the `abstract` keyword and no implementation body.. An abstract method is a method that has a declaration but no implementation, compelling subclasses to provide the concrete behavior.

Q: What is the output of this code?

A) O0I0I1I2O1I0I1. The `outerLoop` label ensures that when `i` is 1 and `j` is 1, the `break` statement exits both the inner and outer `while` loops immediately. The result accumulates character by character until this condition is met.

Q3641 medium

What is the effect of declaring a class as `final` in Java?

Q3642 hard code error

What error will this Java code produce when compiled and run?

java

import java.util.ArrayList;

public class ArrayCovariance {
    public static void main(String[] args) {
        Object[] objects = new String[5];
        objects[0] = "Hello";
        // Attempt to store an Integer in an array originally typed as String[]
        objects[1] = new Integer(123);
        System.out.println(objects[0]);
    }
}

Q3643 easy code output

What is the output of the following Java code?

java

public class MyClass {
    public static void main(String[] args) {
        String fruit = "Apple";
        switch (fruit) {
            case "Banana": System.out.print("Yellow"); break;
            case "Apple": System.out.print("Red"); break;
            case "Orange": System.out.print("Orange"); break;
            default: System.out.print("Unknown color");
        }
    }
}

Q3644 hard code error

What is the result of executing this Java code snippet?

java

public class MultiArrayCasting {
    public static void main(String[] args) {
        String[][] stringMatrix = {{"A", "B"}, {"C", "D"}};
        Object[][] objMatrix = stringMatrix; 

        Integer[][] intMatrix = (Integer[][]) objMatrix; 
        System.out.println(intMatrix[0][0]);
    }
}

Q3645 hard code error

What error will be thrown when `main` attempts to execute the `synchronized` block?

java

public class SyncOnNull {
    public static void main(String[] args) {
        Object lockObject = null;
        synchronized (lockObject) {
            System.out.println("This line will not be reached.");
        }
    }
}

Q3646 easy

Which annotation is commonly used in Java to indicate that a method is intended to override a method in a superclass or implement an interface method?

Q3647 medium code error

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

java

class MyClass {
    int value;
    public MyClass() {
        System.out.println("Default constructor");
        this(0); // This should be the first statement
    }
    public MyClass(int value) {
        this.value = value;
        System.out.println("Parameterized constructor: " + value);
    }
}

public class ConstructorError2 {
    public static void main(String[] args) {
        new MyClass();
    }
}

Q3648 medium code error

What error does this Java code produce when executed?

java

public class Main {
    public static void main(String[] args) {
        String fruit = "apple";
        System.out.println(fruit.charAt(-1));
    }
}

Q3649 easy

What is an abstract method in Java?

Q3650 hard code output

What is the output of this code?

java

public class LabeledBreakWhile {
    public static void main(String[] args) {
        int i = 0;
        int j = 0;
        String result = "";
        outerLoop: while (i < 3) {
            result += "O" + i;
            j = 0;
            while (j < 3) {
                result += "I" + j;
                if (i == 1 && j == 1) {
                    break outerLoop;
                }
                j++;
            }
            i++;
        }
        System.out.println(result);
    }
}

Q3651 easy code error

What is the compilation error in the provided Java code?

java

@FunctionalInterface
interface RunnableTask {
    void run();
}

public class Main {
    public static void main(String[] args) {
        int counter = 0;
        RunnableTask task = () -> {
            counter++;
            System.out.println("Counter: " + counter);
        };
        task.run();
    }
}

Q3652 hard code error

Examine the following Java code. What will be the outcome when executed?

java

import java.io.*;

public class FileWriterError3 {
    public static void main(String[] args) {
        File file = new File("closed_test.txt");
        try {
            FileWriter fw = new FileWriter(file);
            fw.write("First line.");
            fw.close(); // Stream is closed
            fw.write("Second line?"); // Attempt to write to a closed stream
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        } finally {
            file.delete(); // Clean up
        }
    }
}

Q3653 easy

Which method checks if a string has a length of 0?

Q3654 easy

What is a primary benefit of using polymorphism in Java?

Q3655 medium code output

What does this Java code print?

java

class MyThread extends Thread {
    public void run() {
        System.out.print("Thread running: ");
        System.out.println(getName());
    }
}
public class Main {
    public static void main(String[] args) throws InterruptedException {
        MyThread t = new MyThread();
        t.setName("MyWorker");
        t.start();
        t.join();
        System.out.print("Main done.");
    }
}

Q3656 easy code output

What is the output of this Java code snippet?

java

import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        TreeMap<String, String> treeMap = new TreeMap<>();
        treeMap.put("key1", "value1");
        treeMap.put("key2", null);
        System.out.println(treeMap.get("key2"));
    }
}

Q3657 medium code output

What is printed to the console when this code executes, assuming 'nonexistent_dir/file.txt' is an invalid path that cannot be created on the system?

java

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

public class InvalidPathTest {
    public static void main(String[] args) {
        try (FileWriter writer = new FileWriter("nonexistent_dir/file.txt")) {
            writer.write("This should not be written.");
        } catch (IOException e) {
            System.out.println("Caught an IOException: " + e.getMessage());
        }
    }
}

Q3658 medium code error

What error will this Java code produce?

java

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

Q3659 hard

If a method declares a local variable with the same name as an instance variable of the class, how can the instance variable be accessed from within that method?

Q3660 medium

In a method designed to validate user input, if the input is found to be invalid, how should a custom `InvalidInputException` (a checked exception) typically be used to signal this error?

☕ Java MCQ Questions – Page 183