Java MCQ Questions – Page 199 of 200

Q: Which of the following statements about `ArrayList` and thread safety is true?

C) `ArrayList` is not thread-safe; `Vector` or `Collections.synchronizedList()` are common alternatives for concurrent access.. `ArrayList` is not synchronized, making it unsafe for concurrent modification by multiple threads. `Vector` and `Collections.synchronizedList()` provide thread-safe list implementations.

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

C) java.lang.ArrayStoreException. The `toArray(T[] a)` method attempts to store elements of the list into the provided array. If the runtime type of the elements in the list is incompatible with the runtime type of the specified array, an `ArrayStoreException` is thrown at runtime.

Q: What does this code print?

A) FirstSecondThird. Multiple `append()` calls can be chained together because each `append()` method returns a reference to the `StringBuffer` object itself. The strings are concatenated in order.

Q: Which method is primarily used to begin the execution of a newly created `Thread` object?

C) start(). The `start()` method is invoked on a `Thread` object to begin its execution, which then calls the `run()` method in a new thread of control.

Q: What is the output of this code?

B) 20. Since Java 5, covariant return types are allowed. The `Derived` class's `getValue()` method returns `Integer`, which is a subtype of `Number`, the return type of the overridden method in `Base`. Polymorphism ensures the overridden method in `Derived` is called at runtime.

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

A) Compile-time error: MissingRun is not abstract and does not override abstract method run() in Runnable. A concrete class that implements an interface must provide an implementation for all abstract methods of that interface. Here, `MissingRun` implements `Runnable` but fails to implement the `public void run()` method, leading to a compile-time error.

Q: Which statement best describes how immutability contributes to application security?

B) They prevent external code from unexpectedly altering an object's state, reducing vulnerability to malicious manipulation.. By guaranteeing that an object's state cannot change after creation, immutability eliminates the possibility of unexpected side effects or external tampering, making code more predictable and less prone to security vulnerabilities.

Q: What is the runtime error when executing this Java code snippet?

A) java.io.NotSerializableException: . Although MyObject is Serializable, the lambda expression used as a Comparator is implicitly an anonymous class instance that is not inherently Serializable. When TreeSet (which uses this comparator) is serialized, it attempts to serialize its comparator, leading to a NotSerializableException because the lambda isn't declared Serializable.

Q: What kind of error will occur when compiling or running the following Java code?

A) java.lang.StringIndexOutOfBoundsException. The `charAt(int index)` method requires the index to be between `0` and `length() - 1`. Accessing `sb.length()` (index 4 for a length-4 string) is out of bounds, resulting in a `StringIndexOutOfBoundsException`.

Q: Which of the following is the correct way to declare an integer variable named `score` in Java?

A) int score;. In Java, variables are declared by specifying the data type followed by the variable name and ending with a semicolon.

Q3961 medium

Which of the following statements about `ArrayList` and thread safety is true?

Q3962 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) {
        List<String> stringList = new ArrayList<>();
        stringList.add("One");
        stringList.add("Two");

        Integer[] intArray = new Integer[stringList.size()];
        stringList.toArray(intArray); // Attempt to put String elements into an Integer array
        for (Integer num : intArray) {
            System.out.println(num);
        }
    }
}

Q3963 easy code output

What does this code print?

java

public class MyClass {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("First");
        sb.append("Second").append("Third");
        System.out.print(sb);
    }
}

Q3964 easy

Which method is primarily used to begin the execution of a newly created `Thread` object?

Q3965 hard code output

What is the output of this code?

java

class Base {
    Number getValue() {
        return Integer.valueOf(10);
    }
}

class Derived extends Base {
    @Override
    Integer getValue() {
        return Integer.valueOf(20);
    }
}

public class Main {
    public static void main(String[] args) {
        Base obj = new Derived();
        System.out.println(obj.getValue());
    }
}

Q3966 hard code error

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

java

public class MissingRun implements Runnable {
    public void execute() {
        System.out.println("Executing a task.");
    }

    public static void main(String[] args) {
        // new Thread(new MissingRun()).start(); // Line 8
    }
}

Q3967 medium

Which statement best describes how immutability contributes to application security?

Q3968 hard code error

What is the runtime error when executing this Java code snippet?

java

import java.util.TreeSet;
import java.io.*;

class MyObject implements Serializable {
    int value;
    MyObject(int value) { this.value = value; }
}

public class Main {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        TreeSet<MyObject> set = new TreeSet<>((o1, o2) -> o1.value - o2.value);
        set.add(new MyObject(10));
        
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(set);
        oos.close();
        System.out.println("Serialization successful.");
    }
}

Q3969 hard code error

What kind of error will occur when compiling or running the following Java code?

java

public class SBError {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("Test"); // Length 4
        char c = sb.charAt(sb.length()); // Access index 4
        System.out.println(c);
    }
}

Q3970 easy

Which of the following is the correct way to declare an integer variable named `score` in Java?

Q3971 medium code error

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

java

import java.util.function.Supplier;

public class LambdaError {
    public static void main(String[] args) {
        Supplier<String> greeting = (name) -> "Hello, " + name;
        System.out.println(greeting.get());
    }
}

Q3972 medium

Which of the following Stream API terminal operations is designed to return an `Optional`?

Q3973 easy code output

What does this code print?

java

public class MyClass {
    public static void main(String[] args) {
        char grade = 'B';
        switch (grade) {
            case 'A': System.out.print("Excellent"); break;
            case 'B': System.out.print("Good"); break;
            case 'C': System.out.print("Fair"); break;
            default: System.out.print("Invalid");
        }
    }
}

Q3974 easy code output

What is the output of this Java program?

java

public class Main {
    public static void main(String[] args) {
        int result = 5 + 2 * 3;
        System.out.println(result);
    }
}

Q3975 easy

What is the primary purpose of the `FileReader` class in Java?

Q3976 medium code error

What compilation error will occur in the `main` method's lambda expression?

java

import java.util.function.Consumer;

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

public class ServiceRunner {
    public void executeTask(Consumer<String> task) {
        try {
            task.accept("data");
        } catch (Exception e) {
            System.out.println("Caught task exception: " + e.getMessage());
        }
    }

    public static void main(String[] args) {
        ServiceRunner runner = new ServiceRunner();
        runner.executeTask(s -> {
            System.out.println("Processing " + s);
            throw new CustomServiceException("Task failed for " + s);
        });
    }
}

Q3977 medium code error

What is the compilation error in the following Java code?

java

public class FinalVariable {
    public static void main(String[] args) {
        final int MAX_VALUE = 100;
        MAX_VALUE = 200;
        System.out.println(MAX_VALUE);
    }
}

Q3978 easy code output

What is the output of this code?

java

public class DataTypeTest {
    public static void main(String[] args) {
        int myNumber = 123;
        System.out.println(myNumber);
    }
}

Q3979 medium code error

What is the compile-time error for the `Car` class?

java

abstract class Vehicle {
    public abstract void start();
    public void stop() {
        System.out.println("Vehicle stopped.");
    }
}

class Car extends Vehicle {
    // Missing implementation for start()
}

public class Main {
    public static void main(String[] args) {
        // Code won't reach here due to compile error
    }
}

Q3980 medium code output

What does this code print?

java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class StreamTest {
    public static void main(String[] args) {
        List<List<String>> nestedLists = Arrays.asList(
            Arrays.asList("a", "b"),
            Arrays.asList("c", "d", "e"),
            Arrays.asList("f")
        );
        String result = nestedLists.stream()
                                   .flatMap(List::stream)
                                   .map(String::toUpperCase)
                                   .collect(Collectors.joining(" "));
        System.out.println(result);
    }
}

☕ Java MCQ Questions – Page 199