QUESTIONS_AND_ANSWERS_02.md

Java Advanced Concepts - Interview Questions and Answers

1. What is the main purpose of using generics in Java?

   • A) To improve code performance
   • B) To enable dynamic typing
   • C) To provide type safety
   • D) To reduce memory usage

2. Which collection framework interface provides dynamic resizing of arrays?

   • A) List
   • B) Set
   • C) Map
   • D) ArrayList

3. Which data structure is used by the HashSet class in Java?

   • A) Array
   • B) Linked List
   • C) Hash Table
   • D) Tree

4. In Java, which collection allows duplicate elements?

   • A) Set
   • B) Map
   • C) List
   • D) Queue

5. What is the purpose of the Comparable interface in Java?

   • A) To compare two objects based on their hash codes
   • B) To specify the natural ordering of objects
   • C) To provide a way to iterate over a collection
   • D) To filter elements in a collection

6. Which keyword is used to explicitly throw an exception in Java?

   • A) catch
   • B) throw
   • C) finally
   • D) throws

7. In Java, checked exceptions are subclasses of which class?

   • A) RuntimeException
   • B) Exception
   • C) Error
   • D) Throwable

8. What is the purpose of Java Reflection?

   • A) To create new objects
   • B) To access and manipulate class metadata at runtime
   • C) To handle exceptions
   • D) To generate bytecode

9. Which annotation is used to mark a method that overrides a method in a superclass?

   • A) @Override
   • B) @OverrideMethod
   • C) @Overload
   • D) @Overridden

10. Which of the following statements about custom annotations in Java is true?

    • A) Custom annotations can only be used by the Java standard library.
    • B) Custom annotations are defined using the @Annotation keyword.
    • C) Custom annotations can have runtime retention.
    • D) Custom annotations can be used to define new data types.

11. Which interface in the Java Collections Framework provides a way to store elements as key-value pairs?

• A) List
• B) Set
• C) Map
• D) Collection

12. In Java, which collection class is synchronized and suitable for multithreaded applications?

• A) ArrayList
• B) HashSet
• C) ConcurrentHashMap
• D) TreeMap

13. What is the key difference between ArrayList and LinkedList in Java?

• A) ArrayList is faster for random access, while LinkedList is faster for insertions and deletions.
• B) ArrayList allows duplicate elements, while LinkedList does not.
• C) ArrayList uses a doubly-linked list, while LinkedList uses a singly-linked list.
• D) ArrayList is an interface, while LinkedList is a concrete class.

14. In Java, what does the wildcard character "?" represent in the context of generics?

• A) An unknown data type
• B) A wildcard character
• C) An exception
• D) A generic class

15. Which method in the Collections class is used to sort a List in natural order?

• A) sort()
• B) order()
• C) arrange()
• D) arrangeInOrder()

16. In Java, which keyword is used to specify that a method can throw multiple exceptions?

• A) catch
• B) throw
• C) throws
• D) try

17. What is the purpose of the getClass() method in Java reflection?

• A) To create a new class instance
• B) To retrieve the runtime class of an object
• C) To throw a ClassNotFoundException
• D) To invoke a method on an object

18. Which annotation is used to suppress compiler warnings in Java?

• A) @SuppressWarnings
• B) @Override
• C) @Deprecated
• D) @SafeVarargs

19. What is the difference between checked and unchecked exceptions in Java?

• A) Checked exceptions are caught at compile time, while unchecked exceptions are caught at runtime.
• B) Checked exceptions are subclasses of RuntimeException, while unchecked exceptions are not.
• C) Checked exceptions are more severe than unchecked exceptions.
• D) Checked exceptions cannot be caught using try-catch blocks.

20. Which Java annotation is used to mark a class as serializable?

    • A) @Serializable
    • B) @Serial
    • C) @SerializableClass
    • D) @SerializableObject

21. Implement a generic method in Java that counts the number of occurrences of a specific element in an array.

22. Create a generic class called Pair that can store two different types of values. Write a Java program to demonstrate its usage.

23. Write a Java program that uses reflection to create an instance of a class dynamically and invokes one of its methods.

24. Create a custom annotation called @LogMethod that can be used to annotate methods in a Java class. Write a Java program that uses reflection to check if a method is annotated with @LogMethod and logs a message when the method is called.

25. Write Java code to create a generic method that swaps two elements in an array.

26. Implement a custom generic class called Stack that supports push and pop operations. Use generics to allow the stack to store elements of any data type.

27. Create a custom exception class called InvalidAgeException that extends Exception. Write a Java program that throws this exception when the age of a person is less than 18.

28. Write a Java program that uses reflection to inspect the methods of a given class and prints their names and parameter types.

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Answers

1. C - To provide type safety
2. D - ArrayList
3. C - Hash Table
4. C - List
5. B - To specify the natural ordering of objects
6. B - throw
7. B - Exception
8. B - To access and manipulate class metadata at runtime
9. A - @Override
10. C - Custom annotations can have runtime retention.
11. C - Map
12. C - ConcurrentHashMap
13. A - ArrayList is faster for random access, while LinkedList is faster for insertions and deletions.
14. A - An unknown data type
15. A - sort()
16. C - throws
17. B - To retrieve the runtime class of an object
18. A - @SuppressWarnings
19. A - Checked exceptions are caught at compile time, while unchecked exceptions are caught at runtime.
20. D - @SerializableObject

21. Implement a generic method in Java that counts the number of occurrences of a specific element in an array.

public class Main {
    public static <T> int countOccurrences(T[] array, T element) {
        int count = 0;
        for (T e : array) {
            if (e.equals(element)) {
                count++;
            }
        }
        return count;
    }

    public static void main(String[] args) {
        Integer[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 1};
        String[] words = {"apple", "banana", "apple", "orange", "apple", "pear"};

        System.out.println(countOccurrences(numbers, 1));
        System.out.println(countOccurrences(words, "apple"));
    }
}

22. Create a generic class called Pair that can store two different types of values. Write a Java program to demonstrate its usage.

public class Pair<LEFT, RIGHT> {
      private LEFT left;
      private RIGHT right;
   
      public Pair(LEFT left, RIGHT right) {
         this.left = left;
         this.right = right;
      }
   
      public LEFT getLeft() {
         return left;
      }
   
      public void setLeft(LEFT left) {
         this.left = left;
      }
   
      public RIGHT getRight() {
         return right;
      }
   
      public void setRight(RIGHT right) {
         this.right = right;
      }
   
      @Override
      public String toString() {
         return "Pair{" +
                  "left=" + left +
                  ", right=" + right +
                  '}';
      }
}

23. Write a Java program that uses reflection to create an instance of a class dynamically and invokes one of its methods.

public class Main {
    public static void main(String[] args) throws Exception {
        Class<?> clazz = Class.forName("java.util.Date");
        Object object = clazz.getDeclaredConstructor().newInstance();
        Method method = clazz.getMethod("toString");
        System.out.println(method.invoke(object));
    }
}

24. Create a custom annotation called @LogMethod that can be used to annotate methods in a Java class. Write a Java program that uses reflection to check if a method is annotated with @LogMethod and logs a message when the method is called.

import java.lang.annotation.*;
import java.lang.reflect.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface LogMethod {
}

class Test {
    @LogMethod
    public void testMethod() {
        System.out.println("testMethod() called");
    }
}

25. Write Java code to create a generic method that swaps two elements in an array.

public class Main {
   public static <T> void swap(T[] array, int i, int j) {
      T temp = array[i];
      array[i] = array[j];
      array[j] = temp;
   }
}

26. Implement a custom generic class called Stack that supports push and pop operations. Use generics to allow the stack to store elements of any data type.

public class Stack<T> {
    private final List<T> elements = new ArrayList<>();

    public void push(T element) {
        elements.add(element);
    }

    public T pop() {
        if (elements.isEmpty()) {
            throw new EmptyStackException();
        }
        return elements.remove(elements.size() - 1);
    }

    public boolean isEmpty() {
        return elements.isEmpty();
    }
}

27. Create a custom exception class called InvalidAgeException that extends Exception. Write a Java program that throws this exception when the age of a person is less than 18.

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

public class Person {
    private final String name;
    private final int age;

    public Person(String name, int age) throws InvalidAgeException {
        if (age < 18) {
            throw new InvalidAgeException("Age must be greater than 18");
        }
        this.name = name;
        this.age = age;
    }
}

28. Write a Java program that uses reflection to inspect the methods of a given class and prints their names and parameter types.

public class Main {
    public static void main(String[] args) throws Exception {
        Class<?> clazz = Class.forName("java.util.Date");
        Method[] methods = clazz.getDeclaredMethods();
        for (Method method : methods) {
            System.out.println(method.getName());
            for (Parameter parameter : method.getParameters()) {
                System.out.println(parameter.getType());
            }
        }
    }
}

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