Java 17 and Design Patterns Interview Questions (LTS Edition)

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1. What features were added in Java 17?

Java 17 is a Long-Term Support (LTS) release and includes several new features and enhancements:

• Sealed Classes: Restrict which classes can extend or implement a class/interface.
• Pattern Matching for instanceof: Simplifies type casting.
• Switch Expressions Enhancements: More concise and expressive.
• Text Blocks: Multi-line strings with better readability.
• Foreign Function & Memory API (Incubator): Interact with native code and memory safely.
• New macOS Rendering Pipeline: Based on Metal API.
• Deprecation and removal of older features: Applet API, Security Manager (deprecated).

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2. What are Creational Design Patterns? Explain them.

Creational design patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation.

• Singleton: Ensures a class has only one instance and provides a global access point.
• Factory Method: Defines an interface for creating an object but lets subclasses alter the type of objects that will be created.
• Abstract Factory: Provides an interface to create families of related or dependent objects.
• Builder: Allows the construction of a complex object step-by-step.
• Prototype: Creates objects by cloning an existing object.

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3. Program to Implement Singleton Pattern Handling Concurrency, Cloning, and Reflection

import java.io.Serializable;

public class Singleton implements Serializable, Cloneable {

    private static volatile Singleton instance;

    private Singleton() {
        if (instance != null) {
            throw new RuntimeException("Use getInstance() method");
        }
    }

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null)
                    instance = new Singleton();
            }
        }
        return instance;
    }

    // Prevent cloning
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException("Cloning is not allowed");
    }

    // Prevent serialization breaking singleton
    protected Object readResolve() {
        return getInstance();
    }
}

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4. Program using Stream to Get the Second Max Number from a List

import java.util.List;
import java.util.Comparator;

public class SecondMaxExample {
    public static void main(String[] args) {
        List<Integer> list = List.of(5, 3, 9, 1, 4, 9, 7);

        int secondMax = list.stream()
            .distinct()
            .sorted(Comparator.reverseOrder())
            .skip(1)
            .findFirst()
            .orElseThrow(() -> new RuntimeException("No second max found"));

        System.out.println("Second Max: " + secondMax);
    }
}

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5. What is the Time Complexity of Getting an Element from an ArrayList?

Time Complexity: O(1)

Getting an element by index in an ArrayList is a constant-time operation because it is backed by an array and supports random access.

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6. Difference Between HashMap and ConcurrentHashMap

Feature	HashMap	ConcurrentHashMap
Thread Safety	Not thread-safe	Thread-safe
Performance	Faster in single-threaded	Better in concurrent scenarios
Null Keys/Values	Allows one null key and values	Does not allow null keys or values
Synchronization	Needs external sync for threads	Uses internal bucket-level locking
Use Case	Single-threaded apps	Multi-threaded apps

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