Java Interview Question - New FAQs

1. Performance Testing & Setting up benchmarks

Performance testing evaluate how a system performs under load & to setup benchmarks below steps to be noted down.

Steps are:

• Define KPIs: Response time, throughput, CPU/memory usage etc.
• Identify Critical Scenarios: like login, search, submit form
• Use Tools: Gatling, Apache Bench, JMeter (Use to simulate 1000 concurrent users hitting a Rest API and measure the response time
• Baseline Measurement: Run tests under normal load to establish a baseline
• Load Testing: Gradually increase users to find system limits
• Stress Testing: Push beyond limits to see how the system fails
• Bench marking: Compare results against industry standards or previous releases

2. ACID properties in database

It ensure the reliable transaction

• Atomicity: All operations in a transaction succeed or none do
• Consistency: DB remains in a valid state before and after the transaction
• Isolation: Concurrent transactions don't interfere
• Durability: Once commited in DB, changes persist even after crashes

e.g.: JDBC Java code snippet:

Connection conn = DriverManager.getConnection(DBurl);

conn.setAutoCommit(false);

try {

    conn.commit(); //Ensure Atomicity and Durability

} catch(Exception e) {

    conn.rollback(); //Ensure Consistency

}

3. Deadlock in Database & How to avoid it?

Deadlock occur when two transactions wait on each other to release the locks.

Avoidance Criteria:

• Lock ordering: Always acquire locks in a consistent order
• Timeouts: Abort transaction that wait too long
• Minimize lock scope: Keep transactions short
• Use optimistic locking: Avoid locking unless necessary (e.g. use @Version annotation using Hibernate)

4. API Gateway timeout it 10 second & Micro Services response is 15 seconds (Scenario)

Below solutions can be tried out:

• ASync Processing: Return a 202 Accepted and process in background
• Queueing: Use Kafka / RabbitMQ to decouple request handling
• Caching: Serve cached responses for repeated requests
• Optimize Microservices: Profile and reduce processing time

e.g.: Using Spring Boot with async annotation

@Async

public CompletableFuture<Response> processRequest() {

    //Write your long running task logic here

}

5. Explain DB Connection Pooling & Implementation

Connection pooling basically reuses DB connection to reduce overhead.

Benefits:

• Faster response
• Reduced resource usage
• Better scalability

Implementation:

Use jars libraries like Apache DBCP2, HikariCP, C3P0

6. SOLID design principles

S: Single Responsibility 

class ReportGenerate {

    void print(Report r) {

        //Your logic here

    }

}

O: Open/Closed 

abstract class Shape {

    abstract double area();

}

class Square extends Shape {

    //Override area logic here

}

L: Liskov Substitution 

Sub classes should be substitutable for base classes

I: Interface Segregation 

Split large interfaces into smaller ones

D: Dependency Inversion 

Depend on abstractions, not concrete classes

7. Singleton design patterns with and without Synchronized block

Without Synchronized (Not a thread-safe)

class Singleton {

    private static Singleton myInstance;

    public static Singleton getInstance() {

        if(myInstance == null) {

            myInstance = new Singleton();

        }

        return myInstance;

    }

}

Without Synchronized (Thread-safe)

class Singleton {

    private static Singleton myInstance;

    public static synchronized Singleton getInstance() {

        if(myInstance == null) {

            myInstance = new Singleton();

        }

        return myInstance;

    }

}

Best practice: To use double checked locking:

class Singleton {

    private static volatile Singleton myInstance;

    public static Singleton getInstance() {

        if(myInstance == null) {

            synchronized(Singleton.class) {

                if(myInstance == null) {

                    myInstance = new Singleton();

                }

            }            

        }

        return myInstance;

    }

}

8. Implement Linked List in Java (Write a Program)

class Node {

    int data;

    Node next;

    Node(int data) {

        this.data = data;

        this.next = null;

    }

}

class LinkedList {

    Node head;

    

    void insert(int data) {

        Node newNode = new Node(data);

        if(head == null)  {

            head = newNode;

        }

        else {

            Node temp = head;

            while(temp.next != null) {

                temp = temp.next;

                temp.next = newNode;

            }

        }

    }

    void display() {

        Node temp = head;

        while(temp != null) {

            System.out.print(temp.data + "-> ");

            temp = temp.next;

        }

        System.out.println("null");

    }

}