Senior Java Engineer 基础技术笔试 (英文)

Total Point 160 (basic: 130, bouns: 30)

80: passed.100 up: great.

1 (EASY) 10 point

Check out the following java code, and answer the question:

class Data {
int value;

public Data(int value) {
this.value = value;
}
}

public class ReferenceExample {
public static void modifyPrimitive(int num) {
num = num * 2;
}

public static void modifyObject(Data data) {
data = new Data(100);
}

public static void main(String[] args) {
int number = 10;
Data dataObj = new Data(50);

modifyPrimitive(number);
System.out.println("After modifyPrimitive: " + number); // Expected-1: ?

modifyObject(dataObj);
System.out.println("After modifyObject: " + dataObj.value); // Expected-2: ?
}
}

• What is Expect-1 ?

• What is Expect-2 ?

2 (EASY) 10 point

Check out the following java code, and answer the question:

class Resource {
private final String name;

public Resource(String name) {
this.name = name;
}

public String getName() {
return name;
}
}

class Task implements Runnable {
private final Resource resource1;
private final Resource resource2;

public Task(Resource resource1, Resource resource2) {
this.resource1 = resource1;
this.resource2 = resource2;
}

@Override
public void run() {
synchronized (resource1) {
System.out.println(Thread.currentThread().getName() + " locked " + resource1.getName());

// Simulating some work
try { Thread.sleep(50); } catch (InterruptedException e) { }

synchronized (resource2) {
System.out.println(Thread.currentThread().getName() + " locked " + resource2.getName());
}
}
}
}

public class Example1 {
public static void main(String[] args) {
Resource resA = new Resource("Resource A");
Resource resB = new Resource("Resource B");

Thread thread1 = new Thread(new Task(resA, resB), "Thread-1");
Thread thread2 = new Thread(new Task(resB, resA), "Thread-2");

thread1.start();
thread2.start();
}
}

• Are there any issue ? why and how to solve (at least 2 approach) ?

3 (MEDUIM) 20 point

Check out the following java code, and answer the question:

public class IntegerLockExample {
private static Integer count = 0;

public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(() -> increment());
Thread t2 = new Thread(() -> increment());

t1.start();
t2.start();

t1.join();
t2.join();

System.out.println("Final Count: " + count);
}

public static void increment() {
for (int i = 0; i < 1000; i++) {
synchronized (count) {
count++;
}
}
}
}

• Are there any problem? why and how to fix it?

4 (MEDIUM) 20 point

The company often executes the following queries:

-- Query 1: Retrieve all orders placed by a specific user
SELECT * FROM orders WHERE user_id = 123;

-- Query 2: Get recent orders for a product
SELECT * FROM orders WHERE product_id = 456 ORDER BY order_date DESC LIMIT 10;

-- Query 3: Count all orders with status \'Shipped\'
SELECT COUNT(*) FROM orders WHERE status = \'Shipped\';

• Please help me design the sql table index.

• Are there any problem with Query-3 ? Why and how to improve? (Hint: Covering Index)

5 (MEDIUM) 20 point

What is SQL transaction isolation ? Plase list all the sql isolation type and explain what they do.

6 (MEDIUM) 20 point

Scenario:

You are developing a Spring Boot application with a service that saves user data into a MySQL database. The UserService class has a method that creates a user and intentionally throws an exception to test transaction rollback.

However, the user record is still saved in the database!

Buggy Code

@Service
public class UserService {

@Autowired
private UserRepository userRepository;

// Entry point here
public void processUserRequest(Object request) {
createUser(request);
}

@Transactional
public void createUser(Object request) {
// 1. create userDetail
createUserDetailInfo(request);
// 2. create phone
createUserPhone(request);
// 3. create user wallet
createUserWallet(request);

// Simulating an error
throw new RuntimeException("Intentional Exception: Should trigger rollback");
}

public void createUserDetailInfo(Object request) {
// save to DB.
// ...
}

public void createUserPhone(Object request) {
// save to DB.
// ...
}

@Transactional
public void createUserWallet(Object request) {
// save to DB.
// ...
}
}

Tasks:

7 (MEDUIM~HARD) 30 point

• Please draw The B+ Tree Struacure.

• Please explain what is SQL LBCC and MVCC.

• How does MVCC work in READ_COMMIT mode and REPEAT_READ mode ?