CRUD methods (save, findById, findAll, delete) in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to save an entity using Spring Data JPA.

Spring Boot

repository.[1](entity);

Drag options to blanks, or click blank then click option'

AfindById

Bsave

Cdelete

DfindAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using findById instead of save

Using delete instead of save

The save method stores or updates an entity in the database.

2fill in blank

medium

Complete the code to find an entity by its ID.

Spring Boot

Optional<Entity> result = repository.[1](id);

Drag options to blanks, or click blank then click option'

AfindById

Bsave

Cdelete

DfindAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using save instead of findById

Using findAll instead of findById

The findById method returns an Optional containing the entity if found.

3fill in blank

hard

Fix the error in the code to retrieve all entities.

Spring Boot

List<Entity> list = repository.[1]();

Drag options to blanks, or click blank then click option'

Asave

BfindById

CfindAll

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using findById which returns Optional

Using save which saves data

The findAll method returns a list of all entities in the repository.

4fill in blank

hard

Fill both blanks to delete an entity by its ID.

Spring Boot

repository.[1]([2]);

Drag options to blanks, or click blank then click option'

AdeleteById

Bsave

Cid

DfindAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using save instead of deleteById

Passing the wrong argument instead of the ID

The deleteById method removes the entity with the given ID from the database.

5fill in blank

hard

Fill all three blanks to save an entity, then find it by ID and delete it.

Spring Boot

repository.[1](entity);
Optional<Entity> found = repository.[2](entity.[3]());

Drag options to blanks, or click blank then click option'

Asave

BfindById

CgetId

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of findById

Using a wrong method to get the ID

First, save stores the entity. Then, findById retrieves it by its ID using getId().

Practice

1. Which Spring Boot repository method is used to add a new entity or update an existing one in the database?

easy

A. save()

B. findById()

C. delete()

D. findAll()

Solution

1. Step 1: Understand the purpose of save()

   The save() method is designed to add a new entity or update an existing one in the database.

2. Step 2: Compare with other methods

   findById() and findAll() are for reading data, and delete() is for removing data, so they do not add or update.

3. Final Answer:

   save() -> Option A

4. Quick Check:

   Add or update data = save() [OK]

Hint: Remember: save = add or update data [OK]

Common Mistakes:

• Confusing save() with findById()
• Thinking delete() adds data
• Using findAll() to save data

2. Which of the following is the correct syntax to find an entity by its ID using Spring Boot's repository?

easy

A. repository.findById(id);

B. repository.save(id);

C. repository.delete(id);

D. repository.findAll(id);

Solution

1. Step 1: Identify the method to find by ID

   The method findById(id) is used to retrieve an entity by its ID.

2. Step 2: Check syntax correctness

   repository.findById(id); is the correct syntax. Other methods do not accept an ID to find an entity.

3. Final Answer:

   repository.findById(id); -> Option A

4. Quick Check:

   Find by ID syntax = repository.findById(id) [OK]

Hint: Use findById(id) to get one entity by its ID [OK]

Common Mistakes:

• Using save() to find an entity
• Passing ID to findAll(), which takes no parameters
• Using delete() to find data

3. Given the following code snippet, what will allUsers.size() return if the database has 3 user records?

List<User> allUsers = userRepository.findAll();
int count = allUsers.size();

medium

A. 0

B. 3

C. 1

D. null

Solution

1. Step 1: Understand findAll() behavior

   The findAll() method returns a list of all entities in the database.

2. Step 2: Count the number of records returned

   If the database has 3 user records, allUsers will contain 3 elements, so allUsers.size() returns 3.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   findAll() returns all records count = 3 [OK]

Hint: findAll() returns all records as a list [OK]

Common Mistakes:

• Assuming findAll() returns null if no records
• Confusing size() with findById() result
• Thinking findAll() returns a single entity

4. What is wrong with this code snippet if it throws a compilation error?

userRepository.delete(5);

Assuming delete expects an entity object, not an ID.

medium

A. The ID must be a string, not an integer.

B. The method delete does not exist in Spring Boot repositories.

C. You must pass an entity object, not an ID, to delete.

D. You should use deleteById(5) instead to delete by ID.

Solution

1. Step 1: Understand delete method signature

   The delete method expects an entity object, not just an ID.

2. Step 2: Use correct method to delete by ID

   To delete by ID, use deleteById(id) method instead of delete(id).

3. Final Answer:

   You should use deleteById(5) instead to delete by ID. -> Option D

4. Quick Check:

   Delete by ID = deleteById(id) [OK]

Hint: Use deleteById(id) to remove by ID, not delete(entity) [OK]

Common Mistakes:

• Passing ID to delete() instead of entity
• Assuming delete() accepts ID directly
• Using wrong data type for ID

5. You want to update a user's email in the database using Spring Boot. Which sequence of repository methods correctly achieves this?

// Assume userId and newEmail are given
Optional<User> userOpt = userRepository.findById(userId);
if (userOpt.isPresent()) {
    User user = userOpt.get();
    user.setEmail(newEmail);
    // What next?
}

hard

A. Call userRepository.delete(user); then save(user);.

B. Call userRepository.findAll(); to refresh the list.

C. Call userRepository.save(user); to update the user.

D. No need to call any method; changes auto-save.

Solution

1. Step 1: Retrieve the user entity by ID

   The code uses findById(userId) to get the user object.

2. Step 2: Update the user and save changes

   After setting the new email, calling save(user) updates the database record.

3. Final Answer:

   Call userRepository.save(user); to update the user. -> Option C

4. Quick Check:

   Update data = findById + modify + save [OK]

Hint: Update entity fields then call save() to persist changes [OK]

Common Mistakes:

• Assuming changes auto-save without calling save()
• Deleting before saving to update
• Using findAll() to update a single record