CRUD methods (save, findById, findAll, delete) in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the save method do in Spring Boot's CRUD repository?

The save method adds a new entity or updates an existing one in the database. It returns the saved entity with any generated values like IDs.

beginner

How does findById work in Spring Boot repositories?

It looks for an entity by its unique ID. It returns an Optional that contains the entity if found, or is empty if not found.

beginner

What is the purpose of findAll in Spring Boot CRUD repositories?

It retrieves all entities of a given type from the database and returns them as a list.

beginner

Explain what delete does in Spring Boot CRUD repositories.

The delete method removes an entity from the database. You can delete by passing the entity or by its ID using deleteById.

intermediate

Why is Optional used with findById in Spring Boot?

Optional helps safely handle the case when an entity might not exist. It avoids errors by forcing you to check if the entity is present before using it.

Which CRUD method in Spring Boot is used to add or update an entity?

A save

B findById

C findAll

D delete

The save method adds a new entity or updates an existing one.

What does findById return if no entity is found?

A An empty Optional

B An empty list

C An exception

D null

findById returns an empty Optional if the entity is not found.

Which method retrieves all entities from the database?

A save

B findAll

C delete

D findById

findAll returns a list of all entities.

How can you delete an entity using Spring Boot CRUD repository?

A Only by entity ID

B Only by entity object

C By calling save with null

D By entity object or ID

You can delete by passing either the entity object or its ID.

Why is it good to use Optional with findById?

A To automatically delete missing entities

B To speed up queries

C To avoid null pointer errors

D To return multiple entities

Optional helps avoid null pointer errors by forcing checks for presence.

Describe how the save, findById, findAll, and delete methods work in Spring Boot CRUD repositories.

Think about how you add, find, list, and remove items in a real collection.

Explain why Optional is useful when using findById in Spring Boot.

Consider what happens if you look for something that might not be there.

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