@Id and @GeneratedValue for primary keys in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the effect of @Id annotation in a Spring Boot entity?

In a Spring Boot JPA entity, what does the @Id annotation do?

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;

@Entity
public class User {
    @Id
    private Long userId;
    private String name;
}

A Marks the field as the primary key of the entity in the database.

B Automatically generates a unique value for the field when saving.

C Specifies the database table name for the entity.

D Indicates the field should be ignored by JPA.

💡 Hint

Think about what uniquely identifies each record in a database table.

The @Id annotation marks a field as the primary key, which uniquely identifies each record in the database table. It does not generate values or specify table names.

📝 Syntax

intermediate

Which @GeneratedValue strategy generates values using a database identity column?

In Spring Boot JPA, which @GeneratedValue strategy uses the database's identity column to generate primary key values?

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
}

A GenerationType.IDENTITY

B GenerationType.SEQUENCE

C GenerationType.TABLE

D GenerationType.AUTO

💡 Hint

This strategy relies on the database to auto-increment the primary key.

GenerationType.IDENTITY uses the database's built-in identity column feature to generate unique primary key values automatically.

🔧 Debug

advanced

Why does this entity fail to save with a primary key error?

Given the entity below, why might saving it cause a primary key error?

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long orderId;

    private String product;
}

A The @Id annotation is missing from the primary key field.

B The GenerationType.SEQUENCE strategy is invalid and causes a syntax error.

C The database does not have a sequence defined for this entity's primary key generation.

D The entity class is missing the @Table annotation specifying the table name.

💡 Hint

Check if the database supports sequences or if one is configured.

Using GenerationType.SEQUENCE requires a database sequence to exist. If none is defined, saving the entity will fail with a primary key generation error.

❓ state_output

advanced

What is the value of 'id' after saving this entity with GenerationType.AUTO?

Consider this entity saved to the database. What will be the value of id after saving?

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

@Entity
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String name;

    public Long getId() { return id; }
}

A The value must be manually set before saving; otherwise, it throws an error.

B Always null because GenerationType.AUTO does not generate values.

C Zero because the default value of Long is 0 before saving.

D A unique generated value assigned by the persistence provider after saving.

💡 Hint

GenerationType.AUTO lets the provider pick the best strategy to generate a unique id.

With GenerationType.AUTO, the persistence provider chooses an appropriate strategy to generate a unique primary key value when the entity is saved.

🧠 Conceptual

expert

Which statement about @GeneratedValue strategies is true?

Select the only true statement about @GeneratedValue strategies in Spring Boot JPA.

A GenerationType.IDENTITY requires a sequence object in the database to generate keys.

B GenerationType.TABLE uses a separate table to generate unique primary keys and works across all databases.

C GenerationType.SEQUENCE is the default strategy and works without any database setup.

D GenerationType.AUTO disables automatic key generation and requires manual id assignment.

💡 Hint

Think about which strategy uses a table to generate keys and is database independent.

GenerationType.TABLE uses a special table to generate unique keys and works on all databases, unlike SEQUENCE or IDENTITY which depend on database features.