Difference Between CAST and TREAT in JPA

1. Introduction

In JPA, CAST and TREAT are two distinct keywords used to manipulate data types and entity relationships. In this tutorial, we’ll explore the differences between CAST and TREAT and look at examples to illustrate their usage.

2. CAST in JPA

The CAST operator in JPA is primarily used for type conversions within JPQL queries. It allows us to explicitly convert a value from one data type to another. For instance, we can use CAST to convert a String to an Integer or vice versa.

Here’s the syntax for CAST:

CAST(expression AS type)

expression is the value or field that we want to convert, and type is the target data type to which we want to convert the expression.

3. TREAT in JPA

In contrast, the TREAT operator is designed for type-safe downcasting of entities within JPQL queries. It’s particularly useful when dealing with inheritance hierarchies. When we use TREAT, we specify a subtype of the entity, and JPA checks if the actual entity is indeed of that type.

Unlike CAST, TREAT doesn’t change the underlying data type of the value. Instead, it allows us to access the value as if it were of the target type.

Here’s the syntax for TREAT:

TREAT(expression AS type)

expression is the value to be treated, and type is the target data type.

4. Purpose and Usage

In JPA queries, both CAST and TREAT are used to handle type conversions, but they serve different purposes.

4.1. CAST Operator

The CAST is used to convert one data type into another for operations or comparisons. It’s often employed when performing queries that require a data type different from what’s stored in the database.

Consider an example where an entity called Employee, and the salary field is stored as a String in the database. Here’s how our Employee entity is defined:

@Entity
public class Employee {
    @Id
    private Long id;
    private String salary;
    // getters and setters
}

In this example, the salary field is of type String, but we may need to perform numeric operations or comparisons based on this field. To achieve this, we can use CAST to convert the salary field to an Integer type:

Employee emp1 = new Employee();
emp1.setId(1L);
emp1.setSalary("5000");
em.persist(emp1);

Query query = em.createQuery("SELECT CAST(e.salary AS Integer) FROM Employee e");
List<Integer> salaries = query.getResultList();

assertEquals(5000, salaries.get(0));

In this query, we use CAST to convert the salary field from a String to an Integer. The result of this query is a list of integers representing the salaries of employees.

4.2. TREAT Operator

On the other hand, TREAT is used for type-safe downcasting in inheritance. It allows us to work with entities in a way that acknowledges their actual subclass type, even when referenced through a base class.

Suppose we have an entity Vehicle with subclasses Car and Bike, and we want to retrieve only Car entities from a query that selects from Vehicle. We can use TREAT to ensure type safety:

@Entity
public class Vehicle {
    @Id
    private Long id;
    private String type;
    // getters and setters
}

@Entity
public class Car extends Vehicle {
    private Integer numberOfDoors;
    // getters and setters
}

To achieve this, we use the TREAT operator in our JPQL query to cast Vehicle instances to Car instances:

Vehicle vehicle = new Vehicle();
vehicle.setId(1L);
vehicle.setType("Bike");

Car car = new Car();
car.setId(2L);
car.setType("Car");
car.setNumberOfDoors(4);

em.persist(vehicle);
em.persist(car);

Query query = em.createQuery("SELECT TREAT(v AS Car) FROM Vehicle v WHERE v.type = 'Car'");
List<Car> cars = query.getResultList();

assertEquals(4, cars.get(0).getNumberOfDoors());

In this query, TREAT allows us to treat each Vehicle as a Car where applicable. The result is a list of Car instances, even though the underlying entities in the database are of type Vehicle.

5. Exception Handling

When dealing with type conversions and entity casting. Both CAST and TREAT operators have specific behaviors regarding exception handling,

5.1. CAST Operator

When using the CAST operator, exceptions can occur if the data conversion isn’t possible. This usually happens when there’s an attempt to convert a value to a type that it can’t be transformed into due to incompatible formats or data types.

Suppose an Employee entity has a salary value of “5ooo” (which isn’t a valid integer). When we execute a query to cast this String to an Integer, the database attempts to convert this value, leading to a JdbcSQLDataException if the conversion fails:

Employee emp1 = new Employee();
emp1.setId(1L);
emp1.setSalary("5ooo");

em.merge(emp1);

try {
    Query query = em.createQuery("SELECT CAST(e.salary AS Integer) FROM Employee e");
    query.getResultList(); // This should throw an exception
    fail("Expected a JdbcSQLDataException to be thrown");
} catch (PersistenceException e) {
    assertTrue(e.getCause() instanceof JdbcSQLDataException,
      "Expected a JdbcSQLDataException to be thrown");
}

In this test, we’re asserting that a JdbcSQLDataException is thrown when attempting to cast an invalid string value.

5.2. TREAT Operator

In contrast, the TREAT operator handles type-safe downcasting in inheritance hierarchies. Unlike CAST, TREAT typically doesn’t throw exceptions when it encounters an issue with type casting. Instead, it returns an empty result set if no entities of the specified subclass type are found.

Suppose we query for Car instances among Vehicle entities, but the only vehicles available are of type Bike. In this case, the query doesn’t throw an exception but returns an empty result set instead:

Query query = em.createQuery("SELECT TREAT(v AS Car) FROM Vehicle v WHERE v.type = 'Bike'");
List<Car> cars = query.getResultList();

assertEquals(0, cars.size());

In this example, we’re querying for Car entities, but since no matching Car instances exist (only Bike instances are present), TREAT returns an empty list. This approach avoids exceptions and provides a clean way to handle type-safe casting by gracefully handling cases where no entities match the desired subclass type.

6. Criteria API

In the Criteria API, CAST isn’t directly supported. However, we can perform type conversions implicitly using expressions and the as() method, provided the types are compatible. For example, we can convert a field from one type to another if the types are directly compatible.

Here’s how we can use the as() method to cast an expression from one type to another, provided the types are compatible:

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Integer> cq = cb.createQuery(Integer.class);
Root<Employee> employee = cq.from(Employee.class);

Expression<String> salaryExpression = employee.get("salary");
Expression<Integer> salaryAsInteger = salaryExpression.as(Integer.class);

cq.select(salaryAsInteger);

TypedQuery<Integer> query = em.createQuery(cq);

List<Integer> salaries = query.getResultList();
assertEquals(5000, salaries.get(0));

In this example, we retrieve the salary as a String and use the as() method to cast it to an Integer. This approach works when types are compatible, but direct CAST operations aren’t natively supported in the Criteria API.

On the other hand, TREAT is supported in the Criteria API and provides built-in functionality for type-safe downcasting. We can use TREAT to handle entity inheritance hierarchies by specifying a subtype. This example shows how to use TREAT with CriteriaBuilder:

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Car> cq = cb.createQuery(Car.class);
Root<Vehicle> vehicleRoot = cq.from(Vehicle.class);

cq.select(cb.treat(vehicleRoot, Car.class))
  .where(cb.equal(vehicleRoot.get("type"), "Car"));

TypedQuery<Car> query = em.createQuery(cq);
List<Car> cars = query.getResultList();

assertEquals(1, cars.size());
assertEquals(4, cars.get(0).getNumberOfDoors());

In this example, TREAT is used to cast Vehicle entities to Car instances, ensuring type safety and allowing us to work with the Car subclass directly.

7. Summary

Here’s a table that highlights the key differences between CAST and TREAT in JPA:

8. Conclusion

In this article, we’ve explored the difference between CAST and TREAT. CAST and TREAT are two distinct keywords in JPA that serve different purposes. CAST is used to convert between primitive types, while TREAT is used to treat an instance as if it were of a different type.

As always, the code discussed here is available over on GitHub.