The Expression Interface Methods (The Java EE 6 Tutorial)

The Java EE 6 Tutorial

The `Expression` Interface Methods

An Expression object is used in a query's SELECT, WHERE, or HAVING clause. Table 23–1 shows conditional methods you can use with Expression objects.

Table 23–1 Conditional Methods in the Expression Interface


Method	Description
`isNull`	Tests whether an expression is null
`isNotNull`	Tests whether an expression is not null
`in`	Tests whether an expression is within a list of values

The following query uses the Expression.isNull method to find all pets where the color attribute is null:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Metamodel m = em.getMetamodel();
EntityType<Pet> Pet_ = m.entity(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(pet.get(Pet_.color).isNull());

The following query uses the Expression.in method to find all brown and black pets:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Metamodel m = em.getMetamodel();
EntityType<Pet> Pet_ = m.entity(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(pet.get(Pet_.color).in("brown", "black");

The in method also can check whether an attribute is a member of a collection.