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The Java EE 6 Tutorial, Volume I

The Expression Interface Methods

An Expression object is used to in a query's select, where, or having clause

Table 22–1 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. 

Example 22–13 Using the Expression.isNull Method

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());

This query finds all pets where the color attribute is null.

Example 22–14 Using the Expression.in Method

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");

This query finds all brown and black pets.

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