I’m starting a blog.

Today, I learned a difference between using a Java primitive (int, double) and a Java class (Integer, Double) to define a Grails domain class.

class Student {
    String firstName
    double iq
}

Pretty easy, right? IQ is something that I don’t have, but many people do. I want to force new students to define their IQ.

Here’s my test. Make sure that student is invalid if no IQ is defined.

class StudentTests extends GrailsUnitTestCase {
    // <snip> mocking stuff
    def s = new Student(firstName:'Nate')

    // We want to force iq to be defined
    // No defaulting allowed
    assertEquals "nullable" , s.errors["iq"]

    // Test fails -- iq is already 0 !!!
}

When we define iq using Java’s “double” primitive type, the iq is automatcially initialized to zero.

Solution:  Use Double or Integer, etc. classes if you don’t want a property to be initialized automatically.

Better:

class Student {
    // Use the "Double" class instead of "double" primitive
    Double iq
}