Create your own Groovy type conversion

Type conversion the standard way

Type conversion or casting is a programming language method for changing an object’s data type into another.

I’m sure you are familiar with this code that converts a String to an Integer.

def number = (Integer)'1'
def number = '1'.toInteger()
def number = '1' as Integer

If I want to convert the type of my own objects, I need to create a method to achieve this goal. I copy object properties to another object in a generic way; if a property exists on target object, I copy it from the source object.

class User {
    String name
    String city
    Integer age

    def toAdminUser() {
        def adminUser = new AdminUser()
        copyProperties(this, adminUser)
        return adminUser
    }

    def copyProperties(source, target) {
        source.properties.each { key, value ->
            if (target.hasProperty(key) && !(key in ['class', 'metaClass'])) {
                target[key] = value
            }
        }
    }
}

class AdminUser {
    String name
    String city
    Integer age
}

Now I can do something like this:

adminUser = user.toAdminUser()

Type conversion the fancy way

Great, but I want to use this fancy way to coerce one type to another:

adminUser = user as AdminUser

Simple, Groovy supports operator overloading and creating your own type conversion is really easy: we can override the asType() method.

class User {
    String name
    String city
    Integer age

    Object asType(Class clazz) {
        if (clazz == AdminUser) {
            def adminUser = new AdminUser()
            copyProperties(this, adminUser)
            return adminUser
        }
        else {
            super.asType(clazz)
        }
    }

    def copyProperties(source, target) {
        source.properties.each { key, value ->
            if (target.hasProperty(key) && !(key in ['class', 'metaClass'])) {
                target[key] = value
            }
        }
    }
}