OtherJVMLanguages.md

This page collects tips on using CQEngine with other JVM languages besides Java.

Scala

Defining CQEngine attributes in Scala

The following is a summary of a discussion in the forum. See the full discussion for more details.

Consider the following scala code which defines a simple class, with CQEngine attributes to access its fields:

class TestClass {
  private var _x1: Int = 0
  private var _x2: Int = 1

  def x1 = _x1
  def x2 = _x2

  def this(y1: Int, y2: Int) {
    this()
    this._x1 = y1
    this._x2 = y2
  }
}

object TestClass {
  def doSomething(x: Int, y: Int): Int = x * y

  final val X_1: Attribute[TestClass, Int] = new SimpleAttribute[TestClass,Int]("X_1") {
    def getValue(testClass: TestClass, queryOptions: QueryOptions): Int = testClass.x1
  }

  final val X_2: Attribute[TestClass, Int] = new SimpleAttribute[TestClass,Int]("X_2") {
    def getValue(testClass: TestClass, queryOptions: QueryOptions): Int = testClass.x2
  }
}

Problem

If an attempt is made as follows in a different class, to add an index on the attribute:

val testClasses: IndexedCollection[TestClass] = new ConcurrentIndexedCollection[TestClass]()
testClasses.addIndex(NavigableIndex.onAttribute(TestClass.X_1))

..then the compiler might produce error: "Cannot resolve reference onAttribute with such signature" ..even though TestClass.X_1 is of type Attribute[TestClass, Int] (or Attribute<O, A> in Java).

Additionally, if the code above is changed to:

testClasses.addIndex(NavigableIndex.onAttribute(TestClass.X_1()));

..a more detailed compilation error is produced: "no instance(s) of type variable(s) A exist so that Object confirms to Comparable".

Solution

The errors above indicate that Scala's Int is not the same as a Java Integer.

However as Scala Integer is the same as a Java Integer, a solution is to change the definition of the X_1 attribute in the TestClass companion object to:

final val X_1: Attribute[TestClass, Integer] = new SimpleAttribute[TestClass, Integer]("X_1") {
  def getValue(testClass: TestClass, queryOptions: QueryOptions): Integer = testClass.x1
}

The code should then compile and run successfully.

Alternative workaround

Another user mentioned it's possible to work around this issue by tweaking Scala to use Java types rather than the ones from Scala. In the Scala main class, set:

type jLong = java.lang.Long
type jInt = java.lang.Integer

Kotlin

Defining CQEngine attributes in Kotlin

Problem

When using QueryFactory.attribute(Foo::bar) to generate an attribute, the underlying call to TypeResolver.resolveRawArguments might be unable to extract the type information from a kotlin-generated function call.

Solution

An inline helper method in Kotlin, will give you the same functionality, in simple oneliner:

inline fun <reified O, reified A> attribute(accessor: KProperty1<O, A>): FunctionalSimpleAttribute<O, A> {
    return FunctionalSimpleAttribute(O::class.java, A::class.java, accessor.javaClass.simpleName, SimpleFunction { accessor.get(it) })
}

Then you can use:

class Foo(
    val bar: Int
) {
    companion object {
        val BAR = attribute(Foo::bar)
    }
}