Test java classes with ScalaCheck

Question

I'm trying to test a java class with scalacheck. For example, I have a class Queue within ...\src\main\java\

public class Queue<Item> {

private Node first;
private Node last;
private int N;

private class Node {
    Item item;
    Node next;
}

public boolean isEmpty () { return first == null; }
public int size() { return N;}

public void enqueue(Item item) {
    Node oldLast = last;
    last = new Node();
    last.item = item;
    last.next = null;
    if(isEmpty()) 
        first = last;
    else 
        oldLast = last;
    N++;
}

public Item dequeue() {
    Item item = first.item;
    first = first.next;
    if(isEmpty()) 
        last = null;
    N--;
    return item;
}

}

Then I have a scala test class QueueTest.scala within ...\src\test\scala\

import org.scalacheck.Gen.{choose, oneOf}
import org.scalacheck.Prop.forAll
import org.scalacheck.Gen.choose
import org.scalacheck._
import org.scalacheck.Prop._

class QueueTest extends Properties("Queue") {

    Queue<Int> q;

    property("enque") = Prop.forAll { (n: Int) =>
        (q.enque(n) == n)
    }
}

I just need a head start on how to extend a java generic Queue class? All I'm trying to do is to test enqueue and dequeue methods.

I have looked at StringUtils.scala example from Rick Nillson's github but it's still not clear.

Any suggestions would be appreciated

Answer 1

The only possible test for enque is to call it and make sure doesn't throw an exception.

If this were my assignment, I'd define a property for all List[Int] and test enqueuing and dequeuing the items of that arbitrary list, such that the queue returns exactly the same items in the same order as the list.

---

As requested, here are some examples:

import org.scalacheck.Properties
import org.scalacheck.Prop.forAll

object QueueSpec extends Properties("Queue") {

  property("enqueue never throws an exception") = forAll {(xs: List[Int]) =>
    val q = new Queue[Int]
    xs.foreach(q.enqueue)
    true
  }

  property("dequeue always presents elements in FIFO order") = forAll {(xs: List[Int]) =>
    val q = new Queue[Int]
    xs.foreach(q.enqueue)
    xs.forall{_ == q.dequeue}
  }

  property("isEmpty always true after complete dequeue") = forAll {(xs: List[Int]) =>
    val q = new Queue[Int]
    xs.foreach(q.enqueue)
    xs.foreach(_ => q.dequeue)
    q.isEmpty
  }
}

build.sbt:

scalaVersion := "2.11.2"

libraryDependencies ++= Seq(
  "org.scalacheck" %% "scalacheck" % "1.11.5" % "test"
)

result:

$ sbt test
[info] + Queue.enqueue never throws an exception: OK, passed 100 tests.
[info] + Queue.dequeue always presents elements in FIFO order: OK, passed 100 tests.
[info] + Queue.isEmpty always true after complete dequeue: OK, passed 100 tests.
[info] Passed: Total 3, Failed 0, Errors 0, Passed 3
[success] Total time: 2 s, completed 24/09/2014 9:28:10 PM