Re: Tests for several classes implementing a generic interface

Lew <>
Thu, 6 May 2010 06:44:22 -0700 (PDT)
kofa wrote:

I'd like to write a unit test for classes implementing the same
generic interface, Something<T>.
Each class implements Something<T> with a specific class, e.g.
class IntegerThing implements Something<Integer> {...}
class StringThing implements Something<String> {...}

interface Something<T> {
  T createThing();
  void doSomething(T one, T other);
  Set<T> getThings();


public class IntegerThing implements Something<Integer> {
  private int counter;
    public Integer createThing() {
    return counter++;
  public void doSomething(Integer one, Integer other) {
    // ...
  public Set<Integer> getThings() {
    return new HashSet<Integer>();


Then, I'd like to have a test where I only need to change the line
that instantiates the object under test. I've come up with:
public class ThingTest<T> {
  private Something<T> underTest = (Something<T>) new IntegerThing();

Any time you're casting with generics you'll get an "unchecked"

Cast is a runtime operation and the compiler cannot guarantee
compatibility. Your type assertions in that line are wacky - how do
you guarantee that 'T' is compatible with 'Integer'? There's nothing
you show us that does that.

  public void test() {
    T thingA = underTest.createThing();
    T thingB = underTest.createThing();
    underTest.doSomething(thingA, thingB);
    Set<T> result = underTest.getThings();
    // assert whatever...


I don't want to create a whole parallel tree of ThingTest<Integer>,
ThingTest<Special>; this would be used to test each class just one, to
verify puzzle solutions from students. To check each solution, I'd
just replace "new IntegerThing()" with whatever class they used.

Now, this works fine, but gives me a warning: unchecked cast from
IntegerThing to Something<T>. Is there a way to avoid this? At compile

You need to do a proper type analysis. You have not guaranteed in
your code that 'T' is compatible with 'Integer'.

time, it is known that IntegerThing implements Something<Integer>; is
there a way to get the compiler figure out that T is Integer in this

If you can reason through a way to guarantee type compatibility you
can express that in generics, otherwise you're SOL.

You are trying a cast from 'Integer' to 'T', essentially, but you have
given the compiler nothing to guarantee that compatibility.

You probably want something like this untested idea:

  public <T> void test( Something <T> underTest )
    T thingA = underTest.createThing();
    T thingB = underTest.createThing();
    underTest.doSomething( thingA, thingB );
    Set <T> result = underTest.getThings();
    // assert whatever ...


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