Sunday, August 26, 2007

Contract of the interfaces

As I mentioned in my previous post titled Don't test everything, unit testing is very valuable in the software development process, but on some occasions the test expects more that it should. In some cases those expectation work fine, in other they fail.

What the hell am I talking about? If you follow the good practice of coding against interfaces, you know that the interface defines the contract all implementation classes must adhere to. If you write an implementation of an interface your unit tests should test possibly everything that this contract specifies and nothing more.

Take a Set from Java Collections Framework for example. This interface is defined as:

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

At some point I had a unit test that had some input values and my test was assuming the correct results in the set that was returned. I knew that my implementation wa returning a HashSet and I also knew what was in that set. The asserts were quite simple (the code is simplified):

String[] addresses = new String[] {"address1", "address2", "address3"};
Set set = new HashSet(Arrays.asList(addresses));
Iterator i = set.iterator();

It's a good test, you may think. Well, it isn't! This test runs or fails depending on which JDK you use. The problem with this test is that it assumes the order of the elements in the returning set. Set interface does not guarantee the order of its elements. We should not test that.

Let's have a look at the following unit test

import junit.framework.TestCase;

import java.util.Set;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Iterator;

public class TestJdkDiff extends TestCase
    public void testOrder()
        String javaVersion = System.getProperty("java.version");
        String javaVendor = System.getProperty("java.vendor");
        System.out.println("Running " + javaVendor + " " + javaVersion);

        String[] addresses = new String[] {"address1", "address2", "address3"};
        final Set set = new HashSet(Arrays.asList(addresses));
        for (Iterator i = set.iterator(); i.hasNext();)

Execution of the previous test on Java 6 produces the following output

Running Sun Microsystems Inc. 1.6.0_02

The same test executed on Sun's JVM 1.4 prints out

Running Sun Microsystems Inc. 1.4.2_12

Oops! Now I know why my test failed. Even, in JDK the implementation of some classes can change from time to time. As long as the contact is kept all should be fine.

So to fix my initial test one should write

String[] addresses = new String[] {"address1", "address2", "address3"};
Set set = new HashSet(Arrays.asList(addresses));
assertEquals(3, set.size());

Happy coding!

1 comment:

mario.gleichmann said...

interesting one,

but i wonder why anyone would iterate over the elements of a Set and implicitly assert that implementations of Set provide an order of its elements (since Set doesn't assure any order of its elements!) at all?

In that sense, you can alway assert 'to much', but this is not the fault of the class under test, nor the testcase but the 'designer' of the testcase.

So what's the conclusion? As always - think before you act ... ;o)



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