Junit test case for Swapping progam

[Suresh Narayana]

I am new to Junit, i want to know how to write test case for swapping program. Below is a code for which i want o know junit tests case.

public static void swap(int a, int b) {
	a = a + b;// now a is 30 and b is 20
	b = a - b; // now a is 30 but b is 10 (original value of a)
	a = a - b; // now a is 20 and b is 10, numbers are swapped

	System.out.println("Value of a after swapping : " + a);
	System.out.println("Value of b after swapping : " + b);
}

[Paweł Baczyński]

First of all, do you know that your method does not actually swap passed values?
What is being swapped are local copies of those variables (named a and b).

[Jesper de Jong]

That method does not have any effects; if you look at it from the outside, it essentially does nothing. It doesn't return the values of the variables a and b, and since Java method calls are pass-by-value only, if you call this method with two variables then their values will not be swapped. You'll have to return the swapped values in some way.

Because the method doesn't have any effects that are visible outside of the method, it will not be possible to write a unit test for it, because there is nothing to check.

[Tim Cooke]

I moved this into our Testing Forum.

However, Paweł and Jesper are right. Your method has no observable output so you cannot make any assertions as to the correct operation of it.

[Aaron Shawlington]

As the the others said, your method has no side-effects to assert against. I.e. if we put stuff in we need to see stuff come out in order to check if it worked (unless you hooked into the System.out - which I assume isn't the intention).

So lets rewrite your method like this:

   public static void swap(int[] numbers) {
       int a = numbers[0];
       int b = numbers[1];
       a = a + b;// now a is 30 and b is 20
       b = a - b; // now a is 30 but b is 10 (original value of a)
       a = a - b; // now a is 20 and b is 10, numbers are swapped
       numbers[0] = a;
       numbers[1] = b;
   }

Now I might write a test something like this:

   @Test
    public void Swap_TwoPositiveInts_IntsSwapped() {
        int[] nums = {10, 15};

        xxx.swap(nums);

        assertEquals(15, nums[0]);
        assertEquals(10, nums[1]);
    }

The method naming convention I used won't be to many people's taste (I copied it from Roy Osherove), but I think it's the best I've seen generally speaking.
It goes like this: UnitUnderTest_TestVariation_ExpectedResult.

I like to separate the setup, execution, and assertions with blank lines, as above.

You'll hear that it's good practise to only assert of one thing per test. In this case you'll see two assertions, but they are really a compound assertion of a single thing - the swap.

If you have Junit4 on the classpath, and the necessary imports, that test should pass. If you mess with the numbers it'll fail accordingly.