Confusion on Automated Testing

[kib ben]

Im in an intro to java programming course and we're doing a project in Bluej where we need to use automated testing to run our code. The test code is given and cannot be changed, and we have to write code and run the tests successfully. Mine is all running smoothly except for this last part. Here is a snippet from the test code:

    public void test_spray() {
        int currentRoaches = population.getRoaches();
        population.spray(20);     // reduce the population by 20%
        assertEquals((int)(currentRoaches * 0.80), population.getRoaches() );
    }

And here is the method I wrote for it. It compiles but when I run it through the test I get a failed error that says "expected:<160> but was:<180>" I thought I had the formula correct but I guess not.

 /* simulates spraying with insecticide, which reduces the population
    by the given percentage. */ 
    public void spray(double givenPercentage) {
        this.population = this.population - (int)givenPercentage;
    }

I'm not sure what I'm doing wrong here. Any insight would be awesome. Thanks in advance!

[Ron McLeod]

kib ben wrote:I'm not sure what I'm doing wrong here.

It looks like you are reducing by a value of 20, not 20 percent.
    200 - 20 = 180

[Campbell Ritchie]

...and why are you using doubles when you are passing ints?

[Tim Holloway]

... And THIS is why we have unit testing!!!

It's only fair to note, however, that it's very easy to confuse percentage (often an integer from 0-100, inclusive wiith a percentage value (same thing, but divided by 100). So one has to keep that in mind when designing the application code. And the tests, of course!

[Junilu Lacar]

Hint: The unit test itself contains the formula you need to use in your spray() method.

[Junilu Lacar]

Hint #2: You can use the -= operator for this to simplify your code.

[Junilu Lacar]

Hint #3: Be careful with integer division. 1 / 2 == 0, not 0.5 (you can try it in jshell)

Hint #4: Including a double value in an expression will force the conversion of other terms to double.