自动测试和错误公式的问题

Question

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!

Answer 1

You are subtracting givenPercentage as an absolute number. What you are looking for is to subtract percentage of population. 20% of 200 is 40. 200-40=160. Correct code:

public void spray(double givenPercentage) {
    this.population = this.population - (int)((givenPercentage/100) * this.population);
}