How do I write a unit test to cover the case where an IOException is thrown?

Question

I have the following class:

    public class FileLoader {   

         private Map<Brand, String> termsOfUseText = new HashMap<Brand, String>();

         public void load() {
            for (Brand brand : Brand.values()) {
                readAndStoreTermsOfUseForBrand(brand);
            }
         }

         private void readAndStoreTermsOfUseForBrand(Brand brand) {
            String resourceName = "termsOfUse/" + brand.name().toLowerCase() + ".txt";          
            InputStream in = this.getClass().getClassLoader().getResourceAsStream(resourceName);            
            try {
                String content = IOUtils.toString(in);          
                termsOfUseText.put(brand, content);
            } catch (IOException e) {
                throw new IllegalStateException(String.format("Failed to find terms of use source file %s", resourceName),e);
            }
         }

        public String getTextForBrand(Brand brand) {
            return termsOfUseText.get(brand);
        }
    }

Brand is an enum, and I need all the valid .txt files to be on the classpath. How do I make the IOException occur, given that the Brand enum contains all the valid brands and therfore all the .txt files for them exist?

Suggestions around refactoring the current code are welcome if it makes it more testable!

Answer 1

Three options I see right off:

1. Use PowerMock to mock IOUtils.toString(). I consider PowerMock to be quite a last resort. I'd rather refactor the source to something a little more test-friendly.
2. Extract the IOUtils call to a protected method. Create a test-specific subclass of your class that overrides this method and throws the IOException.
3. Extract the InputStream creation to a protected method. Create a test-specific subclass to override the method and return a mock InputStream.