public CompoundPluginDescriptorFinder add(PluginDescriptorFinder finder) {
        if (finder == null) {
            throw new IllegalArgumentException("null not allowed");
        }

        finders.add(finder);

        return this;
    }
@Test
    public void add() {
        CompoundPluginDescriptorFinder descriptorFinder = new CompoundPluginDescriptorFinder();
        assertEquals(0, descriptorFinder.size());

        descriptorFinder.add(new PropertiesPluginDescriptorFinder());
        assertEquals(1, descriptorFinder.size());
    }