2017_07_21_small_interfaces_smarts.md

2017.07.20 - Small interfaces and 'smarts'

We have an interface:

ObjectMapper.java

public interface ObjectMapper {
    <Input, Output> Output map(Input input, Class<Output> outputClass);
}

Sometimes we can't map input so we want to have a possibility to return the type of Optional<Output>.

"Bad":

Add new method to interface ObjectMapper:

    <Input, Output> Optional<Output> mapOptional(Input input, Class<Output> outputClass);

Now we have to implement it in every class that implements ObjectMapper, e.g.:

class DozerObjectMapper implements ObjectMapper {
    /** 'map' method implementation and some other stuff **/
    @Override
    public <Input, Output> Optional<Output> mapOptional(Input input, Class<Output> outputClass) {
        if (input == null) {
            return Optional.empty();
        }
        return Optional.of(map(input, outputClass));
    }
}

"Bad", it depends :-)

Good:

ObjectMapper.java with OptionalObjectMapper class called smart

public interface ObjectMapper {
    <Input, Output> Output map(Input input, Class<Output> outputClass);

    final class OptionalObjectMapper implements ObjectMapper {

        private final ObjectMapper mapper;

        public OptionalObjectMapper(ObjectMapper mapper) {
            this.mapper = mapper;
        }

        @Override
        public <Input, Output> Output map(Input input, Class<Output> outputClass) {
            return mapper.map(input, outputClass);
        }

        public <Input, Output> Optional<Output> mapOptional(Input input, Class<Output> outputClass) {
            if (input == null) {
                return Optional.empty();
            }
            return Optional.of(map(input, outputClass));
        }
    }
}

DozerObjectMapper class has not changed.

Usage:

ObjectMapper objectMapper = dozerObjectMapperFactory.mapper();
Optional<Output> output = new ObjectMapper.OptionalObjectMapper(objectMapper).mapOptional(input);

Topics

• Public APIs or a good practice everywhere?

• Mocking and 'Fake' smart classes

  • Examples: https://github.com/jcabi/jcabi-github, https://github.com/yegor256/rultor

Sources

• Elegant Objects vol. 1 by Yegor Bugayenko (version 1.3, January 7, 2017)
• http://www.yegor256.com/2016/04/26/why-inputstream-design-is-wrong.html - Why InputStream Design Is Wrong