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Stop the spread of nulls

October 4, 2010 at 7:38 pm | blog, software quality | 3 comments


Say we want to write an Android application that listens to everything you say, and tries to find the information that you might need before you even ask for it. For instance, you ask your friend over the phone Is there any good pizzeria near your place?, and this application will perform a query against Google Maps using your friend’s address retrieved from your phone contacts and provide you with a couple of recommendations.

To parse the user commands, after converting the speech to text, and provide the appropriate action handlers, we have these high level interfaces:

public interface Parser {
    Action findAction(String userInput);

public interface Action {
    void performAction();

There are two proposals for the implementation of the parser:

// Altenative A
class DbBasedParserA implements Parser {
    public Action findAction(String userInput) {
        // Find the appropriate action in the DB
        return actionFound ? theAction : null;

// Altenative B
class DbBasedParserB implements Parser {
    private static final Action DO_NOTHING = new Action() {
        public void performAction() {
            // Do nothing

    public Action findAction(String userInput) {
        // Find the appropriate action in the DB
        return actionFound ? theAction : DO_NOTHING;

What are the consequences of this choice? Let’s see the code of the clients [1]:

// Client for alternative A
Parser parser = ParserFactory.getParser();
if (parser == null) {
    throw new IllegalStateException();

Action action = parser.findAction(someInput);
if (action == null) {
    // Do nothing
} else {
// Client for alternative B

And this is contagious. Those if (action == null) are going to keep on spreading over the different layers of the system unless some API stops it. Help save lines of code and NPEs by ensuring no nulls are passed around!

[1] There is also a ParserFactory, with two implementation alternatives that react differently when no parser is found: one returns null, while the other throws unchecked exceptions. Back to post.

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3 Comments to “Stop the spread of nulls”

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