Three-State Booleans in Java

Lukas EderMay 7th, 2014Last Updated: January 5th, 2015

2 63 Less than a minute

Every now and then, I miss SQL’s three-valued BOOLEAN semantics in Java. In SQL, we have:

TRUE
FALSE
UNKNOWN (also known as NULL)

Every now and then, I find myself in a situation where I wish I could also express this UNKNOWN or UNINITIALISED semantics in Java, when plain true and false aren’t enough.

Implementing a ResultSetIterator

For instance, when implementing a ResultSetIterator for jOOλ, a simple library modelling SQL streams for Java 8:

SQL.stream(stmt, Unchecked.function(r ->
    new SQLGoodies.Schema(
        r.getString("FIELD_1"),
        r.getBoolean("FIELD_2")
    )
))
.forEach(System.out::println);

In order to implement a Java 8 Stream, we need to construct an Iterator, which we can then pass to the new Spliterators.spliteratorUnknownSize() method:

StreamSupport.stream(
  Spliterators.spliteratorUnknownSize(iterator, 0), 
  false
);

Another example for this can be seen here on Stack Overflow.

When implementing the Iterator interface, we must implement hasNext() and next(). Note that with Java 8, remove() now has a default implementation, so we don’t need to implement it any longer.

While most of the time, a call to next() is preceded by a call to hasNext() exactly once, nothing in the Iterator contract requires this. It is perfectly fine to write:

if (it.hasNext()) {
    // Some stuff

    // Double-check again to be sure
    if (it.hasNext() && it.hasNext()) {

        // Yes, we're paranoid
        if (it.hasNext())
            it.next();
    }
}

How to translate the Iterator calls to backing calls on the JDBC ResultSet? We need to call ResultSet.next().

We could make the following translation:

Iterator.hasNext() == !ResultSet.isLast()
Iterator.next() == ResultSet.next()

But that translation is:

Expensive
Not dealing correctly with empty ResultSets
Not implemented in all JDBC drivers (Support for the isLast method is optional for ResultSets with a result set type of TYPE_FORWARD_ONLY)

So, we’ll have to maintain a flag, internally, that tells us:

If we had already called ResultSet.next()
What the result of that call was

Instead of creating a second variable, why not just use a three-valued java.lang.Boolean. Here’s a possible implementation from jOOλ:

class ResultSetIterator<T> implements Iterator<T> {

    final Supplier<? extends ResultSet>  supplier;
    final Function<ResultSet, T>         rowFunction;
    final Consumer<? super SQLException> translator;

    /**
     * Whether the underlying {@link ResultSet} has
     * a next row. This boolean has three states:
     * <ul>
     * <li>null:  it's not known whether there 
     *            is a next row</li>
     * <li>true:  there is a next row, and it
     *            has been pre-fetched</li>
     * <li>false: there aren't any next rows</li>
     * </ul>
     */
    Boolean hasNext;
    ResultSet rs;

    ResultSetIterator(
        Supplier<? extends ResultSet> supplier, 
        Function<ResultSet, T> rowFunction, 
        Consumer<? super SQLException> translator
    ) {
        this.supplier = supplier;
        this.rowFunction = rowFunction;
        this.translator = translator;
    }

    private ResultSet rs() {
        return (rs == null) 
             ? (rs = supplier.get()) 
             :  rs;
    }

    @Override
    public boolean hasNext() {
        try {
            if (hasNext == null) {
                hasNext = rs().next();
            }

            return hasNext;
        }
        catch (SQLException e) {
            translator.accept(e);
            throw new IllegalStateException(e);
        }
    }

    @Override
    public T next() {
        try {
            if (hasNext == null) {
                rs().next();
            }

            return rowFunction.apply(rs());
        }
        catch (SQLException e) {
            translator.accept(e);
            throw new IllegalStateException(e);
        }
        finally {
            hasNext = null;
        }
    }
}

As you can see, the hasNext() method locally caches the hasNext three-valued boolean state only if it was null before. This means that calling hasNext() several times will have no effect until you call next(), which resets the hasNext cached state.

Both hasNext() and next() advance the ResultSet cursor if needed.

Readability?

Some of you may argue that this doesn’t help readability. They’d introduce a new variable like:

boolean hasNext;
boolean hasHasNextBeenCalled;

The trouble with this is the fact that you’re still implementing three-valued boolean state, but distributed to two variables, which are very hard to name in a way that is truly more readable than the actual java.lang.Boolean solution. Besides, there are actually four state values for two boolean variables, so there is a slight increase in the risk of bugs.

Every rule has its exception. Using null for the above semantics is a very good exception to the null-is-bad histeria that has been going on ever since the introduction of Option / Optional…

In other words: Which approach is best? There’s no TRUE or FALSE answer, only UNKNOWN!

Be careful with this

However, as we’ve discussed in a previous blog post, you should avoid returning null from API methods if possible. In this case, using null explicitly as a means to model state is fine because this model is encapsulated in our ResultSetIterator. But try to avoid leaking such state to the outside of your API.

Reference:

Three-State Booleans in Java from our JCG partner Lukas Eder at the JAVA, SQL, AND JOOQ blog.